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Are you sure you want Chinese 0-rings securing your M4 firing pin retaining pin?


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Like many of you, I waited a long time to pull the pin for a M4.  I just received a spanking new M4 LE H20.  There seem to be two ends of the spectrum on the first things to do with a new gun, one end is to start blasting away, and the other is to field strip, inspect, clean and lube everything prior to firing.  Being of the clean school of practice, when I got to the BCG, I noticed there was a chunk missing from the little o-ring that holds the 028-J firing pin retaining pin.  See the pic, beautiful US made mil-spec o-ring on the left...damaged 0-ring on right.   No problem, I thought, just order some spares...

Welcome to the world of Benelli parts.  Midwest has the firing pin retaining pin and o-ring for $6 plus $7.99 shipping, and Freedom Fighter Tactical has just the o-ring for $13 plus $16.50 for shipping.  So between $13.99 and $29.50 for this o-ring?  And of course, no one, not even Benelli, reveals the exact dimensions or where the o-ring is sourced.  

An internet search reveals that one common solution is to take the 0-ring to Home Depot, or order something similar on Amazon.  The problem is that these o-rings are from China, which is why you can buy those 300+ o-ring variety packs for $10.  Neither of these options seem very appealing, especially for a weapon that many owners end up sinking at least $3K in to.

I have some interesting experience with o-rings, as I have owned two M35A3 6x6 over the years with the central tire inflation system (CTIS).  Each tire has 11 locations that air can leak...and who knows how many more from the engine driven air compressor to the tires.  So all this requires lots of o-rings.  So when you go to buy parts for these old military trucks, each part has number, and a military spec that goes along with it.  The US Army does not use Chinese o-rings from Home Depot.  

I also saw that some were suggesting Viton o-rings.  These are great for hi-temp applications, but Viton o-rings are ineffective below 5 degrees F as they become hard and inflexible.  Buna-N seals have superior abrasion and tear resistance, making them more suitable for heavy-duty industrial applications.  

So, after a lot of research, I found a source for USA made mil spec o-rings that are the exact dimensions of the 'unknown' Benelli o-ring that meets ASTM D2000, MIL-P-25732, SAE J200 standards.  The Buna-N material and construction meet strict military standards, are resilient and offer resistance to tearing, motor oil, ammonia, ethylene glycol, isopropyl alcohol and mineral oil.  The are sized to the SAE standard AS568. The temperature range is -65F to +275F.  

It doesn't seem that anyone knows what happens when these o-rings fail.  They are called upon to serve as a friction wedge vs their usual job of sealing. The top of the firing pin retaining pin has a mushroom cap on it, and I am only guessing that it could back out until its hits the receiver wall for quite some time.  But if it does back out and break, and somehow the firing pin is released...you are hosed.  

There are plenty of videos on YouTube where the factory rep easily removes the firing pin retaining pin with their fingers.  If yours was not so convenient to pull out, and you pried it out with a tool of any sort, you should probably check for chunks out of the o-ring.  You can also use a Glock tool to easily push it thru from the other side.  

There is really no easy way to buy just one Buna-N mil-spec o-ring.  So I ended up with a few more than I could ever use.  If you are interested in one of these for $3.75 & free shipping, send me an email at [email protected] 






Image 6-28-23 at 11.07 AM.jpeg


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I guess I was really surprised at the response to these o-rings...and the requests for a mil-spec replacement for the 270J Gas plug o-ring.  I have no idea how many thousands of these M-4 are in service with the US military.  The Marine Corps technical manual https://drive.google.com/file/d/0BxN5CusgXy6nbm1udXh5dzYyYUU/view?resourcekey=0-XUTOPgDtyc3Orz9E7imIbg specifically calls out the need to check the o-rings.  I really doubt that the Marine MOS 2111s are driving off base to Home Depot for their supplies of Chinese o-rings.

 If you are interested in firing pin retaining pin o-ring for $3.75 & free shipping, or an o-ring 270J Gas plug o-ring for $3.85 & free shipping...send me an email at [email protected] 


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