An R&D Precision Colt magazine magwell adapter is an inexpensive and reversable way to convert an AR lower to 9mm. Here’s what they are, how they work, and a few fixes to common problems.
This information is intended solely for entertainment purposes only. Do not perform any action based on any of the following information. The accuracy of this information is not guaranteed or warranted. Always bring malfunctioning firearms to a qualified gunsmith for repair. All copyrights and trademarks are the property of their respective owners.
I recently completed a third 9mm AR build, and decided to go with another Colt magazine adapter to convert a normal AR lower to accept Colt-style mags.
My first 9mm AR used an Anderson lower and a Colt magwell adapter. Since I had a spare Aero Precision AR lower hanging around, and I have plenty of Colt mags, I decided to just get another conversion block.
Cost-wise, these adapters usually cost around $70-$100. Paired with an Anderson $50-$70 lower, it’s cheaper than most dedicated 9mm lowers, and in my experience, they work just as well.
The adapter is made of aluminum and steel. They have LRBHO, a feed ramp, magazine overinsertion prevention, and the ejector can be adjusted (bent) to tune ejection. The mag catch still works, so the manual of arms is the same as a normal AR.
I’ve confirmed that the OEM company supplying many (if not all) of these is R&D Precision. Their branded unit is available at Optics Planet, part number RD8292MI1. They’re not very expensive ($70-$100) and they work well. A number of retailers sell their own branded version, such as KAK Industry, AR Stoner, Guntec, NBS, RTB, and Grid Defense/Ghost Rifles.
Promag makes a slightly cheaper plastic version, but I’d avoid them. I’ve seen many reports of them breaking soon after their first use.
These adapters insert into the magazine well just like a magazine, from the bottom (after first removing the top locking screw from the adapter). The mag catch snaps into the adapter and will still work as the mag catch for the Colt mags. You then reinstall the screw and washer in the rear corner of the mag adapter. The washer overlaps the top lip of the magwell and keeps the adapter from dropping out.
On the bottom front are 2 angled holes for 2 set screws that press upward at an angle against the inside of the magwell to try to keep the adapter from moving upward.
Tight Fit:
If you decide to get one of these adapters, be aware that they can fit very tight. My first was tight, but I was able to tap it in place with a rubber mallet. My new adapter would not fit at all.
I sanded the front of the adapter by laying a piece of medium sandpaper on a flat surface and rubbing the front face of the adapter back and forth on the sandpaper until it was able to slide all the way in the magwell.
Unfortunately, after sanding the front of the adapter I now had a new problem. It was slightly loose in the magwell. When inserting a magazine firmly, it hit the overinsertion stop on the bottom of the feed ramp and pushed the whole adapter up in the magwell. This made the top of the feed ramp hit the bottom of the bolt, causing cycling problems. The front set screws that were supposed to grip the inside of the magwell at an angle were slipping on the hard anodized aluminum. The set screws use a 1/16″ allen wrench, and you can’t put much torque on such a small wrench without bending it.
Slipping Fix (aka, the “vampire mod”):
The original Colt 9mm magazine adapters (yes, their 9mm ARs used an adapter in a 5.56mm magwell) were drilled and pinned in place with 3 roll pins all the way through the magwell to ensure the adapter didn’t move.
The R&D adapter uses two set screws that press against the magwell to keep the adapter in place. In my case, the set screws slipped up the smooth inside of the magwell. If only there was an indent or socket in the front of the receiver’s magwell for the set screws to catch into…
DeWalt cordless drill to the rescue!
The set screw holes work perfectly as a drill guide for a 3/32 drill bit.
I removed the set screws and lined the adapter up exacty where it should sit in the magwell, with the top edges flush with the receiver. I held it firmly in place and used the threaded holes as a drill guide. Most common drill bits only cut with the tip, not the sides, so the threads in the holes would not get damaged by using them as guides as long as I didn’t press hard against them sideways.
I slowly and carefully drilled two shallow dimples (vampire bites) into the inside front of the lower’s magwell for the set screws to lock into the lower. The dimples do NOT go all the way through. I just made shallow cuts into the aluminum receiver magwell wall for the tip of the set screws. I’d estimate they’re 1/16″ at the deepest point.
I knew that it could be VERY easy to push too hard and drill all the way through so I went very slowly, stopping and removing the adapter to check the depth of the cuts very frequently (after about every 1-2 seconds of drilling with light pressure).
