Temp insensitive powders and barrel heat

I have been shooting the IMR 4166 powder in my 308 and noticed today after shooting two 10 round strings that the heat in the chamber pushes up velocity and causes vertical stringing. Should those powders that are insensitive to temp do that? My rounds run right at 2525 fps but if you start leaving rounds in the chamber a little too long the velocity climbs up to around 2550-2570 and I get vertical stringing.

 

I don't care what powder you use, you leave it in an "oven" long enough to raise the heat substantially, it will develop higher velocities. In a match, I won't close the bolt on a round until I am ready to shoot, just because of that phenomenon..

 

Shooting at 200yds this is the second 10 round target with the rounds "cooking", the first target they were all grouped around 1 1/4 inch. After I realized what was going on I just went with it to see what would happen. So temp sensitive powders are for ambient temps not accelerated temps cooking in the chamber. Sorry to sound dumb to this but until recently I was never able to get groups tight enough to notice these things making a difference.

 

The stringing may be the barrel warping from the heat or beading problems other than the powder. 200yds may not be enough to show up 25-30 fps.
Billy

 

I recently put the Magnetospeed on my Varmint wt Criterion barrel in 223 and shot a ten round group just to watch for any instability in the load, which was running on the warm side with Varget and a 69 gr Lapua.

Starting with a cold (75* F) barrel and never leaving a round chambered for more than 10 seconds before ignition, my velocities climbed with every successive shot until reaching #8, where upon is began to plateau as the barrel was now warm to the touch.

An important point I'd like to make here is that I single load, I do not shoot from a magazine, and when shooting a warmed barrel, I first close the bolt on the new round and then open again full back (I've removed the ejector) to let the round sit on the bolt head while waiting for the right wind condition. Then close the bolt and fire ASAP.

Yes, there was vertical stringing in my groups too, but I think this brings up a critical point about soaking a charge in the chamber.

In my particular case, this (chamber heat) could not have been a significant contributor to the consistent velocity rise. Something else was a work, and I believe that something else is barrel quench, or the process of the barrel drawing out heat and therefore expansion and pressure from the gas charge burning in the barrel.

Barrel quench is never discussed in any forums I've read, yet most shooters will admit that warming up the barrel before shooting for record is pretty much routine.

Anyone else share my theory about barrel quench?

Inquiring minds are always hungry for the juicy details.

 

I normally single feed mine as well but tonight I was down to the last of these reloads and just wanted to fire them off to finish this batch of brass for annealing. It just caught my attention when they started climbing up the paper like that. My Labradar caught every shot getting just a little faster than the last and towards the end you can see where they stopped climbing and like yours reached a plateau.

 

You can lump this in with the cold bore theory in my opinion.

For the most part I say no to the barrel quench.

For one there might be more variables going on with OP's gun then what we might know at this time.

How many rounds are on the barrel? If the barrel is still relatively new has it settled down? Will it settle down? Does it have a lot of rounds on it?

How is the barrel cleaning? How is it being cleaned(method and what cleaners etc...)?

If you let the barrel cool and with no cleaning does the barrel/rifle repeat exactly what happened before? If you let it cool and it picks up shooting right where it left off with no increase in velocity as it warms up the barrel just might want some fouling in it for it to be consistent from when you started shooting it when it when the barrel was clean.

Need more information.

Later, Frank
Bartlein Barrels

 

Bedding of the stock? Action screws? Barrel touching somewhere? Double check the scope for tracking/holding zero?

Stress in the barrel blank?

What is the contour of the barrel? A sporter type contour or a heavier match contour?

Again need more info.

 

Have you tried a different load?

 

I agree with Frank. I have not seen the barrel heat/vertical stringing phenomena you are describing using heavy barrel contours (M24, MTU, HV). However, more detailed information on your specific setup would be helpful to assist in diagnosing the problem.

It is worth noting that a simple trajectory prediction from JBM Ballistics predicts only a 0.2" difference in drop at 200 yd (from a 100 yd zero) for a 185 Juggernaut at muzzle velocities of 2525 fps and 2570 fps, with the faster load requiring 0.2" less elevation. Your target image shows a vertical spread of greater than 2.0", suggesting that increasing velocity was not the sole cause of the vertical dispersion.

Obviously, barrel heat is potentially a much greater problem with a very thin contour barrel. Another possible explanation would involve the barrel occupancy time for your bullets over that velocity differential, and the corresponding muzzle launch angles. A load that is "in between" accuracy nodes can easily show dramatic vertical dispersion on the target due to the change in bullet launch angle as bullets of differing velocity leave the barrel at different launch angles during the harmonic cycle.

No powder known to man is completely insensitive to temperature change. However, single base powders tend to be affected less than double base powders, at a slight cost of energy/velocity. Because a significant amount of the powder burn occurs within the first few inches of barrel, a hot barrel can accelerate powder burn, even if the rounds themselves are not chambered long enough to heat up markedly. A heavy barrel can absorb more heat and will take longer to reach a given temperature, but it also will take longer to cool down. Heavier contour barrels are typically much stiffer for a given barrel length than thin contour barrels, and the amplitude, or maximum amount of vertical travel at the muzzle will be correspondingly less.

