As I understand primers, the Remington 7 1/2 primer came out to better handle the slightly higher pressures generated by the .17 Rem, over the .222 and .222 Mag. You can also included the current .223 and .204's in that list of higher pressure small primer cartridges.

The Rem. 7 1/2 has the strongest & thickest cup, with the CCI450 magnum primer having the second thickest & strongest, I'm advised. Thick doesn't always mean strongest as the temper of the metal also comes into play. I've had less cratering with Fed primers than with CCI 400's and Win.SR pimers. I have stopped using the WW and CCI 400's due to their being of less cup strength, in my .17AH. I'm now using either 7 1/2 Match or Fed 205 Match primers. The 6 1/2 Rem's, I'm advised, has the thinnest and weakest of the rifle primer cups, with the weakest ingniton in that class of primer.

The problem with the ignition in small pocket ctgs, was due to the over 40 to 50 gr. powder charges of the necked .308 case as it was just too much volume of powder for a small rifle primer to ignite properly in cold weather. This was all before the "Extreme" powders that ADI makes for Hodgdon. Maybe they ignite more easily as well?

Suffice to say, you do NOT want pierced primers as just a single one, can cut an extra hole in your bolt face, errode the firing pin into a spear point (more pierced primers) or widen the pin hole, which promotes more cratering and piercing.

When dealing with higher pressure ctgs. and small primers - do be careful and use the primers having the strongest cups that are available.