Load dev-how many shots does it take to dial in a rifle?

[Jarhead]

Hey!

More progress today. After a cold morning at hilltop- where it was 10 degrees colder than the rest of NSW,

I managed to test the bullet exit sensor with the 6.5- it survived!

I then got off 10 rounds ranging from 2300 fps (39 grains AR2209 with a 147gn ELDM) up to 2625 FPS (42 grains). I measured the bullet exit time and it ranged from 3.8 to 4.2 msec. I had the sensor way out in front of the barrel at the end of the magneto-speed so these exit times are not accurate. I am after a change in time (dt) and the difference in time is accurate so it works for my purposes......

The data tells me that a muzzle velocity change of 750 fps will vary the bullet exit time by 1 msec.

So 750 is "X" in the equation in my last post. Solving that gives me a 2.625 millirad change per millisecond.

What that means is that my barrel needs to be whipping up at 2.625 mrad per msec to completely compensate for point of impact changes due to changes in muzzle velocity.

Next step is to measure the barrel whip on the 6.5, with bullet exit time, plot 2.625 mrad per msec as a tangent line on the graph, and see how close I am to it - like this:

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I am guessing way off as my great groups got bigger recently.

This is great timing, as the MDT chassis arrived yesterday (No issues with it at all Tassie!)(it did feel like Xmas Die Judicii!). I set it up and based on today's results my accuracy has worsened by more than half. Of 5 groups with my (previous) best load, I managed only 1 group under 1 MOA today. Previously I would average 0.3 MOA with a pb of 0.1MOA. The groups are all strung vertically so changing the chassis means I have changed the barrel whip harmonics. The action screws are certainly tighter at 60 inch lbs and the shape of the material supporting the barrel has changed so a harmonic change is no surprise. It is actually a timely opportunity as I can use the measurement rig to dial it in. I never thought I would be happy with being 200% less accurate but I am...


It is also great that I am done testing the rig- it's like a test pilot handing over an airplane to a pilot who will fly it for it's real purpose. Now I am gonna put that thing to work!

[SCJ429]

Is 2625 fps as fast as you can drive those 147 ELDX? Seems very slow as I push a 143 ELDX to 2901 fps from a 260 Rem and 2983 fps out of a Swede. You are shooting a VLD which I am guessing is for longer ranges and giving away 300 fps which are really handy out past 500 metres.

I don't load for the Creedmoor but I am sure you can find a node a little higher up.

As soon as you bolt the Magnetospeed onto your barrel, are you not seeing a change in the frequency that it vibrates at?

[marksman]

"The groups are all strung vertically"

l'll bet the bedding is not right

[Jarhead]

SCJ429,

I can drive them faster-I switched to using 140 grn ELD-M's. I have tested up to 42 grains and just found the primers starting to flatten out. I stayed with 40 as the accuracy was really great and I am happy with not going so fast- this should extend the barrel life a bit. I might explore higher velocities in the next load dev.

I could not get both the Barrel angle sensor and the magneto-speed on at the same time as they both take up the same real estate under the barrel immediately behind the muzzle. I didn't need the angle sensor for the last test as I was measuring velocity and bullet exit variations. I am sure it changed the harmonics- any weight anywhere on the barrel will do that. I plan to test how much the very light polarised film attached to the barrel makes to the harmonics in a my next test. I also want see if there is a measurable shift in harmonics due to temp for Tassie- I need to get a rig to measure temp first. I found my thermocouple but not the associated meter that provides the temp data. pretty sure I loaned that to a friend who is stuck overseas. Hold tight Tassie- we will get there!

I did notice that I was exactly 1 milrad low at 100m with the chron mounted.

Marksman,

Overall, I am very happy with the chassis! I shot a 111 out of 120 at 700 in F-Class (Sporter/Hunter) this afternoon at Malabar.

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I think the bedding is fine (I am always prepared to be wrong!) . When I installed it, I could feel the recoil lug engage with the action screws in - it had about 1mm movement forward and back. I ensured that it was to the rear as I torqued the screws.

Are there any telltales I can look for to tell if it is a bedding problem?

