Does barrel heat affect group size

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Does barrel heat affect group size

Post by Kelsey Cooter » 20 Feb 2018, 3:25 pm

I'm not talking extreme heat, but should I wait for the barrel to fully cool/ come back to the same temp between shots when testing loads or do a five shot group then let it cool?

Asking the question for 223, 308 and 300wm
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Re: Does barrel heat affect group size

Post by JimTom » 20 Feb 2018, 4:29 pm

Again I am definitely no expert mate but from personal experience, my group size does increase as my barrels heat up. Bearing in mind that I only have fairly light barrelled hunting rifles (.223 / .308).
When testing loads I've worked up generally I'll shoot a couple of 5rd groups then alternate rifles or have a spell and let it cool down.
That's how I do it mate, perhaps one of the more wiser (much older) :D contributors could shed a bit more light on the matter.
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Re: Does barrel heat affect group size

Post by marksman » 20 Feb 2018, 5:05 pm

if you have a thin barrel yes, a heavy barrel takes more to heat up but takes longer to cool when heated up
I would do 3 shots in succession to test a hunting load as with the caliber's you have suggested that is probably what you would shoot at one time max
I would also suggest and I do test at different distances from 50-120-180, I do shoot further but when testing these are the ranges I shoot
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Re: Does barrel heat effect group size

Post by bladeracer » 20 Feb 2018, 5:38 pm

Kelsey Cooter wrote:I'm not talking extreme heat, but should I wait for the barrel to fully cool/ come back to the same temp between shots when testing loads or do a five shot group then let it cool?

Asking the question for 223, 308 and 300wm


For load development you want to replicate how you intend to shoot in the field.
If you were shooting long-range precision you'd warm the barrel, and then try to space your shots so the chamber is at similar temps. I think some even use thermocouples to measure it.

I try to shoot a five-shot group fairly quickly as that best suits whatever shooting I might be doing, and gives a more comprehensive picture of potential accuracy than a three-shot group. To zero my Norinco for offhand silhouette I shoot a thirty-round group simply because the group is so big I need more data points to get an accurate point of impact.
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Re: Does barrel heat affect group size

Post by straightshooter » 21 Feb 2018, 8:03 am

The answer is "it depends."
If you happen to have a high quality barrel or even a reasonably good factory barrel you won't see much due to changes in temperature. Far worse effects will be seen from allowing the cartridge to heat up by sitting in a hot chamber for too long.
There is no such thing as a perfect barrel. Some high quality barrels come close though.
Some important factors are: how dimensionally consistent is the bore, how concentric is the bore in the barrel, to what extent does the bore snake down the barrel, how consistent is the twist.
You can't have the same expectations from a factory barrel that may have cost the manufacturer perhaps under $50 and had only a quick visual inspection as you would from a custom hand lapped barrel that these days might cost $750.
If you have a more unfortunate combination of deficiencies in your barrel then you might see stringing of the group as the barrel heats up. This is often misdiagnosed as a bedding problem.
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Re: Does barrel heat affect group size

Post by in2anity » 21 Feb 2018, 8:24 am

I know OP was asking about a bolt, but FWIW a new lever out of the factory usually has fairly tight barrel-bands. Levers can be noticeably affected by barrel heat because as the barrel heats and changes slightly, the barrel effectively warps due to the pulling pressure at the band contact points. That's why it's worth winding your screws back half a turn and having a tiny bit of play around the bands (where possible). This is some of the low hanging fruit when accurizing a lever. Better still, if accuracy is important to you, avoid levers with bands!

Getting back to bolts though, IMO barrel heat should be treated as another variable. For load development I take a second plinker to the range and let the barrel cool between groups; it's all about eliminating variables.
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Re: Does barrel heat affect group size

Post by Gwion » 21 Feb 2018, 10:44 am

in2anity wrote:
Getting back to bolts though, IMO barrel heat should be treated as another variable. For load development I take a second plinker to the range and let the barrel cool between groups; it's all about eliminating variables.


