Skip to main content

Up front, I don't intend this to be a "DIAGNOSE MY MALFUNCTION OVER THE INTERNETZ" exercise. I realize the silliness and futility of such a thing.


That being said, I want to learn more about popped primers in the AR-15 system. I've had my current personally owned patrol rifle for about 4 years. It is a BCM 12.5" barreled upper and BCM bolt/BCG/CH above a no-name lower (believed to be an Aero Precision, rebranded). The buffer and action spring are the same ones that came in it when I bought the rifle from the company that assembled it for me four years ago.


This rifle has been shot quite a bit, but not an absurd amount. It's been through a 3 day Pat McNamara class, a 2 day Kyle DeFoor class, and three years of SWAT trainings that included firing range about once a month. I've not done any maintenance on the bolt/BCG, buffer, or action spring. No parts have ever struck me as needing to be replaced.


I started running it suppressed about 4-5 months ago with my Saker 7.62 with 5.56 endcap. In the past three months, I've had four popped primers (various quality ammunition, including Speer Lawman and Hornady, all 55 grain training style loads). In the preceding four years, I had zero (that I noticed or that caused malfunctions).


My gut tells me:


1) The action spring is getting tired, and isn't putting the same pressure on the bolt/BCG. This is allowing the bolt to move to the rear too soon after firing, and as a result is sending the higher-pressured gas through the casing and busting the primer out the ass. I should replace the action spring with a .... ?


2) The action spring was never strong enough to be used with a suppressor, and I've been running the ragged edge of functionality while playing with fire and popping primers without noticing most of the time. I should replace the action spring with a .... ? 


3) Something with extractors or ejectors. Mumble mumble mumble. I should do.... ?





SO, I'm not trying to ask the internetz to diagnose my issue, but where should I start looking? Are there tests I can try to run and recreate issues to narrow this down? If I do have to replace an action spring... holy fuck. There is a LOT of conflicting and confusing information out there. How does one know what the proper spring is without buying 3 of them and shooting 1000 rounds through each? (I would rate this rifle as 97% reliable right now. I could probably shoot a whole day without issue... or not).


This was today's:




Original Post

You may or may not need to swap out your spring.  I doubt it.


Dropping primers in a suppressed Commando is a function of can back-pressure affecting timing (carrier movement and bolt unlocking).


Rather than the spring I'd give a heavy buffer or carrier weight a try first.  The point of  adding mass (weight) to the carrier is to retard the carrier from moving for a couple of milliseconds, preventing early unlocking while pressure is still high and the case is still obturated against the chamber walls.  You can see there's a lot of crap venting into the upper (which is typical with direct impingement and a can).


Or use GI ammo with crimped primers. 

Last edited by Community Member

I would check your headspace.  Normally blown primers is ammo or chamber related.  Was the ammo stored in your patrol car on a hot day or other drastic temperature shifts?


Regarding adding a suppressor:


  1. Where is your brass ejecting at (with and without suppressor)?


  1. What weight buffer?


Generally, the buffer weight makes a bigger difference then a heavier spring when adding a suppressor. I've only added a heavier weight spring in my 308 ARs, and generally just need a H2 buffer in my SBRs.   


The biggest clue if there isn't enough buffer weight/spring tension is if the ejection pattern is more at the 1 o'clock pattern instead of the 3-5 pattern.



To play it safe, it wouldn't hurt to see a gunsmith/armorer who can go over everything. 

Last edited by Community Member

We cover this extensively in our AR15 / M16 Armorer Courses.  Consider that you are popping primers on different types of ammo from different manufacturers, and the ammo is .223REM of which is lower pressure than 5.56NATO   Popping primers can be a cause of several issues.  Always start with a thorough cleaning, to include using a chamber brush and good solvent, as heavy fouling can cause issues.  Once clean, check the headspace to make sure things are still within spec. 


A worn action spring (buffer spring) will have less resistance, and can allow the bolt carrier assembly to unlock too early before enough chamber pressure has dropped.  There are written specs on the length of action springs that people often refer to for minimum and maximum length, which are an okay guide of when to consider replacing.  The length measurement doesn't always work, as we see springs that meet or exceed the minimum lengths that are too soft in resistance, and were causing problems in the guns.  We show numerous samples of bad springs in our armorer courses, and show the problems they cause, along with damage to the gun that was a result of lack of spring resistance.  In simple terms when it comes to the action spring, it's not always about length, it has more to do with strength (though you wife may say differently).        


When adding a suppressor, most rifles will increase their cyclic rate, thus resulting in an action that unlocks quicker before the chamber pressure has dropped, which can result in popped primers, fail to extracts, unsupported cases blowing up, etc.  If adding a suppressor, to an AR15 / M16 type weapon system, most times we need to slow the cyclic rate down by adding heavier buffers and/or action springs (buffer springs). 


Gas ports erosion can also be a cause of the bolt carrier assembly unlocking too early, gas ports can be gauged to get an exact measurement to see if its within spec.  


Greg Sullivan "Sully"
SLR15 Rifles

(763) 712-0123


Add Reply

Copyright Lightfighter Tactical Forum 2002-2020
Link copied to your clipboard.