You guys that are priming with hand primers may not need to think so much about primer depth, but those who do progressive reloading for gas operated semi-autos do.
Slam fires, in such semi-autos, are usually primer-caused, in less-than-perfect reloads. They are either caused by high primers, or not fully seated primers. Then there are thin-cupped primers (like some Federals)
that can and have caused slam fires, seated flush or even slightly recessed. That means that some assurance is necessary, that you can seat your primers correctly and progressively, for safe reloads.
Head space concerns, when reloading for these rifles is equally critical, but that's another subject. See the Fulton Armory Url below.
My RCBS Pro-2000 has a mechanical stop that is set to seat the primers at the same depth every time. But there's one small problem. The depth of the pocket can vary. That means that they could be "floating" (not bottomed), just right, or crushed. There's two ways to prevent such problems on an RCBS progressive. One way is to prime manually. The other, & the way I do it, is to use the primer pocket uniformer on the Trim Mate, so that every pocket I load
is the same depth. Therefore, I find it useful while setting up, and testing the setup, to
use the tail of my calipers to check primer depth.
Floating firing pins really have caused problems with primers in gas guns. CCI even makes a special "military" primer to make sure the floating firing pins in military rifles don't make for a bad day at the range or on a hunt. They aren't absolutely necessary, but they probably are a safer choice for M1 Garrands and maybe even M1A's.
Important reading from Speer on the subject
And since I've talked about reloading for gas guns, I suggest those who haven't yet, read the following from
Fulton Armory That website hides their internal pages, so once you get to the Fulton Armory site, go to their FAQ page, then to the "Reloading for Gas Guns" page.
Not that I agree with Clint McKee that gas guns shouldn't be reloaded (he manufactures them you know, and what manufacturer condones putting reloads in their guns), but his points still provide the clues to why so many gas guns blow up with reloads, and shooters should learn from, or repeat the mistakes of others. I for one feel safer heeding the warnings.