Primers

Which primer does your load data use? You should try to match the type of primer used in the load data.

Ar or Bolt rifle? AR = CCI #41 to prevent slam fires. Bolt gun you can use std.

What does your reloading manual say?

The AR platform has a floating firing pin. If that's what you're using, the floating firing pin will routinely lightly nick the primer when the cartridge is chambered. So that is one factor that drives putting primers with thicker cups in cartridges fired in ARs: slamfire prevention. As Blue68100 said, that can be the type 41 primer. CCI 450s have the same thickness cup, and I have used them for years because they are easier to get than 41s.

any small rifle primer will work. I'd be a cautious about regular federal primers in an AR because of the slamfire possibility, but other than that, go for it.

As far as magnum vs standard in something other than a Stoner action, let the powder point the way. Some powders work better with more fire.

I normally use standard SR for my AR blackout pistol but you can do tests to find what works best for you.

Small rifle primers. Maybe, in the extreme cold, SRM primers will be a small benefit.

Blue68f100 said:

AR = CCI #41 to prevent slam fires.

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denton said:

The AR platform has a floating firing pin.

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This is a strange phenomena. I routinely see recommendations for CCI41s in ARs but I have never seen a single report alleging a slam fire in an AR. This despite hundreds of millions of handloads through ARs in the past couple decades.

If it was a realistic concern, I would expect to see reports (like we do for M1 Garands) and data reflecting the risk from ever primer manufacturer in the business. Both are conspicuous absent.

The AR pecks the primer on feeding. . . no where near as hard as the Garand, and apparently not hard enough to produce a detectable slam fire occurrence rate in several decades of widespread use.

I agree that I have never heard of a slamfire in an ar either, but it makes me a little nervous, so I don't use the softer federal primers in ar's, sks, m1 garand or carbine. Heck, any semi auto rifle. I'll save them for my lever guns.

Yep I'm a belt and suspenders type of guy. Anything that makes a mark on the primer when loading will get the hardest primer I can to prevent a potential problem. Just sayi'n.

Bolt or Semi , use whatever your book calls for...or whatever you have on hand . Magnum primers are fine if that's what you have , or maybe even called for in some load recipes . If it happens to be semi don't listen to all the slamfire myths . Sierra calls for Remington 7 1/2s in their Service rifle data . They make No mention of harder primer cups being required for ARs . They know a little about Service Rifle bullets and loads .

I've tested for slam fires and it can happen. It all depends on the gun, how free every things moves, powder of the buffer spring and weight of the bolt. The test I do is drop a round in the chamber (fully chambered) then release the bolt. With Federal in my AR-10 it will set std primers off 80% of the time, till I changed out the bolt to JPI HP bolt setup. With the JPI < 5% of the time. With my AR-15's it does happen nearly as often, depending on primer used. Now feeding from a magazine, slows the bolt down enough that this will not likely happen unless your using Fed primers.

That doesn't sound all that safe, is that the way Colt originally built them ?

When loading for the AR platform, I will use either small rifle magnum (CCI is my preference) or NATO/5.56 specific primers like CCI No 41. For bolt guns, I will use whatever small rifle match that I have on hand, especially in CZ's, as they won't reliably punch a magnum or mil primer.

I will say that I've been loading and shooting AR's for over 30 years, and during that time I have loaded a LOT of rounds with standard small rifle because that is all I've had. While I've shot thousands of rounds through AR's with standard primers and have never experienced a slam fire, that isn't to say it won't happen. We've all seen the slight firing pin indentation on the primers of chambered rounds in our AR's, after letting the bolt forward, then clearing the weapon, so you have to at least consider the possibility. I will say that extensive testing shows no statistically measurable difference in the actual performance of the cartridge between magnum, standard, and mil spec/nato primers. Oddly, standard Tula small rifle does measurably reduce the SD...go figure. The tradeoff there is that they are a straight bitch to seat in new brass...and that slightly larger and tighter fit may be the reason for the better SD

Please reference your load manuals for proper primers.

For my .300 BO I’m using CCI SRP and 1680 for subs.

Dale Alan said:

Sierra calls for Remington 7 1/2s in their Service rifle data . They make No mention of harder primer cups being required for ARs . They know a little about Service Rifle bullets and loads .

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7 1/2s have the thicker cup.

Have never experienced a slamfire in any of my ARs. Regardless, my barrel is always pointed downrange when dropping the bolt on a loaded cartridge. I generally use SRP, tried some Fiocchi SRM when I could get them last year for my .300 BLK loads. Haven't noticed any differences between standard and magnum primers. Don't have a chrony, and only load to out pukas in paper, so nothing spectacular to note.

I have used CCI#41, CCI-450, CCI-400, WSR #6 and Rem. 7 1/2 primers in my AR ammo. None have ever had a problem but I don't want to press my luck.

I use either CCI#41 or Rem. 7 1/2 primers mostly and CCI-450 primers with those are not available. I try to stay away from standard SRP for AR ammo.
Info however like the CCI-400 primers when building .223 ammo for my Howa 1500 bolt action rifle.

The only rifle I’ve ever experienced a slamfire in was an FN-49 in 7.92x57mm (Egyptian contract) with Turkish ammo. It was during a long day of shooting and it jammed on the third round. The rifle wasn’t hot enough for it to be a cook off and an examination of the brass showed normal FFE except for what looked like light primer strikes. I was kind of proud the two slamfires were almost touching - even if they were a touch high and left.
I use #34 in the Garand. They work better than standard large rifle Primers

Hmm, I've been shooting .223 and 5.56 in AR's and M16's since the 70's using SRP's and have NEVER had a single slam fire.
Imagine if you went with a heavier firing pin and had a raised primer it could happen, just that I've never experienced it.
Pretty sure the NATO spec SRP's came about when troops starting using SAW's, but do not recall them being used with full auto M16's.
More info from THR circa 2016

I've never had a slamfire in an AR and use Rem 7and a half primers but saw one happen at a match I was shooting in.

I am fairly new to AR's and just loaded a few hundred for them without problems using regular primers. I have been using regular primers with my Minis and bolt 223's for years without problem. Chambering a round in the AR did leave a light primer strike so theres that. When we get back to normalish I will be using 41's in them just for safety sake.