I'm trying to decide which primer is best for an AR15. My biggest concern is slam fires. Deciding between CCI 400 and CCI 41. Thanks for the input.

The CCI 41 is specifically made for use in military style semi-autos with a harder cup to avoid slam-fires. That being said, I've fired hundreds of rounds primed with 400s with no problems, in both 5.56 and 6.8. My 458 socom uses CCI 350s, a softer pistol primer, without issue.

Now days I do primarily use the 41s in ARs just to be on the safe side, the same reason I put a light crimp with a Lee FCD.

I've used basic small rifle primers for years in my AR15. Never had a slam fire.

I have used Rem 7-1/2's for the last 30 years or so with no problems and if I were a betting man, I would say they are as soft as they come. They are copper colored primers and allow me to sort crimped from swaged primer pockets when reloading. I really only learned years later about the CCI military primers when I started loading for my M-1A and saw pictures of the results of a slam fire. I would not hesitate to use the regular CCI's, but the 41 is sure cheap insurance!

I've shot a couple thousand CCI400's in AR-15's. They are not ideal but work well. The problem is they have a thin cup and crater around the firing pin. If the primers pierce the will damage the firing pin and possibly the bolt. My primers flatten due to the thin cup and crater. The only true answer is that if you have a thicker primer go with it but if the CCI400 is all you have than use it. I've also used CCI450's and Wolf Magnums.

CCI 41 or Magnum primer.

Remington and winchester ran Lake City Army ammunition plant for over 60 years before the No.41 and 34 primers were invented.

And the Remington 7 1/2 primer was all that was used when Remington ran Lake City from 1941 until 1982.

It is recommended that you use primers with a cup thickness of.025 to help prevent slamfires.