.310 seems a bit skinny for a this kind of projectile in a 7.62 barrel. For a soft/short lead ball I aim for 0.002-0.003 over the groove diameter (slugged at .314 on my 1918), so .316.
For a short projectile and low pressures I don't think I can depend on inertial upsetting to seal around the bore even for a butter-soft lead projectile, so I went oversize and let the combustion pressure push the ball through. It's the same size ball as I used for slugging the bore, so I know from experience that it doesn't take much force to drive it in at 0.316.
Obviously you don't oversize like this for a longer jacketed projectile. That causes all sorts of trouble.
Ball is a bit misleading, after ring-sizing it looks more like an inflated aspirin pill.
Also, the .316 size gives me a nice snug fit into the necks of the fired/ringed casings, so no resizing is required.
>a bit of Dacron
I did that for the first batch, but then changed to cotton wool. I never observed any trouble, but I know that incompletely burned PET leaves a sticky thermoplastic mess whereas cotton just leaves charcoal/soot that wipes off. So far a couple of cotton balls pilfered from the boss' makeup drawer have lasted me several batches.
Lastly, I noticed that when (jus pour rire) I upped the loading by a couple of grains to see what would happen it went from a light and cheerful -pow- sound to ... angry -crack-. I only fired one of those and broke down the rest. This business of inappropriately fast powders in rifle cartridges is only for very light charges.
Although in hindsight that may just have been the projectile going supersonic. I might try it again.