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Posted: 11/13/2016 1:05:25 PM EDT
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I recently married a 20" upper with a rifle length gas system, with a SW lower with a carbine stock.
I cannot shoot more than one round at a time without having to re-charge the bolt. The rifle is not loading secondary rounds at all. How can I determine if it's: 1. Is there not enough gas pushing the bolt back far enough? 2. Or, is the carbine stock pushing the bolt forward to quickly that the round never has a chance to chamber? In otherwords should I consider getting an A2 stock or variant. Another interesting observation is after I successfully fire one round, if I drop the mag out, the next round to be chambered has a scrape from about the neck all the way(but not totally) to the base of the casing. So i can clearly tell the bolt is cycling like it should but too fast? not far back enough? This is my first experience with a rifle longer than a 16" carbine...Any ideas from the experts would be helpful. |
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I tend to doubt that it's overgassed. Unless the gas port is way too big, a standard 20" rifle-length gas system barrel should operate the rifle at a lower pressure than the typical carbine. It should actually be more reliable than the carbine, even with a carbine receiver extension.
Let's start with the easy stuff. 1.) Use ammunition that's generally considered of sufficient pressure. Federal XM193 or 55gr American Eagle are cheap, readily available, and will suffice. Any rifle should run with those. 2.) Get a brand new magazine from a reputable manufacturer. Load one round into that magazine. Fire it. Does it lock back? (It should.) If it does, your original magazine is probably tired or defective and not getting the second round up in time to be caught by the bolt carrier group as it moves forward. If it doesn't, tell us more about your carbine buffer (is the weight the old standard 3oz, H, H2, H3, something else?) Edited for clarity. |
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You most likely either have an ammunition problem or a gas problem.
I bet you are shooting someone's shit reloads. If not, you need to take a look at the gas system. If it has an A2 style front sight base, that probably is not the problem. Make sure the gas tube is not installed upside down, and make sure it is pinned correctly. Make sure the gas key is tight on the bolt carrier group. If you or someone else installed an aftermarket gas block, it is probably misaligned. If it is an adjustable gas block, make sure the gas port is open or you won't get any gas at all, or not nearly enough. The 20" upper on the carbine lower should have no effect on operation. |
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What is a "sw" lower? Buy a standard carbine spring and buffer from palmetto state armory, they are cheap. Where did you get the upper? You should have zero problem with that set up. What mag are you using?
If you have the standard buffer+spring and a new pmag you should be good to go. Edit: you test it with one round in a mag to see if it locks the bolt back. You may even need a reduced power recoil spring. I had a brand new PSA mid length upper and I had to hand cycle it through about 2 mags before it got broken in and then it would cycle fine. You may be right on the edge of being under gassed. |
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I am running a 20” barrel with a Tubb’s flatwire spring and a H3 buffer with no problems.
If the BCG locks back when fired with one round in the mag consistently then you are not under gassed. Manually pull the charging handle full back, there should be .125” to .250” of the bolt visible in the ejection port. If you are under gassed then you would follow the usual trouble shooting steps for gas system problems. |
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