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SUMMARY: I bought a lower, barrel, bolt from CMMG. Used an existing upper and opened up the ejection port. The gun ran fine for about 1200 rounds, then it started loosing ejected case velocity, they just dribble out, then started FTEing. Contacted CMMG and sent picks of bolt and they sent me a new modified extractor. Also reminded me to use 308 carbine spring and standard carbine buffer. In mean time I started using 9mm Federal Alum ammo $$. I found the ejector spring was weak and same extractor, both are now CS springs. When new I was running flat wire 223 spring and hyd buffer. So my hopes that this gun would be FTE free which to me means 1000+ rounds of new factory ammo with no problems.

I give up! Today I went out thinking everything would be fine since my test with three mags loaded with ten rounds, it cleared them all. Today I went out with three full mags didn't run single one, this is federal alum cases dumping rounds as fast as I can pull the trigger. Ejection is very consistent angle and distance till you find a mangled case sitting on top of half loaded round. Guess I try to get video

No I was very careful to not shorten hook depth, the CMMG extractor had not been head blasted and showed the cutter marks. It also had large flat section right above the edge. So mainly changed the radius and got rid of the ridges also radius the ejector.

Here is bolt with modified extractor and ejector

Alum cases made it more pronounced. I'm going to look into more 9mm PCC that uses a AR extractors, also going to compare cases rim groove shapes, also came across a site that said the ejector pin should have slight radius to make it easier to slide across the case face as it rotates out.

So I ran it today, did the port relief solve the problem? NO! This is running Federal Alum 115gr. Glock 33 rounder, a FTE's on two rounds last eight rounds in the mag. So I inspected the spent casings and the extractor had dug some gashes on the inner edge of the case rim right at the point where extractor ends. So I polished sharp edges back and dropped a round in the chamber and pushed the carrier by hand until the extractor snapped over the rim. Then I slowly pull the carrier to the rear watching it pull the round out of the chamber expecting it to pop out when the round cleared the barrel extension. So what happen NOTHING! The round was wedged at about a 15' angle between the arc on extractor face and the far side of recessed bolt face. It was struck there firmly and it was repeatable. So I inspected the extractor face and compared it two four other 223 extractors on hand. About the same. So with my 4x magnifying eye piece I put a round in the groove and rotated it like it supposed to roll out on extractor face. It bound with the edge of the extractor digging into the inside of the rim. So I rounded the ends extractor and polished the extractor face until the round rolled out with no binding. Performed the slow extraction again and the round hopped right out when it cleared the barrel extension. Loaded 3 mags with 10 each and burned right through them. Also noticed that ejection pattern was now consistent right 3 o'clock instead varying between 2 and 5.

.So I was bored and I was looking over my 9mm PCC and I noticed the alum cases had left some marks on the barrel locking lug face. As soon as the case cleared the inner area of the barrel extension you can see the drag marks across the rear face of the barrel extension. The Ejection port was slightly to the rear of the barrel extension forming a sharp edge for case to catch on and knock it loose from extractor. My Theory is the alum cases are lighter and therefore accelerate faster and didn't clear the ridge like brass does. See pics of modified port.

Her is two things I've done that help, but are no panacea for the problem. Load mags full and let them sit for week to take a set, lots of oil on bolt carrier and into the recoil tube. CMMG support has it problems as you never get to talk with the engineer. But they have been helpful reading off the list of known problems/causes.

Any updates? I've got trouble not feeding 33 round Glocks when full, and few failures to eject. CMMG sent me a new extractor which had locking lug shortened. I'm playing with ejector and extractor tension. I think the extractor looses the rim and the ejector just drives case forward.

So what kind of problems has CMMG FIXED?

On blow back gun yes but on lock breech no. However on lets say MG42 with delay roller and fluted chambers the extractor may not do much. The spring on the extractor on ar-15 can leave you disabled. 6

Recently I switched from federal 115 brass to aluminum cases and both guns started jamming from failure to clear the empty. Aside I tune my recoil springs to control my bolt velocity, I make my brass so that end up 6 to 8 feet away, with aluminum cases they are just dribbling out like 2 3 feet and some cases have been struck by bolt coming home.

I know 2 mpx owners both have had to send their guns back to SIG both are pretty sensitive to ammo. So far my CMMG has eaten everything, reloads, hollow points, however no testing with a can.

One update on my guard, I turned the 8 and a half inch barrel down 5\8" 0D running a comp on the front. Using a 7 inch aluminum hanguard with the aluminum barrel nut also.

AXTS 45/90 WITH spring loaded lock pins only $50 with long and short lever also comes with polished detent pin and spring nice smooth throw combine that with a magpul K2+ grip and you can run the gun just like you can a 1911. On and off the safety without changing your grip. Always thought the safety on was too slow on a AR and with trigger under 3# I like have safety on everytime I move.

Yes, on first reading I thought MPX short piston would be the way to go, but upon seeing them running and the amount of changes and problems. It is easy to over gas them with too fast of a powder, they were tearing themselves apart. Notice how they kill triggers. Extractors, bolts, gas bleeder. Now the CMMG Guard radial delay taps allot energy and has np piston or gas tapping. So it runs cleaner than MPX. My experience is flatwire/hyd buffer setup Guard , versus stock MPX. To me the guard is softer and smoother

I'm running the CMMG GUARD 9mm with flat wire carbine spring and Blitzkriegs hyd carbine buffer with 10.5 barrel with CMMG comp. It feels the most like a DI gun as any I've shot. It throws the case 6 to 8 feet, running rock river tuned 2.5#. It likes to be run wet, a lite spray to bolt/carrier before it match. It feels like a 223 with adj gas block.

I know SIG says don't use it. We tried some and it wouldn't feed where Federal ran fine. Its always feed and gone bang but recently I was having troubles with plates and went to check my sight in and discovered the lousy groups and thought it was the gun, someone at range gave me some federal and accuracy and zero came right back. My guess is the plated bullets, out of 10 shot group have three fliers usually along with 4 inch cluster. Guess I'll just burn them up until the shootoff then run federal

Lately I'm having trouble with lousy accuracy at 25yds about 4 inch groups with two different barrels. Drop in some Federals and it is back under 2 inches

unless that is a flat wire I don't think a carbine buffer, maybe on a extended length 9mm most 9mm buffers extend the head area not spring area, will hold a rifle length spring, are you sure the spring isn't going to solid before buffer bumper hits. If goes solid that will kill the spring. Spring will last a lot longer if you only take them to 90%, gives them room vibrate in..

go to ebay and search for 6" Swivel Pad Lock Pliers buy from whomever, clamp the mag on sides just above base pad and it will squeeze it enough to release the locks and use a pin too push down spring base and they slide right off, dont crush the mag just a little squeeze

Look for how far you eject empty's and angle. If throwing at 1 or 2 oclock too much spring. If more than 15 feet too little spring. Just start cutting coils off the springs till you get good ejection and little nose dive. Also play with quarter spacers if your not running a 9mm buffer.

Brownells

Shoot a CMMG GUARD before you waste a lot of money on the picky SIG MPXS!

You running it too light brass going forward means the bolt is striking the empty when coming forward, and it is supposed to be slower than a blow back, put H2 buffer in it