External ballistics - what matters

[bladeracer]

I was sitting out with the cows, trying to get my head around mirage, got bored, and started running variations through the ballistic calculator on the phone. Thought I might get a feel for which things have negligible effects, and which are critical. I had also seen a discussion on an F-Class forum about loading a variety of charges and shooting the load that most suited the _immediate_ environmental conditions, and I thought it was impossible to make F-Class any more boring

(One comment that I found especially ludicrous was to wait until somebody else fired, see where they hit and adjust accordingly...as if you have any idea where the other bloke was aiming!)

This will be _extremely_ boring for most people, but it was a big job (doing it all on the phone) and I feel like sharing - feel free to click the "Back" button now

I started messing with the environmental factors, altitude, temperature, humidity and air pressure. I've always considered altitude and air pressure to be essentially the same thing when it comes to shooting, and that humidity is irrelevant.

I'm not sure it's entirely realistic to modify just one of these four variables as there is a degree of tie-in between them. Low pressure for example is likely to come with higher humidity and vice versa, but I don't think the link is inextricable, so it'll suffice for some simple experimentation.

I need a basis as a control to work with, and I started working with .22LR, simply because I've been shooting so much of it recently due to powder/primer resupply. But a little way in I thought perhaps working in subsonics might affect things differently, and .22LR doesn't have sufficient long-range accuracy if I decided I might want to test my results in the field.

Been playing with 6.5x55mm as well though so I decided to work with the 147gn ELDM, my preferred long-range bullet. And I chose a mild velocity of 2600fps as my control to give me room to see how velocity increases and decreases matter. I have 88gn ELDM's and 95gn SMK's to try in .223 but haven't even started that yet. With high-velocity stuff, BC really doesn't become relevant until you're out past 300m, so for this, I'll just check on my bullet as it zips through 500m.

So, all calculation based around a 147gn bullet with G1 BC of .697 and G7 BC of .351, launched at 2600fps. Scope is offset 2" (50.8mm) above bore, and zeroed at 200m.

I checked BOM to get realistically-related numbers to operate from.
http://www.bom.gov.au/products/IDV60801 ... 4891.shtml
Yesterday, Feb 12, we saw temps from 14C to 22.5C, humidity from 48% to 93% (1mm rain from 0030 to 0900), and pressure from 1019.4hPa to 1022.9hPa (30.10-30.21"hg). As an aside, winds were 7-30kph, gusting 11-41kph, from NE/ENE.

0000 was 16.8C, 83%, 1021.4hPa.
0600 was 16.1C, 91% (very light rain), 1020.4hPa.
1200 was 19.1C, 60%, 1022.5hPa.
1800 was 20C, 58%, 1019.6hPa.

To give this some relevance, let's make the assumption that I zeroed the rifle midday yesterday, dead on at 200m (219yd) - at 56m (187.7ft) altitude (the weather station, so we know it's real weather) in 19.1C (62.4F), 60%, and 1022.5hPa (30.20"hg) weather.

At 500m (547yd) my bullet is 1449mm (57.06") low, at 1958fps, with 432mm (17.01") drift (10mph wind).

Environmental factors:
Temperature is probably the most obvious change to most people, and it can change dramatically in seconds when clouds roll over. What happens to your bullet if nothing changes except the ambient temperature?

At 0C (it's too bloody cold to be out shooting, but bear with me) my bullet is 1479mm (58.21") low, 1920fps, 465mm (18.30") drift.
At 10C (50F) 1460mm (57.50") low, 1943fps, 445mm (17.52") drift.
At 20C (68F) 1446mm (56.91") low, 1965fps, 426mm (16.79") drift).
At 30C (86F) 1429mm (56.27") low, 1987fps, 408mm (16.07") drift.
At 40C (104F) 1413mm (55.63") low, 2010fps, 390mm (15.35") drift.

So, colder makes the bullet hit a little lower, a little slower, and a little driftier. But even 20C temperature difference only makes a difference at 500m of about 33mm (1.28") elevation, 36mm (1.44") drift, and 45fps. Not of much significance for hunting or competition, your group size is likely around 1MoA or 160mm (6.30") at 500m anyway.

Okay, humidity, it must be harder for a bullet to pass through moist air? (Relative humidity is the percentage of actual water carried compared to the potential amount of water the air is able to carry in it's current state. So 60% humidity means the air is carrying a bit more than half the water volume it could carry.)