[SCJ429]

I don't understand, the rifle won't shoot under an inch at 100 but you did well in a competition at 700? What did you do to improve its accuracy? You must be doing something right, I start out with a rifle that shoots in the .2s at 100 and then fail to hit the target with every shot at 500 shooting Benchrest.

[marksman]

undo the action screws one at a time holding the barrel at the fore end with the rifle standing butt on a table ect.. feel for movement as you do,
also watch the barrel and tang as you tighten
any movement and the action is bending that may be the cause of verticals as the action will snap straight when a shot is fired causing inconsistency in the harmonics

if this is the case you will have more problems at a higher velocity than at lower, the rifle should shoot the same in either stocks/chassis if the bedding is good

[Jarhead]

SCJ429,

The only difference was ammo- the rounds used for 700M were loaded to SSAMI spec (i.e. COAL of 2.82") while the loads at Hilltop were on the lands at 2.91" Same 40 grains of AR2209 in both sets.

Now that I am thinking about it another thing that would make a difference is parallax. At 700M it is set at infinity and at 100M it needs to have the target just out of focus to get no parallax error. I forgot about that until just now- Friday at 100m Hilltop it was set so that the target was crystal clear. I need to try 100m again- checking for parallax error first. I will still try some new loads and try to measure the barrel angle to get it tighter.

OK Marksman- I hear you!

[Jarhead]

Super excited- Friday at Hilltop I will finally be taking measurements (on the 6.5CM) of both bullet exit time and barrel angle with a known goal - 2.89 volts per msec is the target rate of change I should be at.

I will see how close I am to that calculated rate of change.

And see how many other similar rates are nearby (reachable with powder loads of ~ 37-42 grains- I am testing with 40 grains)

I plan to take 10 traces - to verify this is a stable/repeatable setup and also to check if there is a harmonic change with temperature change.

I have ran my OBT numbers and will compare the available tangent line targets to the OBT times and see if there is a better exit time available.

The only problem I have now is determining the start time for OBT and lining it up with my plots. Need to figure that one out. From what I have read the OBT start time is 10% of peak pressure. My instrument plots start when the firing pin hits the primer. I am not to keen to drill holes in this rifle for a pressure sensor. So I am stuck on this next step......

This solution to this problem is to shoot a few rounds of night skeet with my club, have a couple single malt scotches afterwards when I get home, and hope the brain cells line up an idea.

[TassieTiger]

Looks good JH - I’d be listening to MM re checking bedding as this could potentially infect your results.
Just because the chassis is mating to a single point in the lug, doesn’t mean it can’t or won’t move after that from recoil. My smith made up a u shaped spacer that completely encased the recoil lug, to 100% completely fill the chassis / lug interface area. It’s so tight and fits so perfect - I’m not willing to pull it down for a photo - painful to reassemble.

It may be fine regardless, but wouldn’t you want some clear base line shoots across various powder / FPS / accuracy Confirmation regardless?

[Jarhead]

Hey Tassie,

I thought I responded to marksman, but looking back the post is not there. I wrote it out but must not have hit submit.......sigh......

Sorry to leave you hanging Marksman!

I followed your guidance- I felt no movement of the barrel as I undid the action screws. I loosened the rear first and then the front. I could feel that the barrel was still rock solid against the recoil lug and there was no movement until the front action screw came out all the way and the barrel came off the action.

Halfway through that process my wife asked me to, "please don't shoot a hole in our roof." LOL

Safety first - chamber was checked empty- bolt removed. All good.

I don't think the problem is with the chassis mating, I am pretty sure it is with the scope settings. Will verify tomorrow morning at Hilltop and advise.

[Jarhead]

Looks good JH - I’d be listening to MM re checking bedding as this could potentially infect your results.
Just because the chassis is mating to a single point in the lug, doesn’t mean it can’t or won’t move after that from recoil. My smith made up a u shaped spacer that completely encased the recoil lug, to 100% completely fill the chassis / lug interface area. It’s so tight and fits so perfect - I’m not willing to pull it down for a photo - painful to reassemble.

It may be fine regardless, but wouldn’t you want some clear base line shoots across various powder / FPS / accuracy Confirmation regardless?

