D Edwards Posted November 10 Report Share Posted November 10 IT WILL NOT FIRE FULL AUTO WITH A RETRACTABLE STOCK. IS THERE A SOLUTION FOR THIS? ANY ADVICE IS APPRECIATED. DARWIN EDWARDS Quote Link to comment Share on other sites More sharing options...
Colt Chopper Posted November 11 Report Share Posted November 11 No expert here, just experience with mine. LMG upper is designed for a full length stock and a special buffer. On normal M16 uppers with collapsible stocks, and shorter uppers, will require a heavier buffer, are you getting a light primer strike on the second round? Quote Link to comment Share on other sites More sharing options...
Mile High Armory Posted November 11 Report Share Posted November 11 Does it have a full auto bolt? The LMG upper was available with either a semi auto bolt or a full auto bolt. Quote Link to comment Share on other sites More sharing options...
MPFiveO Posted November 11 Report Share Posted November 11 Can you describe exactly how it's not firing full auto? Is it dropping the hammer on a live round? Is it bolt bounce? What buffer weight are you using? The RO750 upper was designed to work with a rather heavy hydraulic buffer and special spring in an open bolt configuration. Despite this I've seen them run fine with standard rifle buffers and springs in the closed bolt configuration. Both of the above configurations utilize the fixed A2 rifle stock. If you're trying to run it with a CAR stock you may need to run a H3 or H6 buffer for best results. Quote Link to comment Share on other sites More sharing options...
Mongo Posted November 11 Report Share Posted November 11 The LMG uppers are notorious for not wanting to run with a collapsible butt stock. Its a known issue and I never bothered trying to solve since I prefer an A2 butt stock when shooting mine. Quote Link to comment Share on other sites More sharing options...
DINK Posted November 12 Report Share Posted November 12 FWIW, a long time ago (probably in the 80s) I was taking a tour of the Colt factory and we visited the customer service area. There was an AR with a 20" barrel and a collapsible stock leaning up in a corner and I asked what the problem was with that one. The head of CS told me that the AR15 just didn't like to run in that configuration and they had no idea why. No matter what they tried, a rifle with 20" barrel and the carbine stock just doesn't run. If Mongo ever does figure it out, he should let Colt know, LOL. Quote Link to comment Share on other sites More sharing options...
johnsonlmg41 Posted November 12 Report Share Posted November 12 Not that hard, you just have to balance out the components just like any other AR platform. Colt can't balance a checkbook either, so when you bring together an upper you didn't design, and a lower you didn't design, it's not a great surprise they had problems? IIRC, for a long time they didn't even make their own lowers? Quote Link to comment Share on other sites More sharing options...
MPFiveO Posted November 12 Report Share Posted November 12 It's simple math. The length of bolt travel does not change in the AR platform. The amount of gas delivered is determined by the size of the gas port. The variable is the weight of the buffer system. A quick search reveal the following buffer weights. Carbine Buffer — 3 oz. For a carbine-length rifle, 3 oz. ... Heavy (H) Buffer — 3.8 oz. There is a wide range of heavy buffers available, starting with the 3.8 oz. ... H2 Buffer — 4.6–4.7 oz. ... H3 Buffer — 5.0–5.4 oz. ... Pistol Buffer — 5.0–8.5 oz. Rifle buffer -- 5.2 oz. Hydraulic LMG buffer -- ? So, again, the upper doesn't know how long the buffer is, it only knows the weight of the buffer. So if the upper is tuned to run a rifle buffer weighing 5.2 ounces, then it should be able to run a carbine buffer weighing 5.4 ounces. The difference is minimal and the gas port is usually in the middle of a weight range, not at the edge of proper function. In almost every case, a mil-spec buffer will be timed for full auto function with a specified port size. A barrel with a larger port size is going to feed more gas into the bolt group moving it faster. If the carrier is moving so fast that it causes the buffer to rebound off of the buffer tube increasing it's forward velocity it can cause bolt bounce. When the bolt carrier strikes the barrel extension so fast it bounces. The job of the buffer is to mitigate this bounce by acting as a dead blow to keep the carrier forward and the bolt locked into battery. A bolt group moving too fast will need more weight in the buffer to counter the bounce. There is a shelf inside the carrier that retracts the firing pin with just a minimal amount of movement. Any movement rearward and the firing pin will not be able to protrude through the bolt face and reach the primer. So timing the movement of the weights in the buffer is critical to keeping all the components in their proper location until the hammer has completed it's swing to strike the firing pin. Quote Link to comment Share on other sites More sharing options...
lonewolftactical Posted November 12 Report Share Posted November 12 USMC did a bunch of testing on this and the only system that met or exceeded the reliability of the full-length Rifle receiver extension and buffer is the Vltor A5 system. Since Eric Kinsel is now with BCM, they also offer the A5 system as well. Forward control designs may also offer A5 length receiver extensions too, but not sure about the buffers. Point being, there are several options out there now for the A5 enhanced carbine buffer system in various colors and buffer weights depending upon your setup. No reason this wouldn't work like a top for your upper. Quote Link to comment Share on other sites More sharing options...
johnsonlmg41 Posted November 12 Report Share Posted November 12 Probably noteworthy if you are using the original hydraulic buffer, some of them are bad by now. I've rebuilt two of mine. The OP is hopelessly devoid of details. Quote Link to comment Share on other sites More sharing options...
MPFiveO Posted November 13 Report Share Posted November 13 36 minutes ago, johnsonlmg41 said: Probably noteworthy if you are using the original hydraulic buffer, some of them are bad by now. I've rebuilt two of mine. The OP is hopelessly devoid of details. Since this is a LMG related post, would you mind sharing the rebuild process? I've never messed with hydraulics but I have one of the buffers and I know they have a limited lifespan. Quote Link to comment Share on other sites More sharing options...
Mongo Posted November 13 Report Share Posted November 13 3 hours ago, johnsonlmg41 said: Probably noteworthy if you are using the original hydraulic buffer, some of them are bad by now. I've rebuilt two of mine. The OP is hopelessly devoid of details. I too would like to hear about your rebuild of the buffers. I have 2, one still unused NOS. Both of mine seem fine still but who knows for how long. Quote Link to comment Share on other sites More sharing options...
johnsonlmg41 Posted November 13 Report Share Posted November 13 Unfortunately that was at least 3 phones ago so the pics are lost. When you get leakage they are toast and have to be rebuilt. They are oil filled with chambers and O-rings like any other shock absorber. I wish my memory was good enough to remember the process. I think I also have an old enedine which is similar. Not sure why no one makes this stuff any more, because they work well on guns that have issues and reduce the ROF pretty well. Quote Link to comment Share on other sites More sharing options...
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