Ammunition is mass produced, why wouldn't tests on the same make and model suffice? In fact, tests done with new bullets would have favoured JB, because any coating is more likely to be intact.
You are only guessing, and guessing doesn't count, not for you, for me, or for any expert, I am afraid...
So, it all boils down to guess work, Bamber is convicted of these murders based on guesswork and how much lead deposit should have been found on Sheila's hands based on the presumption that she handled bullets from one batch, as a result of making comparison of hand swabs taken from volunteers who handled a different t batch of bullets?
Sounds very dodgy to me...