Tassie,

Normally yes, but now I want to use this change as a fresh start for testing the instruments and see if I can find/verify the sweet spot without a spread of different loads......

Also, I rechecked all of my maths, and need a tangent line of 15.75 volts/msec, not 2.625 volts/msec. I divided instead of multiplying. Glad I checked!

[Jarhead]

Hey all,

A semi success this morning. I managed to capture one trace on the 6.5. I planned to measure 10 shots to verify a consistent measurement. After the muzzle blast knocked the bullet exit sensor off the bench I decided I would do just that one shot and come back after finding a better attachment method.

The red line in the below image is the calculated rate of upward barrel flip I need to completely compensate for poi changes due to varying muzzle velocity.

The blue line is where the bullet left the muzzle. It is 1/10 of a division to the left of where the bullet sensor trace drops- that is because I have it 3.2 inches from the end of the barrel. Trust me on this...I laboured over the math for some time.

This trace makes me think I didn't need the bullet exit sensor anyway- as you can see the muzzle flip down rapidly when the bullet leaves.

What I can conclude from this measurement is:

1. I am not achieving complete positive compensation. I need to find a spot with a faster rate of upward flip.

<or>

2. I did the maths wrong and calculated the required rate of change incorrectly. I will redo the maths with a clear brain and a fresh sheet of paper.
I know......this isn't really a conclusion.......


Next steps:

1. Check maths
2. Find a way to secure the bullet exit sensor
3. Retest with multiple shot measurements to ensure a consistent measurement result.

Sorry Tassie- didn't get to the heat measurements. Next time for sure!

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[Larry]

Jarhead can you see any harmonics (ringing of the barrel) or are you just seeing the muzzel jump. The two are quite separate.

[Larry]

The harmonics of a barrel have often been compared to that of a tuning fork. I wonder if you hit the barrel with a hammer or perhaps something a little less intense perhaps a steel bar if you could see the barrel actually resonate. On the scope you could work out the frequency this then could be used in all the maths and accomplish what you are after. Also offer a proof to some of the theories out there.

[Jarhead]

Hi Larry,

I can assure you I am measuring harmonics and not barrel jump. The barrel jump on this plot occurs after 4.5 msec, when the CH1 plot goes off the top of the screen.

The instrument is so sensitive I could see my barrel harmonics on the plot when others nearby would let off a round. It was resonating with a stable waveform diminishing in amplitude.

I am measuring in the time domain, instead of the frequency domain. I can do both. On the trace, you can see the barrel ringing due to internal ballistics. It is not perfectly clear as the amplitude changes over the plot but there is a clear sine wave from 0-4 msec (green waveform) , and then another faster wave from 4-4.5msec (purple waveform).

You can calculate the frequency with formula:

F= 1/t

t being the time between two peaks of the wave.

So the green wave frequency is 1/2msec = 500 Hz.

The frequency of the purple wave is higher at 1/.3msec = 3.3kHz

The desired rate of change of the muzzle upwards has been calculated- 15.75 volts/msec. This can be represented as a tangent line on a time domain plot (red line). To improve accuracy I need to get the measured signal as close to the rate of change of this tangent line as possible. It looks to me that the measured waveform is steeper at 3.5 msec. So if I can make a load that brings the bullet exit closer to that, the vertical stringing should reduce.

I am not sure how a frequency domain measurement will make this easier to express but I am always willing to learn something new.

The theory of the barrel resonating prior to bullet exit is proven- this has been measured and it is consistent.

bas3.jpg (3.15 MiB) Viewed 2707 times

[Larry]

I understand what I am looking at mostly having been in the electronics field of work for 40 years. I think the time domain is the way to go. Using a spectrum analizer just looking at amplitude of frequencies would be very hard to decipher the timings of the different actions.

So did you pull the trigger at 3.4ms and it took about 0.6 ms to travel down the barrel exiting at 3.8 ms? and you wish to have it exit at 3.4 ms? if that is all true knowing the barrel length distance traveled of the bullet the speed can be calculated compared to the chrono for calc checks. The change in velocity that you want to occur can be pretty easily worked out and converted to a charge weight change, then tested both using the same setup you have now and verified using the chrono. I love the project hope it all works out for you. Keep us posted.