This is an interesting article:
Mysteries And Misconceptions Of The All-Important Primerhttp://www.shootingtimes.com/2011/01/04/ammunition_st_mamotaip_200909/
Apparently CCI weighs their benchrest primers. From other threads the CCI benchrest primer has a thinner cup and therefore is not appropriate for semi automatic rifles, as that makes the primer more likely to ignite when a free floating firing pin hits it.
What Bart heard from a Federal Representative is consistent to what a shooting bud of mine has said. His job is oversight of gauging at ammunition plants. Primer cake is still apparently mixed by hand, the consistency of which, from lot to lot, is highly random. The words my friend used was “artifact”. I think he means statistical accident. Workers whose primer mix is used in match primers do get a cash award.
I am of the opinion of the non homogeneity of primer cake is the greatest factor in the phenomena of sensitive primers. Primer cake is a mixture of components, the percentage and purity of which vary between lots. I believe that a thick goopy mix is impossible to achieve 100% consistency through the batch, so it is very possible that some primer cake is going to be more or less sensitive even though it all came from the same mix. And this is true, from seeing drop test data. Primers vary by sensitivity within the lot. It is being recognized now, with enough slamfire reports from shooters, that overly sensitive primers exist. Primers vary in sensitivity and every so often shooters get one that goes off when chambering a round , most often in AR’s and Garands. Until the middle nineties “conventional wisdom” was that the only causes for slamfires were high primers and worn out receiver bridges. That is, it is all the shooter’s fault, either sloppy reloads or a worn out rifle. The concept for primer sensitivity did not exist, there was no such thing, a primer was a primer. However, AR15’s don’t have receiver bridges and lots of slamfires were being reported with factory ammunition in AR15’s. You will still run into fools, particularly owners of M1a’s and Garands, who deign that there are any differences in sensitivity between primer brands. Most of these fools hang out on M14 forums and are very vocal about a primer is a primer is a primer.