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Posted: 8/22/2010 5:21:01 PM EDT
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I need a hand. I have a post sample gun that we put together. It runs good but lately it has started started to malfunction. As you get to the end of a 30 round mag it will strike light a primer light a d the whole show comes to a stop. I thought it may be the ammo so I have swapped brands a few times and the same thing happens. Any pointers on where to start. I have a lot to learn on the M-16 front.
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Quoted:
Try a heavier buffer or replace the hammer spring if its had a lot fo use. +1. If it still has the hammer spring of the host semi, try replacing that spring with a milspec FA spring. Many non-milspec semi springs are slightly lighter (I always run a heavier-than-milspec hammer spring in my '16 because milsurp primers can be hard). And as sig-x notes, even a milspec hammer spring does weaken over age –– I change mine out like I change the oil on my car –– every 10k rounds if I am running a 20" barrel and rifle buttstock, every 5k rounds with a collapsible stock and sub-16" uppers. I don't run 9mm that much any more, but when I did, I swapped in new hammer springs every coupla thousand rounds. If you are running any barrel shorter than 16", you need a heavier-than-standard carbine buffer. H, H2, H3, depending on gas port size and/or ammo used. The barrel chamber is a factor, too –– Colt milspec is for a progressively looser chamber as the barrel gets shorter (yes, they use different reamers). A non-chrome-lined chamber, or an SS barrel, also can be an issue –– the brass just sticks in there in FA, while in a larger-reamed, chrome-lined chamber, it's free to keep running. Despite the design similarity, a semi AR15 is a different mechanical creature from a full-auto M16. Semis need to fire, then have enough residual gas power to cycle the action. Too much gas is just fine, and timing is not an issue at all. In full-auto, OTOH, the fire-and-cycle occurs about twelve times per second, and you need precisely the correct gas pulse, timing, and weight balance to keep it doing so, twelve times every second. That's why we talk about how well semis shoot, and how well MGs run. 
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