In 2010, I bought a little, yellow delrin plug from Midway to use for dry-fire (yawn) with the best of intentions. Fast forward seven years and only now do I realize that dry-fire is actually trigger time at your own convenience and it is free.
The yellow plug provides visual confirmation the weapon is safe for dry fire. The plug holds the bolt out of battery. When the trigger is pulled the hammer strikes the solid underside of the bolt, saving wear and tear on the firing pin. Pulling back the operating rod a quarter of an inch resets the hammer.
Without recoil and noise, your own as well as shooters on either side, a shooter can concentrate on the principles of marksmanship. Cable TV is a target rich environment. So many zombies, so little time. And it's fun.
Midway was where I bought mine but they have since discontinued the item. Sinclair International has them on backorder. Fulton Armory
and Creedmoore Sports
show them to be in stock. Cheaper than a box of ammo. For M1 Garand, M1A and M14.