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Posted: 7/16/2019 9:08:02 PM EDT
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Wanted to exchange the direct blowback barrel and bolt on my 9MM SBR with the CMMG barrel and BCG. Here's how I did it.
Here are the main parts that were exchanged - regular blowback on top and CMMG RDB on bottom. I went with amphibian's recommended buffer setup - Vltor A5 buffer tube, Blitzkrieg standard hydraulic buffer, and Tubbs flat wire buffer spring. 
Removed the DDLES feed ramp, ejector, LRBHO (which barely worked anyway), and mag stop. 
The mags sit about .100" higher and a few degrees closer to vertical in the DDLES lower than they do in CMMG Glock lowers. CMMG on left and DDLES on right. 
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Part 2 of this saga.
In order for the mags to sit a little lower, I replaced the original mag stop with one that limited upward seating depth. I started with a modified part that was about .100 thicker in the notch. It was slow going since I was using trial and error fitting. Install piece, check mag lip clearance, remove piece, file, reinstall, recheck, lather, rinse, repeat, etc. I filed it down slowly until the lower bolt lugs just cleared the mag lips. New one on top and original on bottom. 
I now had to modify the mag release. This picture gave me an idea how much to take off. I removed about .025" of metal, reducing the original from .200" to .175" in thickness. This was another exercise in lather, rinse, repeat metal working. This is actually a new replacement mag release from Odin Works since I got carried away on the original and reduced it too much. 
Before and after pictures of the mag release 
I also had to tweak the NFA LRBHO a little by rotating the mag end of the wire lever a few degrees closer to 90 degrees in order to get the LRBHO to make enough contact with the mag follower to reliably actuate the bolt catch. This was one of the easier adjustments. 
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Part 3. One of the biggest differences between a regular blowback bolt and the CMMG RDB is the CMMG has a bolt cam. I wanted to use the New Frontier Armory upper with the built in LRBHO, but it doesn't have a recess for the bolt when the bolt rotates and locks with the barrel lugs. I took a look at a regular AR upper and cut a recess in with the dreaded "D" tool. I was careful to remove just enough metal so the cam could rotate freely without interference. I didn't want to cut through the receiver wall.
An regular AR upper on the left, unmodified NFA upper center, and modded NFA upper right. 
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Part 4. Now that everything was feeding and ejecting properly, I notice the brass was getting some gouges from the sharp edges of the chamber and the bolt (the lengthwise scratches on the bullets).
I decide to do a minor throat job on the CMMG barrel. Comparison shot of direct blowback barrel on left and CMMG on right. 
Not the greatest throat job but works for me. 
To reduce the scratches by the lower bolt lugs, so I dressed those a little with a fine India stone. I still get rub marks on top of the rounds, but the lugs are no longer digging into the brass. Before 
And after 
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(Went back and cleaned up some of my dyslexia. Should make a little more sense now.)
Part 5. Quick and dirty range report is it works. A little lighter now and a much smoother shooting weapon compared to a direct blowback with a heavy buffer. Might seem like a lot of trouble when it would be easier (but more expensive) to just buy a complete CMMG RDB SBR. But I've already gone through the year long wait and paid the ATF $200 for the DDLES tax stamp. Plus, I already had the upper and lower. And I've found that the New Frontier Armory LRBHO uppers work for me. Now it's time to list the old parts in the Equipment Exchange. 
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Thanks for the detailed write-up! I've tried beveling the corners of the mag release match on my QC10 lower since mags wouldn't seat without fiddling around, but didn't think to actually reduce the height of the latch so the mag would lock in easier. Duh. I'll have to give that a try. I've heard about that NFA upper with the built in BHO mechanism. I'm comfortable with a dremel but tbh I'm not sure i want to go to that much trouble making a pocket for the cam pin but could be worth it for me if the LRBHO is more reliable than the DDLES/QC10 mechanism. I have a full rifle CMMG RDB build using Endomags currently but i plan on doing a pistol with my existing qc10 pistol lower. I'll have to give those mods a try when i get the new cmmg rdb pistol upper built. |
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I never could get the DDLES LRBHO to work reliably. Then I found New Frontier Armory's LRBHO uppers and suddenly things worked the way they were supposed to.
I've had a couple of regular and side charger NFA's LRBHO uppers. I've used them on the DDLES lower and a couple of Spartan Glock lowers, and they worked great. They're easy to adjust - just remove 3 screws and you can easily twist the wire part of the assembly as required. I use a vise to hold the rear of the LRBHO and use a pair of pliers to rotate the hooked front end. The early NFA uppers had fairly thick sidewalls, so I had to file a notch in them to allow a little more travel for the bolt catch. Their latest uppers taper more at the bottom of the sidewalls and don't limit the bolt catch travel as much. I've never had any interference problems between the front end of the LRBHO and the bottoms of my Glock cut bolts and the CMMG RDB BCG, but I have no idea if it would be an issue for someone using different bolts. This first picture was taken about a year ago and shows an Aero Precision slick side upper, an older NFA side charger LRBHO upper, and an NFA regular LRBHO upper with the wire actuator removed. You can see that the sidewall (towards the right in the picture, left side of the upper) is thicker on the NFA uppers compared to newer NFA uppers in the following picture. The sidewall would limit how far the bolt catch could rotate, often preventing it from locking the bolt back on the last shot. I filed a small notch in the upper that matched the bolt catch's position in the lower to allow more rotation for the bolt catch so it would work properly. 
These are some more recently made NFA uppers center and right, plus a regular AR upper on the left. The NFA left sidewalls are now tapered towards the bottom. They are now a little thinner on the left sidewalls than the older ones above and don't prevent the bolt catch from rotating enough to lock the bolt back.
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