CMMG Dedicated 9mm Receiver
- Thread starter nuge
- Start date
Mine broke hammer pins like a mofo until I cut the hammer and ground down the disconnector. If you inserted mags hard, with the bolt open they would go over the feed ramp getting stuck sometimes
Esox
Well-known member
The lower receiver itself has pretty good reviews, but in order to function well and not break pins or bolt catches I would recommend as part of your overall build that:
1. Make sure the bolt you are using is ramped well to more smoothly cock the hammer; this helps prevent breaking pins
2. Utilize quality FCG parts and even consider upgrading to the $4.50 KNS stainless replacement pins (not the anti-walk pins)
3. Ditch the JP silent spring for this project because it just doesn't have the required mass for a 9mm blowback setup; go with a dedicated 9mm tungsten weighted buffer if possible
4. Watch for how much over-travel you have with your bolt to the rear; too much and all that mass going forward slamming into the bolt catch on the last round may break it - the fix is to simply add quarters to the rear of your buffer tube to take up space but not too many that it doesn't go back far enough to engage the catch.
Good luck,
Esox
1. Make sure the bolt you are using is ramped well to more smoothly cock the hammer; this helps prevent breaking pins
2. Utilize quality FCG parts and even consider upgrading to the $4.50 KNS stainless replacement pins (not the anti-walk pins)
3. Ditch the JP silent spring for this project because it just doesn't have the required mass for a 9mm blowback setup; go with a dedicated 9mm tungsten weighted buffer if possible
4. Watch for how much over-travel you have with your bolt to the rear; too much and all that mass going forward slamming into the bolt catch on the last round may break it - the fix is to simply add quarters to the rear of your buffer tube to take up space but not too many that it doesn't go back far enough to engage the catch.
Good luck,
Esox
Esox
Well-known member
Ah yes, I see they have one now: http://www.jprifles.com/buy.php?item=JPSCS-9MM
It looks like the OP will need to make sure his 9mm bolt is one with the removable weight to use it. A lot of the newer/cheaper bolts being sold are solid and they would need to be milled out for the JPSCS to insert inside of it.
It looks like the OP will need to make sure his 9mm bolt is one with the removable weight to use it. A lot of the newer/cheaper bolts being sold are solid and they would need to be milled out for the JPSCS to insert inside of it.
Thanks guys for the feedback. Couple of clarifications: I intend to use the modified FC parts (shown in pic) and expect they will remedy the pin stress/breaking; and the JP captured spring system is the new enhanced version with no block in the rear of the carrier http://www.jprifles.com/buy.php?item=JPBC-9MM-SCS. The carrier is ramped and pretty hefty when combined with the weights on the spring. The heads up on the slam mag loading past the ramp is appreciated. Next step of course is to try it out, but could be a while due to a job assignment away from home.
Was also curious about this dedicated lower below - this may have been my first choice, but the CMMG was available at my dealer when I walked in...
Was also curious about this dedicated lower below - this may have been my first choice, but the CMMG was available at my dealer when I walked in...
Thanks Esox. I think I got 1 & 2 covered. 3 I am going to risk it with the JP system and report back (also have tungsten plugged carriers as back up). #4 is intriguing - yes I think I have read that elsewhere too about bolt catch vulnerability. So a little experimenting required on the # of quarters, but what is typical do you think?
Read the JP page closely. They have a 9mm buffer; I would not say it is weighted correctly. It is part of their LMOS family, low mass. My 5.56 rifles (no adjustable gas system) have noticeably less recoil with the JP standard spring and a rifle buffer, compared to the silent captured setup. I never tried it with 9mm, because taking the weight out of the bolt and running a light buffer didn't make sense.
My 9mm suppressed SBR ended up with the Spikes 9mm buffer after trying both a T2 and Heavybuffers.com's heaviest model. 8.5 ounces worked best in my rifle.
My 9mm suppressed SBR ended up with the Spikes 9mm buffer after trying both a T2 and Heavybuffers.com's heaviest model. 8.5 ounces worked best in my rifle.
