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Posted: 9/14/2013 4:52:15 PM EDT
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Ok guys, two things you need to know before I ask my question... First I am new to AR's, but not shooting. Second, this is a new lower that I bought as an 80% and milled it my self. Using an M4 lower the gun has shot 500 rounds without one issue. I installed the lower I just finished and was test firing it. Out of 190 rounds I got two light strikes. I loaded each one in an empty magazine after it happened, and they both fired. The first one happened at around 50 rounds and the second happened at about 150. To different magazines. For me, the bigger problem occured between rounds 190 and 191. 191 came out of the magazine and tried to load, but 190 went above it instead of ejecting and wedged in the upper. It caught the bolt and locked it up. I had to come home and use pliers to bend the casing enough to free everything up and get it out.
Ok, now my question... What do I need to check and or adjust for these two issues that showed up with the lower? 
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It had a malfunction, but trust me, it was jammed.  Everything was locked up tight. Nothing would move. It took a large effort to get the magazine and round 191 out so I could bring the gun home. The ammo was Independence 5.56x45mm 55 grain FMJ. It is the same ammo I used for 350 of the 500 rounds I put through the CMMG M4 lower. The current lower is an 80% tactical machining unit that I finished. Tactical Machining The first I have ever done as well as the first that I have ever put together. Therefore I am confident that the issue is with something I did or did not do. The lower parts kit is a CMMG AR15. The complete upper is a new USGI assembly that has been in storage (my Father's gun safe) since late 70's. Barrel is USGI SAK MP CHROME BORE, 12 to 1 twist. Upper is a LM stamped A1.
I have another thread that I will edit a link to that has pictures of the first part of the build. Let me know if I missed something the you may require. http://www.ar15.com/forums/t_3_123/616965_New__need_help___.htm http://www.ar15.com/forums/t_3_123/619919_80__jigs_.html&page=2 |
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Go fire it again and see if it's a recurring problem. Brass-over-bolt happens when the gun fails to either extract or eject.
At any rate, I'd check those parts first. Extractor and ejector. Lastly, it happens. While it's a very reliable gun design... if something ain't working right, it just ain't gonna work right. ETA: get some dummy rounds and practice reducing a brass-over-bolt malfunction, it's not the show-stopper most folks think it is. |
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Quoted:
 http://www.youtube.com/watch?v=jzY9jLeI0BU
Great video, thank you. It is what happened and I now know how to clear it should it ever happens again. I did notice when I was cleaning up my brass that every round had a mark on the lip next to the primer. On the flat of the shell. Any ideas on what would cause this or the light strikes? That's a bolt-override, AKA brass-over-bolt. You don't need pliers. Check your ejector and extractor. |
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There is not much about a lower that would cause that malfunction. I would look toward a weak ejector spring, a week extractor spring and the substandard ammo you are using. Although they are not retro parts, extraction can be improved by using the correct extractor spring insert and the Crane o-ring.
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I had one of those malfunctions on a DPMS. It was a lot of fun to clear. I needed a new charging handle and some detents after that one. Take a rod or a live round, reach in thru the ejection port and grab the bolt face with the tip. Pull the bolt to the rear, make sure the charging handle is locked fully forward and make sure you have already removed the magazine. The trapped round/casing should drop free thru the magazine well. |
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Double feed, weak magazine spring is the first thing that comes to mind.
Back in the 90s I was in the USMC out in 29 Palms, CA. A kid in another platoon was from Puerto Rico, and in addition to his weak grasp of English, he had a speech impediment. One day we were out doing one of the 400 series of ranges. I was in HQ at the time, so I chased after the skipper with a radio on my back. We got to 1st platoon and I saw this young kid frown at his rifle, then turn to his team leader and say "Copol, I gotta yubba yam!" "What?" asked the team leader. "I gotta yubba yam!" I leaned over and said "Double feed. He's saying double jam." This was all with M16A2s, so not retro. My apologies if this offends anyone. |
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It can be cleared with the charging handle but that's not ideal.