In general, a well-optimized load with a [relatively] temperature-stable powder such as Varget, combined with a heavy contour barrel and short strings of fire interspersed with sufficient barrel cooling periods are a big part of the recipe to minimize vertical stringing. However, it is not always realistic to allow sufficient time for the barrel to cool completely (i.e. long strings of fire in competition, for example), so proper load development with a powder that is relatively resistant to temperature-induced velocity change is the best place to start.

 

Here’s a question.
How tightly filled is your front bag?
Is the sand being overly compacted as shots continue?
How well does your rifle return to battery?
J

 

If you are thinking of quench as being stress in the barrel blank...that's possible but anytime I could possibly say there was stress in a barrel blank causing problems I've virtually have never seen it string perfectly up and down like that.

Usually if a barrel blank has a lot of residual stress in it and it's common with factory barrels/button barrels....as you shoot the gun and the barrel warms up the shots/groups will walk on the target. Let the barrel cool.....you start shooting it if the first rounds go right back to the original point of impact (that's because the steel has a memory and it wants to back to where it started) again as you shoot it and the barrel warms up it drifts.

That's not leaving the rounds cooking in a hot barrel.

Food for thought.

 

In this study if was found that barrel temperature was a significant factor (in chamber pressure), particularly with Varget.

https://www.shootingsoftware.com/ftp/Pressure Factors.pdf


I'm pasting some results here.

4. In this load, Hodgdon’s Varget is highly affected by barrel temperature, with powder temperature held constant. These data were taken at an indoor range, and, as usual, the rifle was fed single-shot style.

This is a very important finding. Both barrel temperature and powder temperature are important variables, and they are not the same variable. If you fail to take barrel temperature into account while doing pressure testing, your test results will be very significantly affected. As nearly as I can determine, SAAMI does not rigorously control this variable, though individual testers might.

The effect of barrel temperature is around 204 PSI per degree F for the Varget load. Also, the small sample gathered with 4350, before my thermocouple meter went kerflooey, is consistent with this result. Since the 4350 sample is small, the uncertainty is high, but the best estimate is 177 PSI per degree F. If you’re not controlling barrel temperature, you about as well might not bother controlling powder temperature, either. In the cases investigated, barrel temperature is a much stronger variable than powder temperature.


This mirrored my experience where velocity rose gradually and consistently as barrel temp increased.

 

I’d like to see some data on a hot vs cold barrel in conjunction with a hot ammo test. A hot ammo test might not be the best description but they put loaded ammo into a temperature controlled over for X amount of time. Then they run a pressure test. I know it’s been done but don’t have any ready access to the data.

 

This should be a significant effect for any powder where a significant portion of the combustion takes place within the first several inches of barrel, which is to say most, if not all, powders we use. If the barrel is hot, the powder burn rate will be accelerated, even if the loaded round itself is not chambered long enough to get hot.

 

I think that is the often interpreted conclusion, that a hot barrel causes the combusting charge to somehow burn hotter, and therefore raise pressure.

My conclusion is quite the opposite. That the relatively cold barrel quenches the burning charge REDUCING pressure. This is, and I think I'm correct in stating this, a law of thermodynamics. That if you cool a gas in a confined space, it will reduce in volume and therefore pressure. And a cooler barrel will quench the gas charge more than a hotter barrel. It is an indisputable fact that heat from the burning charge is being transferred to the barrel during ignition.

Shooters work with this effect by warming the barrel before shooting for record, and allowing as much cooling time between strings as necessary to keep the barrel within the best operating temperature range.

In the study I reference above, the barrel was not allowed to go above a certain temperature, so it would be interesting to see what happens if the barrel is allowed to continue to heat. Would the rate of pressure rise continue to rise proportionate to the barrel temp rise? Or would it plateau, and if so, at what temperature?

At some point you'd get cook-off of the round, but that is typically the primer detonating. What if the bullet is chambered and fired quickly, before primer cook-off can occur? Its burn qualities must be effected by temperature of the primer compound, in fact the author of the above study ends his paper by bringing up this very subject. Is it primer temperature, and not barrel temp that has the most effect on pressure rise?

Inquiring minds are sure to find the answer.

 

Great, another reason to blow away more components. Ive been doing three. Now its eight you say.

 

Good point, how many rounds is enough when warming a barrel to operating temperature? It probably depends upon many factors, so measuring it for yourself is the only way to know the answer to that.

This thread has got me re-thinking my warm up process.

 

I think a lot depends on the ambient temperature. I notice here in AZ's hot dry days, it only takes a few to warm it up... especially if I'm not taking much time between shots (like 2 or 3).

And if one measures the barrel's temperature, as I often due, that seems to help with knowing just where you're at and give one some consistency.

 

I live in the desert and 100 degrees by 10am. How about a cartridge straight from the cooler. Hot barrel and cool round. Silly?