At 0% my bullet is 1452mm (57.15") low, 1956fps, 434mm (17.10") drift.
At 33% 1450mm (57.1") low, 1957fps, 433mm (17.05") drift.
At 67% 1449mm (57.05") low, 1959fps, 432mm (17.00") drift).
At 100% 1448mm (57.00") low, 1960fps, 431mm (16.95") drift.

So, humidity alone does nothing measurable. The difference between 0% and 100% humidity at 500m amounts to 4mm drop, 3mm drift, and 4fps.

Air pressure then? Barometric air pressure generally stays in a very narrow window at sea level, 980-1030hPa. At the relatively low altitudes inhabited by most of the population, the pressure decreases by roughly 12hPa per 100m increase above sea level, it doesn't change by large degrees, at least not within the few thousand meters altitude that humans operate in.

At 980hPa (28.94"hg) my bullet is 1433mm (56.43") low, 1982fps, 412mm (16.23") drift.
At 1010hPa (29.82"hg) 1460mm (56.89") low, 1966fps, 426mm (16.77") drift.
At 1030hPa (30.41"hg) 1453mm (57.20") low, 1954fps, 436mm (17.15") drift).

Within the extremes of what we're likely to see without breathing apparatus, a variation of 50hPa (1.57"hg) at 500m affects elevation by about 20mm (.77"), windage by 10mm (.40"), and velocity by 28fps.

Obviously these factors can add up, but they can also subtract. Finding myself a nice 4000m-high mountain with ultra low air pressure gives my 500m 1958fps bullet another 19fps, 100% humidity adds another 1fps, drop the temperature to zero and I'm making 2023fps, ahead by 65fps, or 3.3%. Not exactly into .264WinMag territory And my bullet hits just 49mm higher at 500m than when I zeroed it.

If I had zeroed the rifle on a boat in the Dead Sea, at the height of summer, then gone chasing snow-bound Nepalese mountain goats, at 500m ranges, I might be in trouble...if I wasn't sensible enough to re-zero when I arrived there.

For myself, I think I can set the environmental variables around the averages for my area and ignore them. If I travel hundreds of kilometers to hunt or compete, I'm going to zero when I get there anyway.

There are some other things of interest that can affect my bullet's flight, like accurately reading wind direction and wind speed, accurate ranging, internal factors like different ballistic coefficients, different velocities, and statistical deviation in muzzle velocity, and external factors like scope-bore offset. Next time I'm watching the cows I'll have another play

[in2anity]

The F-brigade tend to over-complicate things, in a manner that adds no value. Just watch the upstream flag 2/3 of the way down for droops/taughtness, or directional changes, and don't shoot when it's changed - try and wait for it to return back to its constant. If you are running out of time you have to try and guess what it has changed to - at the risk of dropping an integer.

Sight picture should alternate between the chosen flag and sight, or perhaps flag->spotter->back into the sling->repeat. Be a little aware of "overall" flags across the whole course if the wind is being fishy.

Watch for changes in the mirage rather than trying to get too caught up on "reading" the mirage - moments of clarity through some glass probs means a gust of stronger air. Quickly increasing boil probably means a dip in wind speed. Nothing beats time on the trigger, under all conditions, at long distances, observing the mirage and how that translates to your groups at long range.

Hold/adjust your sights strategically - fractionally into the wind if the wind is kicking, thus "gusting" you more into the center V/X ring. Do the opposite if the wind seems to be dying.

Mentally call every shot before obtaining feedback - if there's a mismatch between your call and where the shot landed, then think about what may have caused it, and adjust accordingly, in a little mental circle. Trust the target, keep adjusting.

Oh and course, pull the trigger at the right time. Easier said than done. Even for the most supported of "bechrest" type setups and experienced shooters.

[bladeracer]

The F-brigade tend to over-complicate things, in a manner that adds no value. Just watch the upstream flag 2/3 of the way down for droops/taughtness, or directional changes, and don't shoot when it's changed - try and wait for it to return back to its constant. If you are running out of time you have to try and guess what it has changed to - at the risk of dropping an integer.

Sight picture should alternate between the chosen flag and sight, or perhaps flag->spotter->back into the sling->repeat. Be a little aware of "overall" flags across the whole course if the wind is being fishy.

