Guys I have a Colt Mk18 Mod0 and Mod1 uppers which are both short stroking on my Colt M16A1. Uppers are all Colt and were built with new parts.

Using a Colt H2 buffer with PMC 556 55gr ammo.

I swapped to a H buffer and the short stroking stopped but still would get an occasional fail to lock open on last round.

Shooting Colt 11.5" barrels with the H2 I have no problems.

Any help is much appreciated!

Is it particularly cold where you are shooting?  

I assume you are running a carbine stock on your M16A1 lower, so you have a carbine length spring to go with those buffers.

I have never run 55gr out of my issued Mk18s or M4s.  It was always M855 or Mk262, either of which had no trouble cycling with H1 or H2 buffers.  That said, PMC is usually on the hotter side, so I wouldn't think their M193 55gr loads would have trouble with cycling the bolt on your setup, all things being equal.

Was mid 50s today.  Using Colt carbine tube/spring.

Oh, and I meant lower, when I typed upper above.

It's possible the cold air is messing with things, especially if you are at altitude.

H3 buffer and mil spec factory ammo should solve your problem.   Adding a can will also solve this problem.


    I was issued very early Mk18's (Cut down M4 barrel) around 2003, they came from Crane with H3's and we ran 77 grain bullets in combat.  The SOPMOD cans, which were knight's would make even the most sluggish gun run.  In training in we used green tip and it ran fine.  I vaguely remember that you should use a different buffer with 55 grain ammo, but I don't remember specifics anymore.  Perhaps one of the younger 18Bravo's on the forum can jump in to this thread.

There was also a rubber donut placed around the extractor spring that enhanced reliability.  


18Z50

How would adding a heavier buffer solve the problem? That would make the problem worse not better.

What is a cab?

He probably typoed “can”, since c and b are next to each other on the keyboard.  Below is my “retro” mk18 from AFG.  Cycled perfectly with H1 buffer and no can.

Very nice! I am thinking this has to be an ammo thing. I'm going to take an assortment next time to see what results I get. I always thought PMC was good to go, but maybe I was wrong.

Sounds like an undergassing issue.  If you used an original Colt 10.3 MK18 bbl this may help. The 10.3 was orginally a request by SOCOM for a CQBR length upper vs the 11.5 which was the standard at the time. The original gas port sizes for the 10.3 were .092 (which was way overgassed) and required the use of a H2 or H3 buffer and an increased weight buffer spring to slow down the bolt carrier and prevent bolt bounce related failures. They then experimented with a .067-.068  gas port and found to get it to run correctly the weapon needed a regular carbine buffer and spring and had to use hotter 5.56 ammo like the m855/ss109 or run full time suppressed with an H buffer. Eventually the guys at Crane found the best combination was a .070 gas port with a H1 buffer to run suppressed. If running unsuppressed, it needed a regular carbine buffer and 5.56 ammo. FN/DD and even Colt are now making the barrels with a .074 gas port, which provides enough gas to cycle with .223 or 5.56 ammo, Using a H buffer both suppressed and unsuppressed. They also added an O ring under the extractor to aid in proper extraction.

If your barrel was cut down from a longer one, then gas port size could be the issue as well since longer barrels use smaller gas ports.  The longer barrel allows more dwell time for gas to feed the system.

I guess the solution to your problem lies in figuring out what size your gas port is. If it is the .070 (the correct Crane spec), or .068, you may have to run it with a regular 3.0 oz carbine buffer to get it to run unsuppressed.

At one time the CQBR (Army) and MK18 (Navy) shipped with two or three buffers. I cant recall for sure. specialized Armament used to be my town and my LE unit bought weapons from Ken so we talked a lot. If i remember, there was carbine for M193. H2 for green tip and H3 for suppressed.

I have a Colt from Ken (semi) with a “real” MK18 upper that i have since put on a SA adjustable gas block and longer forend. I have a standard carbine spring in it and a buffer with NO weights at all. Adjust gas and it runs great

Thanks for the info. I have a factory Colt 10.3 barrel and a cutdown Colt M4 barrel. Both gas ports are measuring at 070.

Finally got back out to the range today.

It cycled Winchester and Federal M193 with no issues and locked back after last round each time. I always thought PMC XTAC was good ammo but apparently I was wrong.

I run a Colt H3 in my factory MK18 per Ken Elmores advice works 100%.