Because heat will move the point of impact in a barrel, whether it be a rifle, shotgun or cannon barrel. In trap shooting years ago we found by shooting the point of impact on a hot day shooting much shells, the point of impact will move in a shotgun barrel as much as 12 inches. If you wonder how the military does this on sniping or cannons and stuff they have all there barrels Cryogenic Frozen. It's a process where by the temp in the barrel is brought down to minus 300 degrees fairenhight, then taking the same about of time taking it down to that temperature it is brought back to room tempature. If down properly we found that 12 inches will change to about 3/4 of an inch. The miltary has all there cannon's etc, down this way, i believe they can do this up to either 88's or 105's. By doing this with a rifle barrel you don't have to (as it is a shoulder mounted cannon) sit around and wait. This process takes the stress out of the barrel. Does not harm the barrel in any way. I send mine to 300 below in Illionis. The main thing is to find a reputuable company. I know this about the miltary as this company i use has miltary contracts. They also do the rifle barrels. the cost is about $99 per barrel, but i have found in the later winter they do specials like 2 for one.
