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Posted: 10/2/2008 11:46:10 PM EDT
| I'm looking for a list of all the possible or probable reasons a AR will have bolt over base feed issues. I checked the FAQ and didn't find anything on bolt over base failures. Thanks for the help. |
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First off, Pull the mags apart and give then a good cleaning. In the case of used mags, fouling. And in the case of new mags, assembly debries that may be causing follower binding on the inside of the mag. As for mags, best luck will be with USGI types since they are produced to specs a great deal better than the cheap knock off type. From there with only a single round loaded into a mag, insert/charge the singe round into the rifle and fire for effect. The desire is that the bolt has now locked back on the bolt catch since the mag is now dry. This proves that the rifle is correctly full stroking, and you can mark off a huge to check for thing from the list of problems. With full stroking, and clean working mags, then it may just come down to the height that the mags are being held upwards from the mag catch, and maybe even relationship of the upper and lower in the case when using a front pivot adapter. |
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Thanks Dano. Checked for short stroking as soon as I thought there might be a problem. Locks open every time. Just checked to see if the mags sit high enough. They are fine. I don't think that the followers are getting hung up, but I could be wrong. I am using USGI bodes with CS springs and Magpul followers. And for the record the gun is a LMT MRP 10.5 bbl. H buffer and regular bolt and carrier, not the enhanced. What it looks like is happening is that as the bolt travels rearward the top round in the mag gets flipped up. And then on the return of the bolt it slams into the side of the round. Maybe it is not correct to call it a bolt over base. If it was a pistol I would call it a high feed. In the past I had a 6.8 rifle that had a bolt that had sand cuts in the lugs. It would catch the cartridge rim and flip it up the same way I think this rifle is acting. That rifle was a easy fix. I replaced the bolt. I'm thinking the LMT bolt is doing the same thing, but have never seen this with a standard bolt. I'm also looking at the feed lips on the magazines. Maybe the lips are not holding the round in a proper position. If you pull rearward on the rim of the cartridge with your finger nail while it is in the magazine you can get the round to pop up. The problem is that every mag I'm using will do this weather USGI, P mag, or HK. I can't tell if some are easier to get the round to pop out than others. For that matter I'm not totally convinced that there is a problem. It only happened three times in a matter of about 300 rounds. I have gotten the last 100 threw it without failure. But I require my rifles to work better than that. I just want to check every thing out to see if I can find a problem. That is why I am looking for a laundry list of things to look for. Anyone else have any suggestions? Thanks |
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With the rounds getting flipped in the mag, the followers have not broken in yet, Hence not keeping constant force of the top round against the bottom of the mag lips. Pull the followers and check the skirts for production burs that may be allowing the follower to bind to the sides of the body as the rounds rise in the mags. Also, a touch of CLP left in the mags when you clean then will not hurt a thing to allow the mag followers to break into the mag bodies. Load a few rounds into the mag and press down on the rim of the rounds, then let them let them snap back up all by themselves. If the back end of the top of the round is not coming back up against the feed ramps cleanly, then you have a follower binding problem (try two rounds, then three to confirm both sides). Note: shorties do have a quicker cycle rate, and a mags in question may not be able to keep up with it (read top round recover in time). Not sure if this is a SBR or full auto rig, but slowing the cycle rate down with a heaver buffer may be just the trick to allow mags to better recover. Also could be that the mag bodies are toasted (cracks in the Rear U section). Cracks here will allow the lips of the mags to spread open every time. |
Very probable. I will check them out.
Not running this one FA yet. I do have a RDIAS. I might just put that lower on, and dump a mag with the timer going to see how fast she is running. Any recommendations on how fast is to fast? Thanks again. 
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I like to mac-jack the shorties, so around 1200 works for me. This allows me to burst a few rounds on threat with a quick single trigger trigger pull, and move onto the next threat to deal with it. At this speed, there is very little climb (mod'd Branson brake helps) and it just feels like a long single shot for the bursts (think HK G-11 for the auto pulse feel). If you going to mag dump, then a MGI rate reducer will slow the cycle rate way down to control even a dump into tight groups, even without a mod'd brake (think sedate like an AK-47 recoil, just without the feeling that the thing is beating it's self to death like AK's do). As for mod'g a Branson brake, pay with the slot index to get the unit to recoil straight downward (not to one side like an Ak-47 with slant brake). Once you have that, increase the muzzle opening until it no longer recoils downward, just straight backwards (think MP5 FBI brake feel, and the cycle speed and opening will go hand in hand, so pick your cycle speed before mod'g brake). While I'm on the subject of the AK's, a black jack buffer goes a long way to take that clanking metal on metal feeling like it's going to self destruct out of the rig, especially if you are pulling the retarding to speed up the cycle as well (read into full auto SKS range). |
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