In the military, we do so much dry firing in training and in the armory without snap caps that after almost 19 years of being a firearms intructor and cop, I highly doubt you'll need any snap caps. We just dry fire the weapons upon turn in to the armory to unload the hammer springs and put 'em in the racks. The vast majority I've done this with have been M16 series, but I've done the same thing with Garands and M14's with no problems. JMHO...
I use those red plastic spring-loaded dummies sometimes for dry-fire practice. If you match a few of them to each rifle the brass head allows the shooter to verify that the bolt is still locking up right and the firing pin's still dropping in the same place. Dummies aren't critical to dry-firing these rifles but they do buffer the hammering the firing pin takes against the rear of the bolt. At today's cost of USGI parts the longer you can keep a gun in service = $$$ saved.
I use a M1/M1A Dry Fire Device from MidWay USA for trigger practice. It's a plastic thingy that fits in the chamber and keeps the bolt about halfway open. You dry fire, work the op rod to re-**** and dry fire again. Seems to work okay.
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