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Posted: 7/23/2024 3:04:35 PM EDT
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I built a side-charging upper in .22lr (CMMG bolt+barrel) that uses a modified FM style front charging system. Here it is on two different bufferless lowers utilizing FM bufferless setups. This is mid-project prototype, so mea culpa on less-than ideal handguard length, barrel length, and muzzle device for this setup. Same for missing handguard screws.     As a prototype, I saw no reason to risk damage on a failed project using an expensive upper, so I grabbed a raw unfinished upper I had sitting around for this type of experimentation. I also had an FM lightweight handguard sitting around, so I used that. (FM now sells their forward charging PC rails a la carte, so that's an even better choice to replicate this.) FM's lightweight handguards have the same cross-section profile under the picatinny rail as their forward-charging pistol caliber rails. This allows the FM forward charging handle to be sufficiently supported in either rail. To utilize a charging handle in the lightweight rail, the charging handle action rod and action rod spring need to altered, and the rail needs to have at least 1 set of vertical supports of the picatinny rail removed to allow sufficient movement of the charging handle. These pictures show where a set of vertical supports was removed from the front rail, and how some sanding and filing the bottom opened up a channel for the paratrooper charging handle to smoothly move in.   I'll get some pictures of my modifications to the rod and spring system the next time I disassemble it. I also modified the upper, but until I disassemble it, it'll be hard to get pictures of those alterations. Essentially the forward charging handle action rod at a height where it would line up with the top of the opening in the front of a charging handle, instead of right in the center of that opening as a gas tube would align. So I expanded the gas tube opening on the front of the receiver upwards to allow free movement of the action rod into the upper. But at that height, the action rod would go right over the top of the CMMG bolt -- so the action rod needed an adapter to allow it to interface with the bolt. A standard charging handle can charge CMMG style bolt, but there's no gas system utilizing the opening in the front of it. To give the action rod something to press against, I plugged the front of the charging handle with a steel rod. I cut about 1" long section of the closest oversized steel rod I had, and ground it down until it fit snugly. I then tapped it into the charging handle, drilled a hole for a roll pin (from the bottom), and pinned the rod in place. Then I cut off the end of the steel rod, and ground the front face flat, and profiled the portion of the rod in the charging handle to match it using a grinding wheel. Lastly I cut the charging handle off behind the wings that keep it aligned in the upper.  I then cut out the middle section of the charging handle, allowing the front part to move freely while the rear portion remains in place. I retained as much of the charging handle as possible to reduce gas issues and provide support for the upper rail/spring of the CMMG bolt/buffer spring system.   I also modified FM parts to make two lowers: a Griffin MK2 utilizing a modified FM buffer plug+picatinny rail, and a FM Ranch lower (which uses Remington 870 pattern stocks/grips) utilizing a 25 cent spacer. (This FM lower is built out with a birds-head grip because: whenever possible build as pistol first.) Ideally I'd like to do 22 rifle using the ranch lower and a proper stock. The FM Ranch lower is really easy to convert to work with a CMMG style bolt. First, remove the round, black buffer-pad.  Second, insert quarter. This makes for a snug fit, and I one might need to sand some of George's face off, depending on how flat the weld is on the back of your CMMG-style bolt. ETA: Bolt length is different in the PSA version of the carrier, and a quarter was too thick of a spacer for that version of this bolt.  Modifying the FM buffer-plug for a standard lower was slightly more work. First I installed the plug, marked where the bottom was on the plug, and removed the assembly. Then I cut/ground off the material above the bottom 3 index points for the buffer retainer. The material removed here 1) stops a standard upper from closing on a lower with this plug installed, and 2) takes up the same physical space as the CMMG-style bolt. Fortunately, the CMMG bolt's rear isn't round and fits perfectly with only the side and top material removed from the plug's index holes. The buffer pad, similarly must NOT be installed, as it occupies the same space the rear of the bolt does.    And, I think that's everything I have pictures of at this point. Other parts that are used here include a Griffin Hammer Comp muzzle device, Griffin MK2 lower, LaRue MBT trigger, Strike Industries charging handle. Both lowers, by re-inserting the buffer pads, still work as intended with the FM bufferless uppers they are designed for. (My other uppers for these lowers include .350 Legend and .300 BLK, both re-barreled FM bufferless uppers.) So, here we have it, one last view:  eta: shrunk pictures for readability. |
