Re: Hot Barrel-POI Change
Velocity reduction is a common effect of fouling.
Powder granules are coated with an antistatic/deterrent/burn rate regulation agent, overwhelmingly commonly composed of graphite. This remains largely unburned and ends up constituting nearly all of what we call 'carbon fouling'.
This graphite fouling provides a dry lubricant effect (which many attempt to create with moly, but actually end up complicating the issue by introducing a second dry lubricating agent) to the bore, reducing bullet friction and consequently reducing pressures. This, in turn, decreases velocity.
Therefore a hot bore, with no other effects present, should shoot lower.
This phenomenon is important for load development because it affects bore transit time, and therefore, affects the ability of a load to match up with barrel harmonic nodes. Folks who customarily shoot one shot need a different load optimized for a cold bore, from folks who customarily a lotta shots and need a load optimized for a hot bore. This is one of the reasons why some see accuracy loss when barrels get hot, and others see accuracy improvement under such conditions. Barrel heating, to the degree that competitive shooters experience, has practuically no effect on barrel harmonics, but it (and fouling) does change bore times. Doing load development with a similarly hot bore can give better accuracy in sustained fire strings.
Greg