Watch for changes in the mirage rather than trying to get too caught up on "gusting" the mirage - moments of clarity through some glass probs means a blast of stronger air. Quickly increasing boil probably means a dip in wind speed. Nothing beats time on the trigger, under all conditions, observing the mirage and how that translates to your groups at long range.

Hold/adjust your sights strategically - fractionally into the wind if the wind is kicking, thus "pushing" you more into the center V/X ring. Do the opposite if the wind seems to be dying.

Mentally call every shot before obtaining feedback - if there's a mismatch between your call and where the shot landed, then think about what may have caused it, and adjust accordingly, in a little mental circle. Trust the target, keep adjusting.

Oh and course, pull the trigger at the right time. Easier said than done. Even for the most supported of "bechrest" type setups and experienced shooters.

Yep, a shot that is actually a dead-on hold is a rarity I think, a waste of time watching where other people's shots are going as you have no idea what their hold was when they launched it.

I think I would like to be a shooter than can look at the wind, decide it's 7-minutes, dial it in and hold dead-on with the first shot. I shoot more by feel. I might think it's 7-minutes, but I don't want to lock that in, I want to be able to fudge it a bit right up to releasing the shot. I think it's learned from shooting in wind that varies from second to second

I was shooting last week and it felt like I was getting a fairly steady breeze for a change. But I put the meter up and it was moving between 2mph and 5.5mph. The transitions weren't sudden like gusts though, it was more subtle. Sneaky it was.

I'll spend a few weeks shooting and developing a feel for the wind, then we'll graze the cattle through, and the three-foot grass becomes three-inch grass. It changes my overall perception of what the wind is doing. When the whole field between myself and the target is swaying on a breeze, the wind is very obvious. Then I go out again and it's all dead still, but the wind is still there, very annoying

We don't have many trees, and no flags, so wind perception comes mainly from watching the grass.

Calling shots is a must, every time. I was practicing on Silhouettes last year with a scope on the Henry .22 and several shots were not going where I thought they should. Had a look and the rail had slid forward in the dovetail and popped off the front of the receiver

That's an issue with a bloody zinc receiver cover, you don't want to tighten anything to the point of crushing the metal

Mirage is a complexity I'm still playing with. Last week was the worst I've ever had here. I think I'd like to set up a camera at the target that I can view on my phone. Then I can compare the impact with my point of aim and get an idea of exactly how much effect various levels of mirage have on the shot. Everything I've read says that 2-minutes deflection is about the worst you can get, but I haven't seen it precisely measured as yet. I suspect military snipers probably do measure the effects of mirage in training but I haven't seen anybody doing it publicly. As range increases, the mirage can become so disrupted that the target disappears completely so you have to wait for it to reappear.

Last week I had the rifle resting on the bipod and rear bag. Looking through the scope, without touching the rifle, the target dot was bouncing all around the reticle, when it wasn't disappearing and reappearing. I don't recall experiencing it at such a level ever before. The shooting position played a part I think. My line of sight happened to follow the ground very closely - between 150mm and 300mm max - for a particularly long distance - something like 75m to 100m - before starting to drop away slightly. The target was 2500mm above ground so I doubt there was much haze at the end.

Actually, I just thought of something I'll have to check! In that paddock there is the wall of a steel shed that blew down. It might be very close to my line of fire, possibly even under it. But being about 6mx4m corrugated steel sheeting it would have been gouting heat upwards. Perhaps that was the issue?

[in2anity]

I think I would like to be a shooter than can look at the wind, decide it's 7-minutes, dial it in and hold dead-on with the first shot. I shoot more by feel. I might think it's 7-minutes, but I don't want to lock that in, I want to be able to fudge it a bit right up to releasing the shot. I think it's learned from shooting in wind that varies from second to second

No shooter will perfectly read the wind first shot every time. Remember military shooters often have a figure 12-sized target (torso), not the 1/2moa F-class X ring to deal with. Even still, at long distances, even the best snipers miss their targets. Meanwhile to hit a pig at 300m first shot - that really isn't out of the question - if you're gear is up to it. I've said it before, but I use the fullbore apporach where i multiply estimated wind-speed * wind-direction coefficient * distance coefficient, and rarely will the initial guess be any more than 3moa incorrect, then you just fine tune from there.

