MachIVshooter
Member
I always figured that a gun is made for a cartridge at a given pressure, which is usually a certain range under the the maximum rated. Thus, a load around that pressure--which will be near maximum even if just at a technicality (90% is 'near maximum' after all)--will usually be more accurate.
Pretty much this. Though every gun is different, most tend to perform better with the bullet and operating pressure/velocity they were designed around.
Simply put, it is a "window of efficiency", something that is not unique to firearms. Machines work best when operated within their design parameters, and performance often degrades when they're run below or pushed beyond those parameters. Think of under or over loading cartridges in the same way you would lugging or overrevving an engine.