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Quoted: Neat work. Does the FM side charge have a lock back function? Could a notch to hold the bolt back be cut into the handguard? The FM forward side-charging setups (Gen 1 or Gen 2) do not have a lock-back mechanism. FM's uppers are designed to use the bolt catch on the lower receiver. Gen 1 side chargers are all pistol caliber and require buffer tubes. Gen 2 side chargers are all rifle caliber and have the buffer spring system contained in the upper receiver, above the bolt carrier group. As far as adding such a notch, with their Gen 1 (standard rail height) receivers and rails, it's not an option. The Gen 1 action rod has a cross-section shaped like a sideways H, and it runs on guides on the sides of the handguard to keep it aligned. As such, it can't rotate. Gen 2, which has a higher rail height than mil-spec, and uses a proprietary receiver, bolt carrier and gas key, has a circular cross-section action rod, and would probably be able to be notched so the handle can lock back. But Gen 2 lacks a traditional charging handle, so at least with a CMMG style bolt, it's not an option for this particular project. The end of the action rod.  This image shows the end of the rail, where the guides are obvious. The allen wrench is in there to loosen the set screw that holds the charging handle in the action rod.  This picture is from the rear of the receiver. The guide rails for the action rod are visible through the expanded opening for the gas tube.  And here's that same modified gas tube opening from the front.  To make sure the spring on the action rod didn't bind, I considered a washer, and specifically cutting a small section of the H-shaped portion of the action rod. Instead I switched up the springs slightly. I chose a short spring with a slightly narrower ID and a thicker wire. I then ground the ends of the short spring flat to create a better interface with the front of the upper receiver and eliminate binding of the spring in the opening to the upper receiver.  I combined this heavier spring with a trimmed original spring. Here's an original action rod and spring, and below the shortened action rod, and altered spring system I'm using.  eta some clarification eta shrunk images -- got smarter about imgur |
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An update, with a "final product" model using a slight update of this technique (stock refinishing planned): I left the rear charging handle intact for this, and re-cut the angle on the rear of the receiver's pocket for the charging handle latch catch. The rear charging handle still latches, but it does not lock, so the fore charging handle can actuate the rear charging handle. It acts more like a ball-detent creating some resistance now. The rear charging handle usage does not affect the fore charging handle. Parts: Remington 870 12ga youth stock, LPA sights (fiberoptic front), Foxtrot Mike Productions lower w/Hiperfire trigger, AT3 Tactical billet upper (modified), CMMG bolt + barrel, U.S. Mint George Washington spacer, BoreBuddy collar, buffer & weights, buffer pad, Railwoods checkered walnut panels, FMP handguard (modified) and fore charging handle assembly (modified), Breek Arms Warhammer charging handle (modified). Griffin Armament taper mount EZ-Brake. Swivel studs for traditional hunting sling. Right side:  Disassembled:  Left side:  Gallery with full-sized images, some images not posted here. |
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Lotta design and build work there - congratulations! For the side charge, bufferless AR pistol that I have; for one of the .22LR uppers I have a Gibbz G9 side charge upper with a CMMG 4.5 in. bbl./collar/bolt/recoil assembly/frame and for LRBHO and bolt holdback, I use the ARCatch22 V2 bolt catch adapter. For 5.56x45 I have an OA93 upper and take off the V2 adapter. MHO, YMMV, etc. |
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Quoted: Thanks for the compliment and suggesting the Catch22 V2. I will pick have to one or two of those up. Are they only available from manufacturer? I go mine from Brownells - same price as the manufacturer. You might also want to consider getting one of the AR Catch22 Lightened Bolt Catch Springs ($1.25 - about the same cost as a standard bolt catch spring) if you're only going to use the lower with your .22LR upper. My experience has been that the LRBHO doesn't get tripped reliably even with .22LR mags that have feed springs that are on the strong side with the standard LRBHO spring. I'm not a competitive shooter, so I pretty much only use the BHO manually to ensure that I can easily see that the chamber is empty, and so I've left the standard LRBHO spring in. Best of luck. MHO, YMMV, etc. |
Thanks again. Regarding the bolt catch plunger spring, this upper does have rifle caliber uppers that go on it. I have one semi-auto upper mid-build (waiting on .400 Legend barrel -- playing with the newest thing, yaeh, yaeh...), and have one other upper finished for this lower, a .300 blk bolt action upper. Wilson Combat barrel, Griffin Armament Hammer Comp, SD Tactical handguard (modified), Hightower Glock Sight Bases w/ Hi-Viz fiberoptic sights, Spikes Tactical featureless slick side upper (modified), a United Defense (el-cheapo) bolt carrier group (very heavily modified to include spring system under handguard, no tail, etc) and side charging handle. 5 lbs 5oz. Okay, enough typing... ...there's football games that aren't gonna watch themselves. |
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