^^^ This ^^^

Also, as mentioned above, don't leave your rounds chambered while setting aim at the bench. Get set for your 'Natural Point of Aim' (NPA: this is where the rifle is at rest and pointing at your intended POI; even if you take your hands off the rifle in the rest, the cross hairs should stay on the bull). Chamber your round and take your shot. Immediately eject your round and leave the bolt open to allow air flow through the barrel & chamber. Re-set NPA. Chamber. Shoot. Eject. Repeat.
After your desired number of shots for the group, it is ideal if possible to stand the rifle up with the bolt open to allow the heat to rise and draw cool air in the chamber/receiver. This may not be allowed at the range but even having the muzzle higher than the chamber will help the 'chimney to draw' and cool more effectively.
As in2anity suggests, having a plinker to fore off some fun shots or another rifle to test and the same time will allow more time for the barrel to cool. Steady is the key in my book. Not too fast, not too slow.
I see people sitting with a round in a hot chamber while waiting for a shot and re-setting NPA and cringe just a little because all that hard work they are doing to pull a tight group is being counter acted by building fluctuating pressures from powders heating up to varying degrees in the chamber.

Again, this is just for precision shooting like in f-class or when testing loads & tuning rifles. Of course in many applications you will have a round chambered while taking aim. When testing loads you want to see how consistent it is and the only way to do that is to fire each round under as consistent conditions as possible. Once you are happy with your load, then you can do some rapid fire tests to observe how the load/rifle behaves in those situations (say in a rapid fire event or busting up a mob of ferals).
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Re: Does barrel heat affect group size

Post by bladeracer » 21 Feb 2018, 12:17 pm

straightshooter wrote:Far worse effects will be seen from allowing the cartridge to heat up by sitting in a hot chamber for too long.



I thought this had been pretty thoroughly debunked as urban myth these days? If the chamber were extremely hot (like rapid semi-auto fire) I could see it potentially causing problems though. Do F-Class shooters generally take measures to keep their ammo in a temperature-controlled environment?

I've found some .22LR (particularly SK and Eley) doesn't take well to being left in the sun, the lube melts, sometimes making it a real struggle trying to load a magazine with them :-)
I haven't tried testing "melted" .22LR alongside cool stuff but I might have to do so sometime just to see if there's an effect.
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Re: Does barrel heat affect group size

Post by Gwion » 21 Feb 2018, 12:23 pm

bladeracer wrote:
straightshooter wrote:Far worse effects will be seen from allowing the cartridge to heat up by sitting in a hot chamber for too long.



Do F-Class shooters generally take measures to keep their ammo in a temperature-controlled environment?



Yes. It might be as basic as keeping it out of direct sun but it is still preventing a change in pressure & velocity.
Some newer powders may be more temp stable but adding heat in a confined environment is still going to change the burn characteristics of the powder and therefore the pressure and ipso facto the velocity and thereby the POI.
I don't know how that could possibly be debunked??? :unknown:
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Re: Does barrel heat affect group size

Post by sungazer » 21 Feb 2018, 12:59 pm

Yes to the F class shooters I have seen MTM boxes that have aluminum foil glued to the sides. They nearly always leave the round sitting in the action and only close the bolt when ready to fire. Some will have closed the bolt but not locked it down until just before firing 10fps at 1000yrds is the difference in an X and a 6
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Re: Does barrel heat affect group size

Post by bladeracer » 21 Feb 2018, 1:16 pm

Gwion wrote:Yes. It might be as basic as keeping it out of direct sun but it is still preventing a change in pressure & velocity.
Some newer powders may be more temp stable but adding heat in a confined environment is still going to change the burn characteristics of the powder and therefore the pressure and ipso facto the velocity and thereby the POI.
I don't know how that could possibly be debunked??? :unknown:



I would think you'd probably get more temperature variance in the chamber due to the varying time taken between shots than a few degrees in ambient temperature. If you have to wait thirty seconds for the scorer to raise the target after your first shot, forty-five seconds after the second, and fifteen seconds after the third the chamber temp could vary significantly?

As to how it could be debunked, I've read and seen video of precision shooters testing it and finding no measurable difference. I haven't seen or read of anybody doing the test and finding it made a difference, do you know of such a test?
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Re: Does barrel heat affect group size

Post by Gwion » 21 Feb 2018, 1:24 pm

Nope. Never seen a test for it.

The varying temps in the chamber is why you only chamber the round when you are ready to fire, minimising the time any heat in the cha mber can affect the powder.
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Re: Does barrel heat affect group size

Post by bladeracer » 21 Feb 2018, 1:48 pm

Gwion wrote:Nope. Never seen a test for it.

