Primer choice

[Mebe007]

Realistically what primer should I be using? I use cci #41 on my 5.56mm and cci 450 in my grendel. Should I be using the same cci 450 in the arc given that it is an ar to avoid slamfire or should I just use cci400? I see the hornady book says federal 205. I thought federal primers were a nono in semi automatics

[grayfox]

Slam fires have been known to occur although I think nowadays they are rare, but then again I don't know your AR setup. Frankly a lot of those slam-fires could have instead been b/c of light trigger pull (allowing trigger to reset/fire without locking back) - just an opinion since the double-fire-light pull issue has arisen again in some posts.
450's and #41's are best IMO but if 400's work in your - I don't know that I can say you should absolutely avoid them.
The issue with 400's is the relatively thinner metal cup, 0.020 if I recall. 450/41's have a thicker one, 0.025 so it's harder. High pressure/hot loads can pierce the 400's quicker than their thicker brothers.
I use 450's mostly in my AR-grendel and 6Arc, and #41's for 223 and BO...
But I have some 205M's for a bolt action 6Arc that appear to be the best choice, I'm going through that testing right now.
One rifle that cannot use the #41 is my howa Grendel, the firing pin apparently cannot cleanly fire them every time... so I don't use 41's in that one.
Bottom line, in today's environment you probably should use what you have, in rifles you can trust them in.
450's have been available a lot lately, so I have a bunch of them. But want to use my others where I can, too.

[Old Bob]

Here's a chart with small rifle primer dimensions. CCI #41 primers are the same thickness as CCI 450s.

I use CCI #41s in my .223 Rem/5.56 NATO AR15 reloads; CC450s in my Grendel/6mm ARC AR15 reloads; Federal 205Ms in my Grendel, 6mm ARC & .223 Rem bolt gun reloads. I may use Rem BR 7.5s in my bolt guns as well. Found a full brick of them stashed away I forgot I had. I have some CCI 400s too that I might use in my .223 Rem bolt gun. I have a small stash of Winchester SRs (less than 100) that will be used for fire forming.

[GRH]

Plain ol' Fed 205, ES 15 / SD 5.4 with A2520 pushing a 105gr out of my 20' gas gun.

If there is any concern using them, second place goes to Fed 205MAR ES 21 / SD 7.

Then there's Titanium firing pins for bragging rights.

I have chambered rounds and ejected it to see if there was a dimple or any mark, and if there was I honestly don't remember seeing one.