Excavation of and old project
Wow a lot has changed over the past 5 years around colt style 9mm subguns. Nevertheless I have just completed this "blast from the past" straight blow back build out of my CMMG MK9 receiver.
Here's what I ended up assembling with the receiver: custom FA FCG parts to prevent pin damage, SS pins as insurance, ramped & weighted bolt carrier, carbine length (commercial) receiver extension, magpul CTR stock, Tubb flat wire lightweight buffer spring, RB5007 buffer, steel RDIAS, and an all-colt 9mm A3 SMG upper. Now to get to the range/desert and make it all work or make changes as needed.
Wow a lot has changed over the past 5 years around colt style 9mm subguns. Nevertheless I have just completed this "blast from the past" straight blow back build out of my CMMG MK9 receiver.
Here's what I ended up assembling with the receiver: custom FA FCG parts to prevent pin damage, SS pins as insurance, ramped & weighted bolt carrier, carbine length (commercial) receiver extension, magpul CTR stock, Tubb flat wire lightweight buffer spring, RB5007 buffer, steel RDIAS, and an all-colt 9mm A3 SMG upper. Now to get to the range/desert and make it all work or make changes as needed.
The good news is that this system runs pretty smooth and slow in FA (no measurements at this time) with no beating up of pins and certainly no trigger slap. Not so good good was the RDIAS required some timing for FA with a make-shift shim. Also a vigorous mag thrust with bolt locked back did lead to minor sticking a couple of times due to overtravel as Chili warned.
A&S Conversions
UZI Talk Life Member
Glad to hear that you finally got it together. When I got my DIAS, I went straight to the source. I bought a Colt pre ban 9mm carbine that was drilled for the standard front push pin. The steel sear block was removed. The lower was gray and matched the 10.5" Colt subgun upper. The biggest reason why I went with the Colt, at that time Colt used a larger diameter hammer and trigger pins. I had M60joe ream out the M16 fire control to fit the larger pins. I did do the "V" notch work myself. I bought a HK Fleming sear just after the transfer of the drop in. I could never get the blowback Colt system as smooth as the roller locked mechanism. I still have the Colt. In 2008 I found a LE 9mm carbine. Same gray drilled for the push pin and large fire control. Colt didn't bother with the sear block for the LE carbines. The bolts are ramped and I got a tungsten weight from Joe when he ramped the bolt.
Luckily I found a set of pristine M231. I had Garrison Machine modify a Colt carbine upper with some of their reproduction DOE parts and made me a DOE upper. With the drop in that is going to be as close to a DOE gun as I'll ever get.
Scott
Luckily I found a set of pristine M231. I had Garrison Machine modify a Colt carbine upper with some of their reproduction DOE parts and made me a DOE upper. With the drop in that is going to be as close to a DOE gun as I'll ever get.
Scott
My ideal lower for a sear gun would be an OEM Colt 9mm with pinned in block, standard pin sizes and sear ready shelf. But I don't think that exists.
True, the colt 9mm is not as smooth as an MP5, but I would like to get mine as close as possible and still stay with the straight blowback. I am encouraged by the progress in smoothness with the minor mods such as FCG, buffer, buffer spring etc. The CMMG MK9 set up is pretty smooth at first trial. I used the carbine length receiver extension and the kynshot RB5007 buffer. This left a gap of 3/16" at full compression between the bolt and BHO - big enough to ensure lock back while minimizing the force against the BHO and ensuring no casing can get into the lower during operation. Since the diameter of a 9mm case is ~ 3/8" I am thinking that is probably the upper limit for this gap.
Just checked this gap in my Colt HT (SMG) and its too big at 7/8". There I am using an RB5005 since the OEM receiver extension is shorter than carbine length. So I'll be most likely adding a spacer. It seems like this gap distance could also be a variable that affects RPM and smoothness - may be worth a look...
