Barrel Temperature and Accuracy...?

[TomG160]

Hi everyone, I've been reading here for a while and have really enjoyed the wealth of knowledge everyone shares so freely... But now I have a question of my own.

I have a new bolt action Savage 110 in 30-06. At 100 yards I can get a fairly tight group with my first few shots (1-1.5 inches). After about 7 or 8 rounds, my group seems to really open up. If I wait a few minutes (10 or so), I can pull off another decent group. Right now I feel like blaming it on my barrel being hot to the touch, or is it potentially something else? Sorry if this has been asked before, but I've looked up and down with no luck...

Thanks for any input
Thomas

[Raymond Losli]

Heat from the barrel will conduct to the chamber causing variation in dimension due to expansion. Also a heated chamber will warm the powder in the case causing the burn rate, pressure, and velocity to change.

I have read this theory a few times and it make much sence.
.

[Zak Smith]

As the temperature of the powder is increased, the resulting combustion pressure and bullet velocity will increase. But these are unlikely to cause accuracy problems unless the temperature change is very very high.

The more likely culprit is simple thermal expansion of parts in the gun and warping of the barrel steel when heated. As barrels are machined, there is residual stress which can change the shape of the barrel as it is heated. Examples of thermal expansion would be a change in the relationship of the receiver and barrel or barrel and stock bedding as the gun heats up.

If your stock is properly free floated and bedded, there isn't much you can do about barrel warp. If there is something loose in the gun which is exacerbated by heat, a gunsmith might be able to locate it, or he might not.

One way you could test your theory is to determine if the number of rounds it takes for the groups to open up increases on cold days and decreases on warm days.

[Death from Afar]

Zac has hit the nail on the head. Also bear in mind that if not cleaned afetr the correct number of rounds ( which depends on a swag of factors such as the type of jacket and velocity of the round) then the barrel will foul and your accuracy will deteriorate too.

[ksstargazer]

Another effect I have noticed is that the heat rising off the barrel affects the appearance of the sights. The heated air causes a wavy view of the sights. That may account for some of the accuracy.