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Posted: 2/14/2005 9:06:53 AM EDT
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I have an upper that will extract a spent case when there is no magazine in the rifle (ie: single shot). But, when I insert a loaded magazine, the spent case will only be extracted half way and the bolt will continue to move back and then begin to feed a fresh round, cloging up the works. This problem is the strangest thing as the upper functions fine single shot but when a mag is insert, it malfunctions. It is not the magazine or ammo as I have tried several USGI mags and several types of ammo. Any help is appreciated. Scott |
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A +1 for short stroking! Let me guess - it's an SBR? What ammo are you using? I've found that underpowered ammo will cause this in an SBR. How does it work with M193? Important question - if you load one round in a magazine, close the bolt and fire it, does the bolt automatically lock open after that shot? If the bolt doesn't lock open it's short stroking. Before you resort to enlarging the gas port do yourself a favor and go to the FAQ's in the troubleshooting forum and follow the checkout proceedure step-by-step. You may wind up having to enlarge the port, but I would exhaust all other possibilities first. Just my .02 |
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I have used Federal M193 in it. It is a DPMS 7.5" upper with a permantely fixed fake silencer. I haven't tried firing one round with the mag in the rifle. But, if the rifle was short stroking, why would the bolt move all the way back to feed a fresh round? Like I stated in my original post. Scott |
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scotty, check your ejector and spring. push it in with the tip of a punch or something to check if the spring is still good. I had the same thing happen to me, turns out my ejector spring had shattered into a bunch of little pieces and there was not proper spring tension to eject the spent round, and it was getting hung up in the upper when the bolt came forward with a new round. hope i could help.. CT |
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As a test, disassamble a mag, and insert the bare body into the piece. Then, cycle the thing slowly by hand, and see if you can feel the mag lips dragging on the underside of the carrier. If you can, that may well be a source of your problem. It just takes a few thousanths of an inch out of spec on the slot for the mag catch in the lower, or too shallow in the grooves for the mag clearance on the carrier, to cause this. In a full size rifle, it might not be a problem, but since pistol function is marginal at best, it wouldn't take much drag to cause your malfunctions. If the piece passes that test, then I would have to go w/the "too little gas" crowd. 
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I still think its an extraction problem. If it cycles far enuf to strip a fresh round yet is unable to clear the spent case it is usually an issue with the extractor (assuming the ejector is functional). It can be caused as the extractor loses control of the case during recoil, such that the ejector can not spit the case thru the ejection port since the case is free of the bolt face. The fact that it happens with a mag in place could indicate that the dynamics of this are being changed by the friction of the carrier running over the mag. O-rings are not band-aid for SBRs, they are required equipment. |
Your description is the closest to my problem, except the case is not comming out of the chamber all the way and the ejector doesn't even have a chance to spit the round out. |
| I have to revise my morning diagnosis as I missed the "feeds a second round" comment. Since that is the case, I vote for extractor tension. The bolt/carrier group may work fine in other uppers, but I doubt they have 7.5" barrels. The shorter barrel yields a higher pressure gas tap point, resulting in higher carrier acceleration. The next round in the magazine (or follower) is dragging on the fired brass and increasing the tension required to extract it. Boost the extractor tension or use something to dial back the gas. |
The bolt can move far enough back to strip a round out of the magazine but not quite far enough to lock the bolt open on the last round. That's a classic example of short stroking! One test that Tweak told me about is to hold the bolt release out with one hand as you fire it (with the magazine installed). You should feel the bolt trip the bolt catch during recoil. If you don't, the bolt is not coming back far enough, AKA short stroking. I tend to go with the theory that you've got barely enough gas (with that 7.5" bbl) to begin with and the added drag of the loaded magazine is just too much for the bolt to overcome resulting in a short stroke when a loaded magazine is put in. Online diagnosis is a bitch, but this is the best that I can do under these circumstances  .
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