I think the flags are good at training you because the wind speed is extremely obvious as soon as you are on the range - so you get to marry what you are feeling on your body and face and other tells with what the value the flags a providing. Mirage is still largely a mystery to me, but the more you study it, the more you will start to be able to read it.

The digitial feedback is also invaluable because it's giving you imediate feedback.

[Biscuits]

I haven't quite figured it out, but I think you can just use density altitude instead of temperature and air pressure for external ballistics. That means you only have one variable for comparing data, rather than two. But this is _after_ you have your muzzle velocity.

Your muzzle velocity is temperature sensitive, for among other reasons because the propellant burns at a different rate depending on what temperature it is. I have chrono'd several types of ammo, but looking at my spreadsheet just now, the biggest temperature difference in my data is the same 155gr 308 match factory round with a SMK bullet. At 9 degrees Celsius, MV was 821m/s with SD of 6.4m/s. At 29 Celsius, MV=839m/s SD=4.4m/s. Remember, this is measured at the muzzle, so air pressure or density altitude should not make any difference.

Also often overlooked, is wind. Not wind for your windage calculation, but the headwind or tailwind component. A headwind will increase the drag on your bullet, requiring you to dial more elevation. A tailwind will do the opposite. A 20km/h headwind compared with nil wind will mean an additional 0.2 milirad of elevation at 800 metres. If you are shooting at a small target and you have got everything else right, that 0.2 mil is the difference between a hit and a miss.

Next purchase will be a Kestrel weather meter. I have been getting atmospherics off the phone, but that isn't accurate enough for me, plus it relies on a phone signal.

[bladeracer]

I haven't quite figured it out, but I think you can just use density altitude instead of temperature and air pressure for external ballistics. That means you only have one variable for comparing data, rather than two. But this is _after_ you have your muzzle velocity.

Your muzzle velocity is temperature sensitive, for among other reasons because the propellant burns at a different rate depending on what temperature it is. I have chrono'd several types of ammo, but looking at my spreadsheet just now, the biggest temperature difference in my data is the same 155gr 308 match factory round with a SMK bullet. At 9 degrees Celsius, MV was 821m/s with SD of 6.4m/s. At 29 Celsius, MV=839m/s SD=4.4m/s. Remember, this is measured at the muzzle, so air pressure or density altitude should not make any difference.

Also often overlooked, is wind. Not wind for your windage calculation, but the headwind or tailwind component. A headwind will increase the drag on your bullet, requiring you to dial more elevation. A tailwind will do the opposite. A 20km/h headwind compared with nil wind will mean an additional 0.2 milirad of elevation at 800 metres. If you are shooting at a small target and you have got everything else right, that 0.2 mil is the difference between a hit and a miss.

Next purchase will be a Kestrel weather meter. I have been getting atmospherics off the phone, but that isn't accurate enough for me, plus it relies on a phone signal.

Yes, as I mentioned early on, I've always considered the environmental factors to be miniscule compared to the real factors that affect your shot, wind being the biggest at longer ranges. I don't consider ambient temperature to matter either, at least not with modern powders. You 60fps difference is not worth consideration for the vast majority of shooters and hunters. With some chamberings, 60fps is probably less than their normal shot-to-shot extreme spread. Your chamber temperature (thus your powder charge) will likely be more than 20C higher for your second shot anyway. And unless you averaged those velocities and SD over many, many rounds, the sample size is likely too small to be significant. Near enough for ballistic calculation, but so close that both could be within the same window. One reason I particularly enjoy shooting .22LR at long ranges is the amount of shooting you can do means you can gather a _lot_ of data very quickly, and very cheaply, and the effects are magnified so they are more obvious and more easily measured.

Yes, wind is the biggest issue. Shooting .22LR at long distances really brings it home. Even an imperceptible wind has huge effect on the bullet at 350m and further. Not an issue if you are simply walking your bullets onto the target from missed splash feedback, but trying to make a first-shot hit can be extremely difficult, even when there appears to be no wind.

But wind can never be measured precisely, it is alive, changing by the millisecond, and it is different at every millimetre of the bullet's flight. We can only ever make an experienced guess at the ultimate effect its perceived properties might have on the bullet from one shot to the next.