True, the colt 9mm is not as smooth as an MP5, but I would like to get mine as close as possible and still stay with the straight blowback. I am encouraged by the progress in smoothness with the minor mods such as FCG, buffer, buffer spring etc. The CMMG MK9 set up is pretty smooth at first trial. I used the carbine length receiver extension and the kynshot RB5007 buffer. This left a gap of 3/16" at full compression between the bolt and BHO - big enough to ensure lock back while minimizing the force against the BHO and ensuring no casing can get into the lower during operation. Since the diameter of a 9mm case is ~ 3/8" I am thinking that is probably the upper limit for this gap.
Just checked this gap in my Colt HT (SMG) and its too big at 7/8". There I am using an RB5005 since the OEM receiver extension is shorter than carbine length. So I'll be most likely adding a spacer. It seems like this gap distance could also be a variable that affects RPM and smoothness - may be worth a look...
A&S Conversions
UZI Talk Life Member
It is my understanding that there were a very limited run of guns during the change in anodizing colors from gray to black around 1999 to 2000. They were called Green Label guns. These are extremely rare but I have seen a few listed for sale over the years. Personally I think that the larger diameter fire control pins are a big plus since the standard fire control hammer pin is known to be a weak spot of the blowback system. But to each his own.
M60joe did pin a set of the two peice Colt 9mm magwell adapter into a Bushmaster lower so I would have a black one. I have never had an over insertion problem with the Colt two peice pinned magwell adapter. I have hung onto my original pre 94 Colt 9mm mags because after the ban ended Colt switched to plastic followers. The pre 94 mags have a steel follower. PSA had a sale on 9mm mags. They were steel and $10 each with free shipping for 10. I wish I had bought more. They are made exactly like the pre 94 Colt mags with a steel follower. Something to keep in mind when looking for magazines. The blued Colt LEO ONLY mags with the plastic followers never worked as well as the pre 94 Colt mags with the steel followers.
Scott
Those were made by Metalform. I bought a pile of them. They run 100%. Out of stock at PSA.
https://palmettostatearmory.com/psa-9mm-smg-magazine.html
But Greg Cote has them for $26+shipping+tax
https://gregcotellc.com/cart/ar15-m...r15-9mm-32-round-magazine-9smg32s-p-1539.html
Had to brush up on the Colt FC pin size thing... Yeah, the beefier pins may be better "absorbers" of the shock of straight blowback - all else being equal. Based on the diameter info I could find the larger pin has ~20% more mass than the smaller, and a larger pin hole has ~10% more surface area along its circumference. So for two pins and 4 holes that's ~40% more pin mass and ~40% more hole surface area in the larger pin set up. Whether that would eliminate pin breakage and hole egging for high volume 9MM FA I don't know. I assume that one would still use stainless steel larger pins for 9mm as insurance.
A&S Conversions
UZI Talk Life Member
Admittedly, I didn't go into the science. The pin breaking is the problem. It is my understanding that the stainless steel pins are more forgiving than the hardened steel pins. The original pins in the pre 94 had a ss pin in the hammer, with a standard steel pin in the trigger. The LEO 9mm came with two ss pins. I got fresh replacements from KNS. I had the pre 94 clearanced for LL use, but I found a deal on a DIAS. I haven't used the pre 94 in a while. I keep thinking about selling the pre 94 and the pre 94 mags to someone over the boarder in MA. One of many projects that I should get to.
Scott
Scott
I like the "dedicated" 9mm lowers for a RDIAS host as well. The magwell is properly dimensioned for the magazine and they come with 3-position markings.
One of my RDIAS' sits in a DDLE (now QC10) 9mm lower and I have a couple of Spikes dedicated 9mm lowers as backup for a host and as a project gun
As for trigger groups for my RDIAS hosts...if I don't have a 4-position group installed, then I go with the POF modular drop-in f/a trigger group. I have had this installed in my dedicated 9mm lower for some time without issue.
That spikes dedicated lower looks good, but wait times are loooong... Also got the cmmg working great with my Dias. Today I compared it with a colt smg - both functioned great, at about the same rpm and relatively smooth. Both have custom FCG components to avoid stress on pins and disconnector. The POF drop in looks good, may acquire one for plug and play comparisons with my current 9mm set ups, then use for 5.56.
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