What causes it?  I have had it happen to me twice now on my RRA Entry Tactical 16" which I've had for 3 years.

Is there something that isn't working properly?  Does it have to do with the buffer spring tension?  I'd like to remedy this issue.

Its just a quick 2 rounds, but it has me worried for my dog's safety.  

Luckily I shoot at a secluded private place.

ETA:  ammo used was Hornady 55gr.

Free-floating firing pin combined with a batch of ammo with possibly weak primers.

High mass buffer and strong recoil spring can go into play as well if present. That's why HK have firing pin safety in HK416 (high mass HDTP buffer and strong spring is part of package).

For reloaders, it's usually caused by high primers.

Under what circumstances did your slamfires occur?

Are you SURE your finger was off the trigger?

Inadvertant bump?

I'm pretty sure the firing pin channel is clean enough that it shouldn't get stuck when it cycles.

Its only happened twice in 3 years,  the first time was about 3 months ago. Is it just getting worn?  I put about 1000 rounds per year through it.

It was new factory Hornady 55gr.

It was a sustained suppressing fire drill.  About one shot per second.

I am sure it was one pull of the trigger.

Garbage/crud/debris around the firing pin is the only time I've seen this happen. It does get your attention in a hurry.


 

When a rifle "doubles", how can you know it was a slam fire rather
than a trigger malfunction, e.g. disconnector not grabbing?

Hornady produces very good ammo, so I'd be hesitant to blame the ammo.

I'd guess FCG problem or user error, such as an accidental bump fire.


What kind of FCG is in it?  'Mil-spec' 1 stage?  Rock River 2-stage?  Other?  Have you monkeyed with the parts at all, filing anything?

Accidental bump is the first thought that comes to mind.  Next is a dirty bolt-internally.  Clean the bee jee zus out of the insides of that bolt, and while you're at it clean the firing pin extremely well too.

Had that happen to me several times.  Just happened with my FN this week.  Several doubles.  All due to my fancy trigger work.  Even happened with an AR that was on a bipod  Just like bump firing.  I don't need the attention.  I also wonder if it happens on guns that sit a while and the oil congeals, then the fcg gets gummed up and doesn't work right.  I see  you only shoot about a thousand a year.

The use of WD40 can cause trigger malfunctions. If sprayed on thick and left long enough it becomes varnish like. The only solution isto spray it on thick again, let it soak a while (15 minutes) ten scrub it with a tootbrush. Afterwards spray with brake cleaner/gun scrubber and use CLP thereafter. I had "doubles" and slam fires with an AK until I cleaned that gunk out.

AR can double, and usually will, if the trigger has been worked on to make it lighter. Sometimes soon afterwards, sometimes years later, it will begin to double. I've replaced a hammer and fixed one for a friend. It seems the lighter the pull, te quicker the failure. They actually fire one round when you pull the trigger and another round when you release it. Fire your rifle very slow holding the trigger to the rear. Once recoil has ended and your sights are lined up again, release the trigger. That is your problem if it fires again.

This post gets into the slam fire issue in good detail starting about 3/4 down the first page.

Slam fire discussion