As the above poster indicates, he is having good luck with 264 diameter bullets. I have had good luck with 264 diameter bullets as well, especially in 160 and 140 grain weights. The lighter bullets don't seem to do well.
I don't think it's because of the fast twist but because of the long and generous diameter freebore that is there to facilitate the longer bullets the rifle was designed to handle.
Carcanos are capable of very good accuracy. Their soft nosed bullets go all the way through a deer and black bear with aplomb. It also takes down a moose very well, as long as you do your part.
Back in the day, the 6.5 Carcano cartridge and its long for calibre bullet were very progressive and forward thinking designs. Still not a lot wrong with them. The recoil is light and the effect on game is very telling.
Why don't you get a mould and cast up some lighter bullets. Then you will never run out and, you can satisfy your curiosity.
Contact smellie, he loves Carcanos and has likely tried everything that you are looking to do.
I suspect, you are trying to get more velocity out of that old girl. The action is strong enough to take stouter loads but the case capacity is on the small side. If you want more velocity, pick up a Swede 6.5x55. Most Swedes will handle the lighter bullets very well