The varying temps in the chamber is why you only chamber the round when you are ready to fire, minimising the time any heat in the cha mber can affect the powder.


Found this article: http://bulletin.accurateshooter.com/2017/07/how-ammo-temp-affects-pressure-velocity-and-point-of-impact/
This was shot with ammunition that started at temps of -3.6C, 21C and 54.4C and shows a significant difference in velocity and PoI.
I guess it's not a myth after all, my bad.

I would've liked to see the chamber temperature monitored as well though. Feeding a frozen cartridge into a chamber that's just been fired is likely to see a very rapid climb in temp compared to feeding a 50-degree cartridge in?
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Re: Does barrel heat affect group size

Post by sungazer » 21 Feb 2018, 1:55 pm

If you are waiting for a certain condition to come back you may wait 5 min or more so not putting the round in the chamber every time until you are ready is a good practice to get into. from you 25deg dope or even 30 deg dope table and the temps it can get to when left in the sun can easily be the 20 deg difference then if you fired 5 or 6 quickly even every 30 seconds the barrel would probably be getting toward 60-70 in parts.
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Re: Does barrel heat affect group size

Post by marksman » 21 Feb 2018, 1:56 pm

I myself want to see how the rifle is going to perform while testing and do the 3 shots in succession or rapidly, snap shooting at different targets for hunting practice
no different to benchrester's who shoot 1000 and choose to fire rapidly to beat the wind, like Stewart Elliott
www.youtube.com/watch?v=03qtXpoObfM
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Re: Does barrel heat affect group size

Post by Gwion » 21 Feb 2018, 2:11 pm

marksman wrote:I myself want to see how the rifle is going to perform while testing and do the 3 shots in succession or rapidly, snap shooting at different targets for hunting practice
no different to benchrester's who shoot 1000 and choose to fire rapidly to beat the wind, like Stewart Elliott
http://www.youtube.com/watch?v=03qtXpoObfM


Sure. That's all part of testing but I wouldn't do it in load development or when sighting in because rushing shots (rapid fire) just increases your chances of error and fliers, giving false data as to how the load and rifle are performing. A well tuned load and rifle will shoot straight whether fired slowly or quickly but the shooter error will increase as speed increases: generally.
Long strings with light barrels: say, more than 5 shots in a minute: is when you are going to see significant change in POI due to a change in barrel dynamics.
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Re: Does barrel heat affect group size

Post by sungazer » 21 Feb 2018, 2:17 pm

Yes there is no way you can compare a tikka lite barrel with a Heavy Varmint profile in a custom rifle 1.25 in for 5 inches then straight taper to .9 at 32 inch for instance.
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Re: Does barrel heat affect group size

Post by brett1868 » 22 Feb 2018, 2:52 pm

I prefer setting a cadence for fouling shots then lead into the scoring rounds being fast or slow depending on the wind. I actually got this idea from a state champion pistol shooter but seems to be working for my rifle shooting as well.
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Re: Does barrel heat affect group size

Post by brett1868 » 22 Feb 2018, 3:02 pm

To answer the question this time....Yes, barrel temp will affect group size but it's one of a number of factors contributing to blowouts. For a light barreled hunting rifle then set your zero on the first cold bore shot cause that's probably how it'll be most of the time. Ammunition should be kept at a consistent temperature cause it'll move around as temps vary, not a lot @ hunting distances but enough at distance to hurt your score. I took the time to make a timber ammo box for the big gun just to insulate the ammo from the days temp variations and to keep them suspended pointy end down.

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Re: Does barrel heat affect group size

Post by Gwion » 23 Feb 2018, 10:45 am

That's great on electronic when you can fire in a chosen patch of wind but when you're stuck in the medieval period with clunk manuasl targets, your shooting pace is somewhat dictated by who ever is in the butts...
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Re: Does barrel heat affect group size

Post by sungazer » 23 Feb 2018, 10:56 am

:D :lol: There is great debate over that amongst some that are against the rapid fire technique and want delays put in electronic targets. Personally I think let the electronic targets be the technique can work for you and against you. There was never a level playing field with manual targets.
Then there are those that question the accuracy of them. I have been at comps with manual targets and some competitors question the marker repeatedly I find that a pain, then sometimes they are right and the marker was making mistakes. So errors are just as present if not more with manual markers.
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