Originally Posted by DWalt

The #34 primer is less impact sensitive, to resist slam firing in M1 and M14 military rifles. There are different stories as to how it is made less impact sensitive, including a thicker cup, a shorter anvil, and a less sensitive primer mix, but I personally don't know how it is done. In any event, there is really no significant difference in primer performance between a standard LR primer and the #34. I have tested standard CCI LR primers against #34 primers in a .30-'06, and there is no detectible difference in average MV. I do use #34 primer in loading for my M1, even though I have never had a slam fire incident using regular primers in it.

BTW, I once did a fairly extensive test of CCI Standard LR primers against CCI Magnum LR primers in a .270 using IMR 4350 and both 130 and 150 grain bullets. Some loading manuals recommend using Magnum primers in the .270 with slow propellants, but my tests showed there was very little difference in average MV (with standard primers producing a slightly higher MV), and in fact I found the Standard Deviation of the MV was much lower when using standard primers. Maybe somewhere in the arctic, Magnum primers may be better, but I have to call BS on the loading manual recommendation to use magnum primers in the .270.