Hot barrel E.S.

[Topshot]

I have noticed that as the barrel of my rifle heats up, the readings from my chronograph also increase. Has anyone also noticed this trend?

As part of my testing I fired 10 shots in quick succession over my chronograph starting with a cold barrel. The rifle was a .338 Sako with a standard weight 24" barrel.

The first four shots all showed an increase in velocity over the preceding shot, then once the barrel was quite hot the velocity readings for the remaining shots all hovered near each other. This resulted in an extreme spread of about 50 fps.

The next test I fired 10 shots and waited for the barrel to totally cool between each shot. The E.S for this string of shots was in the low teens.

I am wondering if this is due to the light weight barrel and if the same thing happens with a heavy barrel?

 

[royinidaho]

You're to be congratulated for the thorough in your investigation. Good work.

Not knowing for sure, I'd say that your results may well show that the trend is more closely related to barrel temperature increase than anything else.

I'd say that you have found the reason that heavy barrels are used for more serious long range shooting.

However, for LRH its the cold bore shot that counts. Your rifle seems pretty fair with that.

I recommend giving up on more testing and go harvest something.

Good luck......

 

[Cruizin]

I've noticed the same thing and a heavier barrel will help some because the heat dissapation will take longer to heat the entire barrel up, but on the flip side it will take longer to cool as well. I think that once that chamber heats up your round gets hotter due to conduction. The other thing that's happening is the powder is still burning as it goes down the barrel. If the barrel is hot the burn is probably more efficient. I think this is probably most of what is causing the increase in velocity.

 

[Mikecr]

Does it take you 10shts to hit/kill game?

All barrels do this, even heavy, cut rifled.
A heavy barrel stretchs the affect over time a little.
But here it's the first shot that matters, and not 5 or 10 after the barrel is stable.
So if there is a problem it aint your barrel, but rather -application.

Is this a hunting rifle?
Have you developed a cold bore load yet?

 

[Boss Hoss]

That is Very common with factory tubes. The only fix is to change it out. My sporters as well as competition rifles maintain the same avg ES even when too hot to really touch otherwise they would be of no use.

The other secret is not to chamber the round until you are ready to fire otherwise the powder will heat causing a higher pressure condition. There are no powders immune from the increase in pressure when heated.

 

[Topshot]

My main interest in this was in developing a low ES load for my rifle. I started off trying with five shot groups and the last one or two shots always blew the extreeme spread way out. I then began wondering what was causing this to happen. I then worked on ten shot groups to get more positive data.

Some of the thoughts I had were barrel expansion resulting in less resistance to the bullets travel down the barrel and also the temperature of the powder altering its burning rate. I use ADI powder and didn't think temperature would play such a big part.

I have now developed a long range load based on shots only fired from a cold barrel using 70 grains of AR2209 and a 250 grain Hornady BTHP Match bullet at 2724 fps. ES is in the low teens. Accuracy is very good.