I built a 9mm CMG radial delayed blowback upper, and I really enjoy shooting it.  It allows the use of a standard AR lower with a 9mm PMAG conversion.  I see that they also sell a .45 RDB barrel and BCG combo that I would like to build, but it doesnt look like I can use a standard AR15 lower for .45.  Does anyone have experience on compatible .45 lowers that would work with a .45 CMMG RDB upper?

@MadMachinist

Cav Arms Mk 2/GWACS/Sabre Defense lowers may work, albeit you'd likely have to modify the Grease Gun mags or the mag catch.  Think MM has magazines that will work in a standard lower.

To add on 'backbencher's' recommendations, I have a local buddy that is using the 'Madmachinist' / Macon Armory mag block and mags in a standard lower (full auto actually).  Works great but only holds 20 rounds.
I like that his setup also locks the bolt back after the last round: https://maconarmory.com/collections/all

If Glock mags are your thing then there is of course CMMG's lower that holds the mags at the right height.  You will hopefully get better support using their lower of course.
'Madmachinist' recommends the Angstadt Arms lowers for his gas operated builds which should feed at the same height as the CMMG RDB.

I am gathering the parts to do a .45 RDB setup myself but I'm going to be using an old MGI GG magwell that I was able to acquire used.  I had one before and sold it and have been looking for one for years.  It is designed for straight blowback as well.  However, I dropped my friends CMMG .45 RDB upper on mine and cycled some dummy rounds through it and was surprised there was no rubbing of the mag on the carrier and fed fine.  I have details on that magwell here:http://www.c3junkie.com/?page_id=76
I like that this magwell obviously works with my MGI lowers but also you use unmodified GG mags.

Thank you both for the great feedback.  Looks like there is no cheap way to do this at this time.  Maybe I'll build a 5.7 RDB upper while I wait for someone to figure out a cheaper way to build a compatible .45 lower for the CMMG upper.

I'd love to see an Endomag-equivalent option for more of the CMMG RDB calibers.  That seems like an untapped market to maximize the existing investment in a traditional lower.  I'm sure there are many buyers that defer buying another caliber if it means they need to add an additional lower (or can't leverage their NFA investment).  I personally prefer the RDB uppers over other blowback/DI caliber options.  I'd definitely buy a .45 RDB to add to the 9mm.

I agree.  I dont mind building another lower, but the .45 lowers are all $300 just for a stripped lower by the time you pay shipping tax and FFL fees.  I might as well buy a legit CMMG guard for that kind of money.

I still wish somebody would revive the MGI platform....
One lower that you SBR and get whatever magwell you want.   I'd love to have a Scorpion magwell.

I thought Wyndam licensed the design?

I'm presuming there must have been some sort of collaboration as the lowers and magwells are interchangeable but I don't recall ever seeing a patent on the design.  Mack Gwinn does have several patents on gun parts, I just haven't seen a patent mentioned on the design.  There is a patent for the one by Cobb Firearms that is similar as well as Colt has a modular lower patent as well but the way the magwells are secured are all different from the MGI one.

I don't care who comes out with one, I'd ditch my MGI if something better would come along.

Windham doesn't seem to want to do anything with it.

Back in the day MGI offered the small and large frame Glock magwells, AK-74, the Grease Gun magwell then later they only offered the AK-47, Colt 9mm and standard AR15....which is all that Windham shows and for those 3 options, it isn't worth it.

Hell, I’ve got a Glock mag 45 lower just laying around here, if only I could get it to Work with RDB!!

@Mad-Machinist

45ACP using PMAGs.

I have tested this with Macon Armory 11" di upper and a standard AR15pistol lower


https://youtu.be/IC7B7AO06vQ

What the hell?  Just ordinary PMags, load up .45 ACP in em?  Stoner and Browning are laughing their asses off @ us right now.

Depending on what lower you have you may have to lower the mag a little or it may be drop in.
I think Rudy (Mad Machinist / Macon Armory) recommends the Angstadt Arms lowers for his 45 gas operated uppers...which require the same feed height as the CMMG RDB.
Everyone I know that has a .45 RDB has had no issues, even running in full auto.
One of my buddies has one that he is using with an old Bazooka Brothers Grease Gun lower and it runs fine in full auto as well.  He also has the Macon Armory mag block and 20 round mags that also work great.

There was a fellow on here years ago that was fooling around with 3D printing an insert and new follower that could be put in PMAG bodies to make them truly .45 ACP.

That's been done for 9x19mm.  CMMG is using it.

ETA:  No caffeine yet - it's the Endomag.

Mine is a NFA lower.  I was psyched to build it, but everything I had read early on, it wouldn’t work.  I gave up and abandoned the idea.  
Maybe there is hope yet.

I think it would be fine if you are OK with possibly having to lower the catch...order a spare catch from NFA..

Has anyone else tried a standard lower with a .45 RDB upper and standard 5.56 PMAGs?  I just ordered an 8" .45 RDB barrel and bcg to try myself with a standard lower and PMAG.