There's no mystery to a key holing problem Rob. It just means the rifle has been shot extensively with cordite ammo and the wear pattern in the leade or throat will no longer stabilize the projectile. The problem really only presents itself when boat tail projectiles are used. I really wish one of the big manufacturers would produce a flat base Mk.7 projectile. It's been discussed at length here before. I went 'round and 'round with Sierra about it. I even sent them pulled projectiles for study. Unfortunately, I don't have the thousands of dollars it will take for them to tool up. In North America we can shoot the 180 grain Pro Hunters and their equivalent but they're no good for you since soft point hunting projectiles are prohibited for target shooting. Manufacturers like Remington and Winchester have the tooling and have been producing excellent Mk.7z ammo for ever but only Remington-UMC puts out the ammo with a batch produced every now and again. Winchester doesn't even offer it.