I use a cheap wind meter that has a temp gauge, so it gives me wind speed, direction and ambient temp. I also have some higher-quality wind meters with remote vanes, but I rarely bother taking them out, the smaller one lives on my belt fulltime with my rangefinder and is more than adequate. I have used apps on the phone that claim to give me environmental measurements. Fun to watch when driving around the hills, and I'm sure it can give me a temperature reading with reasonable accuracy, but I don't trust it with elevation, air-pressure and humidity as I've never tested it. It also gives me lux, but I have no idea how it reads it or how it relates to the light entering my scope. Right now I have one app telling me I'm at 233m, I am actually at 161m ASL - two other apps are correct in altitude. It also says I'm at 985.25hPa which may be correct - the airport is reading 1002hPa and is 8.86km from me at 55.7m ASL.

Thankfully, these factors have virtually zero impact on shooting, at least within 1000m or so. You can do all sorts of measurement and calculation from environmental measurements all along the flight path, but the wind will still toss your numbers in the bin.

If it's a very cold morning I can aim a few inches high, if it's 45C I'll hold a few inches low. Take note how far off my reticle the bullet hits, and stow the data for next time

[in2anity]

Thankfully, these factors have virtually zero impact on shooting, at least within 1000m or so. You can do all sorts of measurement and calculation from environmental measurements all along the flight path, but the wind will still toss your numbers in the bin.

Exactly this. Perhaps such parameters may come into play for that ELR stuff (I wouldn't know), but inside fullbore distances out to 1000yds, just worry about the wind and use your phone app at the very start if you are being super pedantic about the opening shot.

[Biscuits]

With regards to muzzle velocity, a 60fps error in your muzzle velocity will translate into a miss at intermediate ranges.

I've plugged in generic 308 data with a G1 BC = 0.45 and run two cases. Both identical, except one has a 2700fps MV and the other has a 2760fps MV. 10mph crosswind for both.

https://shooterscalculator.com/ballisti ... ate+Graph+

and

https://shooterscalculator.com/ballisti ... ate+Graph+

Let's say you want hit a 12 inch target at 800 yards. You have been there before in the middle of winter and hit the target smack in the middle with a certain elevation setting. S You go back in the middle of summer and your MV has gone up by 60fps, but you don't adjust your elevation. With the 12 inch plate, you can be off by 6 inches any direction since your last session and still hit the plate.

Using the 800 yard data, you are going from 215.6 inches of drop to 204.7 inches of drop. A difference of 10.9 inches. You've gone from hitting the centre of the plate, to missing the plate by 5 inches. Even if you get your wind calls perfect, you are still going to miss. That's just the difference in MV due to the temperature of your cartridge/power before it is fired. On top of that is the change in barometric pressure.

On the cartridge heating up in the chamber; obvs the cold bore shot might be different. But in general I only chamber a round when I'm about to fire, I don't normally leave it sitting in the chamber for any length of time, so it won't heat up much. I've certainly had my share of misses due to not getting the wind right, but why add to those misses by not accounting for temperature and atmospherics?

[bladeracer]

The discussion isn't really directed at people who use a ballistic application when they're shooting, if you're doing that you'll put all the info in, get the answer, and still get screwed by the wind. This is more for a shooter to make guestimates in the field without using a calculator, just a basic DOPE sheet. You know your bullet drop every 50m say, do we need to be concerned with the environmental factors? I say no, not for hunting to 500m or so, it's wasted effort and unnecessary distraction. Concentrate on studying the wind and making the best shot you can. Your elevation will be within a couple inches (which is covered by simply holding high if it's cold, low if it's hot), the windage could be anywhere up to a meter away if you aren't reading the wind correctly all the way along the bullet's flight path. Where you are in your handy little hiding spot the wind might be reading next to nothing, the grass at the target might be indicating a very light breeze, the invisible wind howling along the valley you're shooting across might be doing 25mph, don't worry about the temperature. The video I posted this week of Mark shooting the .303 at 3000yds was probably a wind hold of 40m or more.

