Thread: Substituting projectiles

Lee f/l dies are not neck sizing dies if you remove the rod they will outside diameter the neck in a very aggressive manner . This is to accommodate brass of all thicknesses , the final neck sizing is done by the expander ball as it is drawn back through the neck . If the OP were using a bushing die then things may be different.
The cases may grow if the die sizes the body without engaging the case shoulder , not will , may . So just in case , how difficult is it to check a case or two will chamber .
The ideal would be to use a Lee Collet neck sizing die with the decapping rod lifted up out of the way . The rod itself works as a mandrel to help keep the neck concentric.
Personally, I'd air on the side of caution . The propellant is unknown and changing bullets even to one of the same weight may change the internal pressures, especially if the bullet is longer then seated to the same c.o.l , as you may be reducing case capacity . I strongly suspect mil'spec fmj's will be considerably shorter than modern hunting bullets, with their boat tail and polymer tips .
Anyway , it's entirely up to the OP how he proceeds and whose advice he takes .
I'm done on the subject.