OK, make this simple for me, I am easily confused. I have two 7.65x53 (Belgian) Mausers (you can call them Argentines if you prefer). I do NOT have a Lee Collet in .303...just a plain Lee resizer, maybe the collet die has much larger internal dimensions. A once fired 7.65 Norma case will only enter the .303 die about an inch where it binds up tight..with Imperial lube. Measuring the shoulder of the 7.65 is .036" larger than the .303...rim to shoulder length is about .090" longer on the .303. Without a .303 collet die in hand, I'd say probably not.
Now to the bullet thing...I have been shooting the 7.65 Mauser since the '60's...it needs, as designed a .313 bullet (discontinued by Speer years ago), it will shoot well with a .312. In my experience, the smaller bullet diameters is pretty much a waste of time and money. It may prove to be adequate, but never great , accuracy wise. Are you guys thinking of using .308 dia bullets in the 7.65 (next to last sentence of your op)? I know it was done way back when...but I think it was the same IQ of reloader that decided all 8x57 bore dia's were created equal.
I hope you appreciate this, I trudged through snow in sub freezing temps to go to my toxic superfund epa site known as my shop to test your theory. Poor me.
