Quote Originally Posted by Group Therapy View Post
Or is the drop on heating also a sign of a worn out barrel?
Hmmm? Plausibility check?
Coefficient of thermal expansion of steel is about 12 millionths per degree C at human environment temperatures.
So taking a 0.30 barrel out of the freezer at 0 C and firing it so rapidly that it heats up to boiling point - 100 C - will increase the bore by 12 x 100 x 0.3 millionths of an inch.
Or about 0.36 of a thou.
More plausible would be from 10 C on a cold day up to 50 C (when you can't hold it any more).
About 0.12 of a thou.
If that makes the projectile so loose that there is a noticeable loss of muzzle velocity, then the barrel might indeed be marginal, i.e. very worn.
Not worn-out - it's not a bench-rest system! Worn-out is when the rifle is useless for the intended purpose. The 100 yd group seems OK to me.

Just shoot slowly (1 min between shots) and regularly, and see if the grouping then becomes tighter.