I knew a couple of blokes just a few years older than me that have gotten hooked on long-range deer shooting, 750yds or more. But they have found themselves a spot where they shoot a lot and have studied and learned from shooting in that environment. They know what the winds do in certain places at certain times of the day simply from shooting in those conditions and recording what happens. And they use ballistic calculators, and hunt together so they can discuss the shot beforehand. If you're chasing a pack of goats that you often see in a spot you can't stalk into, take some shots at that spot under different conditions when they're not there and record your DOPE. I did this on the rabbit warrens I liked to shoot over with the .222 when I was a kid, I could go out there anytime and have a very good idea of where my bullets would be hitting when the bunnies were outdoors. Would I try a 300m shot on a goat in an area I haven't been shooting in before, possibly, if I was confident of my read of the wind - at 400m, nope.

We can get 20C+ variation just in one day, no need to wait six months. Monday last week we had a minimum of 16.1C and a max of 36.5C at my local weather station. Go out at 0600 then go back at 1400 if you want to practice in temperature variation. As I said, if it's colder than when you zeroed just hold a few inches high, if it's warmer then hold a few inches low. Your 65" wind hold is what will cause the misses, not the 10" elevation due to temperature. If you need a first-shot hit then do the calculation for sure, but you've got to be very, very good to call the wind accurately enough for a first-shot hit even at 500m. If you're happy with a second-shot hit then don't bother, fire shot one and correct from it, if you can see where it hits. I think all of the extended-range steel shooting I've seen is all about walking your bullets onto the target, so none of this is relevant at all. You can generally ignore all of the variables and simply fire a well-aimed shot, see where it hits in the paddock, dial your scope across to the impact, and you're good to go. You do need to visualise the impact on a flat surface at the target distance of course, if the impact is in the dirt 20m behind the target, depending on the angle it might appear to you to be two-inches above the gong, but if it's 20m behind it you need to come down about 20-inches. Range the impact if possible as well as the target. If you're trying to lase a small target, lase other things around that area to backup your target range. If there's a tree nearby, lase that and estimate its range compared to the target, same with any rocks you can see. Don't take a shot based on a single laser measurement if you can avoid it.

Take a measurement on a fresh round, direct your IR thermometer into your cold chamber, fire a round, extract it and take another measurement. Chamber a fresh round for ten seconds, remove it and take a measurement on it. The fired case is likely hot enough to burn your skin, probably 50-60C. It's brought a lot of heat out of the chamber with it, but the chamber is still very hot. The unfired round is probably more than 20C warmer. Ten seconds is fairly quick aiming in long-range shooting.

I was shooting .22LR back in December. The ambient temperature was warm, but not extreme, probably mid-thirties. But the direct sun was making my timber benchtop very uncomfortable, so I grabbed a thermometer just to assuage my curiosity. The bench top was 47C from memory. My ammo on the benchtop was 54C. Did it affect my 210m shooting, possibly, but not enough that I noticed any difference from when I started - when the ammo was probably 30C cooler from my office. With .22LR the difference in 40C ambient air temperature at 210m should be around 2.2", still not worth worrying about. I doubt 40C powder temperature variation has any measurable effect, at least not with modern powders.

With regards to muzzle velocity, a 60fps error in your muzzle velocity will translate into a miss at intermediate ranges.

I've plugged in generic 308 data with a G1 BC = 0.45 and run two cases. Both identical, except one has a 2700fps MV and the other has a 2760fps MV. 10mph crosswind for both.

https://shooterscalculator.com/ballisti ... ate+Graph+

and

https://shooterscalculator.com/ballisti ... ate+Graph+

Let's say you want hit a 12 inch target at 800 yards. You have been there before in the middle of winter and hit the target smack in the middle with a certain elevation setting. S You go back in the middle of summer and your MV has gone up by 60fps, but you don't adjust your elevation. With the 12 inch plate, you can be off by 6 inches any direction since your last session and still hit the plate.

Using the 800 yard data, you are going from 215.6 inches of drop to 204.7 inches of drop. A difference of 10.9 inches. You've gone from hitting the centre of the plate, to missing the plate by 5 inches. Even if you get your wind calls perfect, you are still going to miss. That's just the difference in MV due to the temperature of your cartridge/power before it is fired. On top of that is the change in barometric pressure.

On the cartridge heating up in the chamber; obvs the cold bore shot might be different. But in general I only chamber a round when I'm about to fire, I don't normally leave it sitting in the chamber for any length of time, so it won't heat up much. I've certainly had my share of misses due to not getting the wind right, but why add to those misses by not accounting for temperature and atmospherics?