CMMG radially delayed bolt and barrel test

[Les Snyder]

just got back from a comparison of a CMMG radially delayed bolt and barrel combo (not a complete CMMG rifle) on a back up lower to my blow back 9mm carbine... the results are promising enough that I will switch the barrel and bolt to my current PCC... to utilize the hydraulic buffer, and will run a 3/8 drill through a Miculek comp from one of the 5.56 guns... only problem so far was the need to clearance the rear of the magazine to fit the slightly wider lower bottom lug on the carrier that strips the cartridges... a MagPul Glock style mag did not need clearance... a slight clearance cut on an ETS and Glock mag also received a slight trim on the height of the follower....still in the development stage.... regards

[Roy T]

Looking forward to trying that setup since food will be the feature at the next group meet I guess it will be the 31st. just when I thought we had this figured out here we go down another rabbit hole. I will bring ammo

[Les Snyder]

Roy... I should have it pretty much together by then... I had the test bed on a straight H2 buffer and spring no muzzle device... stuck Howard's carbine hydraulic in and his comp that was 1/2 x 28 for a 9mm....will need to get the okey doke from Leo... see you the 24th

 

 

[Roy T]

What load did you test with? One of the guys at the range has a Banshee and i took it for a test run of the box new it ran but the feel was strange did not have enough time to determine if that was good strange or just strange interested to see your version. The one I fondled was a pistol so that may have contributed to the strange part.

[Les Snyder]

I shot it with my 135 grain 146 power factor coated Bayou bullets, and with Howard's 100 grain plated...perception wise, I can hold A hits at 25yd double taps with very little effort.... watching Howard shoot, there was virtually no observed recoil... had a very consistent ejection pattern with the plunger ejector in the bolt....there is an option to add additional weight to the carrier and probably a little room for playing with the internal bolt spring

[NPDriver]

I just filed a little material from the top of my magazine release rather than trim the magazines. A normal blowback bolt doesn't have an actual bolt with lugs to clear, so I found that lowering the mag in the magwell made it much easier to charge if you inserted the mag on a closed bolt. The lugs weren't gouging the hell out of the brass on the backstroke either.

[Les Snyder]

NPDriver... thanks, I trimmed the followers in addition to checking for rear clearance, but if causes a problem will lower the shelf on the mag catch... I had problems with only the rear of the first 2 mags I stuck in the carbine, naturally....did you actually measure the amount you removed? and if so, how much?....

[NPDriver]

I think it may depend on the manufacturer, but I can only speak to what I know. I have a Foxtrot Mike's build kit, it used to be sold on Primary Arms but they discontinued it. Upper, lower, 8.5" handguard & all the springs, detents & pins were included. It ran fine when I put it together if you inserted the mag on an open bolt, so I knew right off I didn't need to remove much material.

 

I talked to people using different brand lowers and they said that they needed to remove as much as 1/8" from their magazine release. I was nowhere near that. I just scratched a reference line on my mag release where it looked like I needed to be, clamped a file into my vise, then just dragged the release along the file making sure I was holding it as level as I possibly could. 

 

When I got close to my witness marks I checked the fit, then wrapped the file with emory cloth to slow down the amount of material being removed. I just did this & kept checking the fit until it cycled smoothly. It doesn't need a lot of contact to scoop a round off the top of the mag.

 

I may have removed around 1/16" of material altogether. It didn't take a lot, just some patience and fitting. I could have taken it to my friend's and had him mill it off but I felt confident I could just do it by hand if I went slow and held it steady. Like I said, it ran but if you tried to charge it on a full mag, the lugs on the bolt wanted to rip the round off the top of the magazine and gouged the s#!t out of the brass. I never touched any of my magazines or followers and I've run Glock mags, Magpul 21 rounders & a KCI 50 round drum. Smooth as butter!

[Les Snyder]

thank you .... it looked as though around .100" would do it... I get a couple of small scratches on brass....only shot a couple of hundred rounds through it,  so far has run 100 percent... I use an ETS 31 rounder with a Taylor Freelance extension for competition

[NPDriver]

Yes sir! You have it running so obviously it's close. I wouldn't touch the mags myself, I wouldn't want any variation between any of my magazines. Give the release a little shave, you know it's close if it's cycling. What lower are you working with?

[Les Snyder]

early New Frontier Armory billet, without bolt hold open... at the worst, I'm out 4 Glock followers... thanks for the help...my preliminary trials were with a second lower, a PSA billet (made by NFA), and did not want to change anything that could not be replaced in a timely manner until I got it checked out in the primary rifle

Edited July 25, 2019 by Les Snyder

[NPDriver]

It's always easier to remove material than add to it, so just go easy once you get close. Did you open up your ejection port at all? 

[Les Snyder]

I did not notice any brass marks, and watching a friend shoot it, there was no indication with any ejection port striking... I have access to a milling machine if needed in the future

[NPDriver]

Yeah, I never did anything with mine either. The only time I had failures to eject was when using a brass catcher. Still gonna open it up though or get one of those PCC specific upper recievers with the oversized ejection port. Been eyeballing this one myself but there are a few out there.

 https://www.rainierarms.com/arms-republic-9mm-stripped-billet-upper-receiver/

[Les Snyder]

please keep me informed with any modifications you make in the future... regards

 

Les

L747

[NPDriver]

Will do Les. So far I've added a LaRue MBT-2S trigger, great 2-stage trigger for under $100, a PWS enhanced buffer tube/QD endplate w/ locking castle nut. The anti-tilt lip on that tube really helps. The BCG is already started into the tube when you close it up. Got their PCC comp too, nice piece. I also used a POF roller cam pin, my goal was to keep this thing as smooth cycling as possible. Got a Geissele charging handle for it a few weeks ago. Next is a Kynshot hydraulic buffer, I'm told the #5005, AR10 model is the best for this application. I'll get around to it sooner or later, just got back from vacation & those paychecks tend to be a little light, but I'll post here with my results.

 

Good luck with the fine tuning!

Jerry