Remove the magazine, and insert a tool through the ejection port and pull back on the bolt face to release the cartridge that is stuck in between the bolt and charging handle. If that's a live roung generally it's a bad magazine. If it's an empty you have a cyclic rate, extraction or ejector problem. |
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Quoted:
Take a rod or a live round, reach in thru the ejection port and grab the bolt face with the tip. Pull the bolt to the rear, make sure the charging handle is locked fully forward and make sure you have already removed the magazine. The trapped round/casing should drop free thru the magazine well. Quoted:
Quoted:
I had one of those malfunctions on a DPMS. It was a lot of fun to clear. I needed a new charging handle and some detents after that one. Take a rod or a live round, reach in thru the ejection port and grab the bolt face with the tip. Pull the bolt to the rear, make sure the charging handle is locked fully forward and make sure you have already removed the magazine. The trapped round/casing should drop free thru the magazine well. Mine was locked up tight. I collapsed the stock, and slammed the stock on the ground while pulling on the charging handle and it still wouldnt budge. I ended up taking the stock off to clear it. |
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Quoted: Double feed, weak magazine spring is the first thing that comes to mind. Back in the 90s I was in the USMC out in 29 Palms, CA. A kid in another platoon was from Puerto Rico, and in addition to his weak grasp of English, he had a speech impediment. One day we were out doing one of the 400 series of ranges. I was in HQ at the time, so I chased after the skipper with a radio on my back. We got to 1st platoon and I saw this young kid frown at his rifle, then turn to his team leader and say "Copol, I gotta yubba yam!" "What?" asked the team leader. "I gotta yubba yam!" I leaned over and said "Double feed. He's saying double jam." This was all with M16A2s, so not retro. My apologies if this offends anyone. |
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Mine was locked up tight. I collapsed the stock, and slammed the stock on the ground while pulling on the charging handle and it still wouldnt budge. I ended up taking the stock off to clear it. Quoted:
Quoted:
Quoted:
I had one of those malfunctions on a DPMS. It was a lot of fun to clear. I needed a new charging handle and some detents after that one. Take a rod or a live round, reach in thru the ejection port and grab the bolt face with the tip. Pull the bolt to the rear, make sure the charging handle is locked fully forward and make sure you have already removed the magazine. The trapped round/casing should drop free thru the magazine well. Mine was locked up tight. I collapsed the stock, and slammed the stock on the ground while pulling on the charging handle and it still wouldnt budge. I ended up taking the stock off to clear it. You don't mortar a bolt override, you don't pull on the charging handle, you make ANOTHER charging handle! You mortar a stuck bolt/case. |
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Magizines are new, 3 USGI 30 round, 2 after market 30 round and 3 USGI 20 round. The malfunction happened on USGI 30 round. As for cheap, sub par ammo, it is all I can get my hands on for now. If I want to shoot, I shoot this untill supplies come back.
The magazines had a hard time locking in this lower. If I opened the halves, they went right in. I played with the magazine release adjustment, but it had no effect. The magazines were all tight from the fresh anodizing, but the magizine well started to loosen the more I used it. Is there a chance that this is just break in on a "one off" home made lower? I intend to give everything a good cleaning and re-assemble and inspection. I will well oil per the maintenance books. |
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Quoted: Magizines are new, 3 USGI 30 round, 2 after market 30 round and 3 USGI 20 round. The malfunction happened on USGI 30 round. As for cheap, sub par ammo, it is all I can get my hands on for now. If I want to shoot, I shoot this untill supplies come back. The magazines had a hard time locking in this lower. If I opened the halves, they went right in. I played with the magazine release adjustment, but it had no effect. The magazines were all tight from the fresh anodizing, but the magizine well started to loosen the more I used it. Is there a chance that this is just break in on a "one off" home made lower? I intend to give everything a good cleaning and re-assemble and inspection. I will well oil per the maintenance books. Your issues are being caused by extractor, ejector, magazine (doesn't matter that they are new they could still be junk, try a p-mag), or ammo issues. If the lower is causing the problems then it is probably too far out of spec to be helped.
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| Disconnector spring is large end down, per the book. I took everything apart and cleaned it very good. There was brass on the extractor and the bolt in general. Everything was gold dusted. I could not find any obvious signs of issues. The trigger spring is installed correctly. The lower was dryer then I thought it should be, so I lubed it up. Could just be ammo, but it ran so well last time out with all of the same items but the lower... |
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Remove the magazine, and insert a tool through the ejection port and pull back on the bolt face to release the cartridge that is stuck in between the bolt and charging handle. If that's a live roung generally it's a bad magazine. This is your answer but leave an empty mag in so when you push the bolt back it will lock then get rid of the offending mag. |
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This is your answer but leave an empty mag in so the when you push the bolt back it will lock then get rid of that mag. Quoted:
Quoted:
Remove the magazine, and insert a tool through the ejection port and pull back on the bolt face to release the cartridge that is stuck in between the bolt and charging handle. If that's a live roung generally it's a bad magazine. This is your answer but leave an empty mag in so the when you push the bolt back it will lock then get rid of that mag. No need to do that. The case/round will fall out thru the mag well. |
| After completing a build, I will generally run a couple of hundred rounds through it, clearing any malfunctions along the way before I tear into it. Most builds and new guns need one to two hundred rounds to smooth out the ridges, edges and burrs created during construction. Another thing to consider is never running a new build without lubing the crap out of it. You want friction to be your friend getting everything broke in properly. |
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"I gotta yubba yam!" Yup. I was squadded with a member of the National Guard from Puerto Rico at Camp Perry a few years ago. We pulled one of those pit rotations that seemed to last all day. He was a very talkative individual, and I could just about understand maybe 25% of what he was saying by the next pit change. Didn't stop him though, and it was entertaining, sorta. |
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