Since I was drilling at an angle against a vertical surface, the holes were elliptical. I drilled the holes only until the “top” edge of the drill bit cut just deep enough to barely complete the top curve of the elliptical hole.
I reinstalled the set screws with a little blue loctite and now the adapter doesn’t move at all, even with a very firm slap of the magazine.
It’s a simple fix, and it holds the adapter exactly where it should be. The adapter is still removable and the new small dimples do not affect the use of the magwell for standard AR magazines.
Another locking option:
A reader sent me an email with a picture of their solution to lock the adapter in place:
They set the adapter where they wanted it, drilled through the magwell and into the adapter, tapped the threads and installed a 6/32 set screw. The set screw is hidden behind the front takedown pin to conceal it from view when the rifle is assembled.
Set screws just spin without tightening:
If the set screws go in and they just keep turning without tightening, a longer set of set screws may be needed. This could happen if the dimples mentioned above are deep enough that the screws protrude out of the receiver-side of the adapter too far (or if you somehow manage to strip the set screw sockets). The included set screws are 6-32 x 1/4. Using 6-32 x 3/8 should hold it. If the dimples are very deep, 6-32 x 1/2 set screws may work as well.
Longer set screws also allow more purchase on the threaded socket to prevent stripping. I think it’s a good idea, and I upgraded all my adapters. I found the screws at Home Depot.
Bolt Compatibility Fix:
If you decide to try one of these adapters, please note that not all bolts will “play nice” without a little modification to the adapter.
I have found the bolt shape design used by B. King’s Firearms (best value IMHO), Kaw Valley Precision, Faxon, Angstadt Arms, Brownell’s hybrid Colt/Glock, and FailZero DOES seem to work well with these adapters with no modification needed. They all look the same. For identification purposes, note the “gas key” shape and flat charging handle shelf at the front of the bolt:
Some bolts’ bottom contour will hit the outside top edges of the feed ramps or the top rear of the ejector, preventing the receivers from closing or dragging on the adapter during cycling. A reader has confirmed that the bolt used by Dirty Bird will hit the feed ramp, but I’d expect that any bolt with a similar profile would also have problems, since they usually all come from the same OEM supplier and are just re-branded by different sellers:
I can also confirm that the FM-9 bolt may hang up on the outside top edges of the feed ramp, binding the bolt.
To fix this problem, I made a few passes with a dremel grinding stone on the top angled outside edges of the feed ramp. This provided enough clearance for the bolt to work normally. It was a very simple fix, and could be done with a hand file if necessary.
Broken ejector:
If the ejector gets broken for any reason, a new one can be obtained from R&D Precision. I contacted the owner and cost at the time of this writing is $8.00 each plus shipping.
Simply separate the two halves of the adapter using the hex screws – one on each side – to open it up and replace the ejector.
Magazine retention issues:
I have never had this problem, nor seen any reports this problem, with the R&D Precision adapter. I thought it would be best to add it here in case someone did. I have seen this problem reported with the Hahn adapter.
If the mag pulls out easily without pushing the mag release, or falls out on it’s own….
- Try a different AR mag catch. A new one may be on the better end of the tolerances your setup needs. Yours may be out-of-spec.
- Make sure the mag catch is seating all the way into the receiver slot. Removing the coating around the outer edges of the mag catch usually fixes seating issues.
- Make sure the mag catch is fitting all the way through the adapter slot and isn’t catching on the adapter, holding it back from fully engaging. The adapter may be sitting a little high/low and dragging on the mag catch.
- The magwell is a little wide, or the adapter is a little narrow. Shimming the right side either between the lower/adapter, or adapter/mag will push the mag toward the catch.
- Last resort: Remove the mag catch. Where the threaded rod attaches the catch, a dremel can be used to carefully remove some of the shelf around the round rod. This should allow the catch to sit deeper in the lower, making it go further into the mag.
Colt’s AR-15 and 9mm AR/SMG’s use the same mag catch. They’re identical. However, Rock River Arms makes their RRA “9mm” mag catch, which sits about .012″ (3mm) deeper in the receiver to provide more positive magazine lockup through the side of the magwell adapter.
Many people claimed that the RRA 9mm mag catch pad was longer, but careful measurements have confirmed that the mag catch pad is about the same height as a standard mag catch. The RRA 9mm catches actually sit deeper in the side of the receiver.
Blowback9.com 