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Posted: 3/26/2010 6:34:52 PM EDT
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After a frustrating day at the range I figured I would come here and see if the hive could provide some answers.
The build: Mega Billet Lower DPMS LPK Magpul ACS Stock w/ carbine buffer and spring BCM 14.5" carbine upper S&W BCG (Not sure if they outsource, got it from another new rifle) Magpul MOE handguards The Issues: Rifle wont eject spent case about 75% of the time Wont feed new round 100% Bolt wont cycle and "cock" the rifle about 50% Attempted Fixes: Thought rifle was a little dry, lubed a little heavier than normal Swapped upper and bolt with functioning (factory) S&W 16" carbine w/ semi-auto BCG. Swapped lower with functioning (factory) S&W M&P15 lower Magazines used: PMag 20 and 30 rounders, Brownells USGI Steel mag 30 rounder Ammo used: Remington 55gr (50 round pack from Wal-Mart), XM193, Hornady TAP 75gr So anyone have any suggestions. I also checked the gas tube and it seems secure but due to moving out of state I don't currently have the tools or space to take apart the rifle and check the gas hole. I am also really confused because the upper wont work with a known good lower and the lower wont work with a known good upper and BCG. I am kind of new to actually building these rifles, am I doing something wrong or did I miss something? I followed the how-to step by step when I put this together and the upper was put together by Bravo Company minus the CH and BCG. Any help would be appreciated. I can also post some pics if you think it may help. Thanks. |
| So it's a BC built upper less the CH and BCG. And, it wont work on a known working lower. I'd check the gas key and be sure its tight and have a look at the rings on the bolt. I would think that the upper problem is with the BCG. As for the lower sounds like something isn't installed right. Does it pass the function test with no upper on it? Hammer cocks and safety works, with hammer release when trigger is pulled? Be sure not to let the hammer fall on an unprotected lower (stick your finger in there so it doesn't strike the lower). |
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Quoted:
So it's a BC built upper less the CH and BCG. And, it wont work on a known working lower. I'd check the gas key and be sure its tight and have a look at the rings on the bolt. I would think that the upper problem is with the BCG. As for the lower sounds like something isn't installed right. Does it pass the function test with no upper on it? Hammer cocks and safety works, with hammer release when trigger is pulled? Be sure not to let the hammer fall on an unprotected lower (stick your finger in there so it doesn't strike the lower). The BCG does work on another rifle but I checked anyways. The gas key is tight and staked. The rings are loose and can be spun by hand. Is that normal? The grooves were staggered like I read they should be. The lower passes the function checks. I just checked again to make sure though. |
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Sounds like a gas issue. Swap a new BCG into this build. If it works fine, then your BCG was bad. If it doesn't work, then it's in the gas system of the upper. Double-double check the lower parts, hammer spring installed correctly and disconnector spring installed correctly (wide end down).
I'm sure about this - but what buffer spring do you have in the lower? You said a 14.5" upper. Not sure a rifle spring will fit in there but maybe that's the case? And it's too much force for the BCG to overcome? That's a wild ass guess. Try the first part first though. Can't just randomly swap parts around or a differential diagnosis is impossible. One this at a time, a know quantity. Just because your BCG works in another rifle doesn't mean it will work in yours. |
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Quoted:
Sounds like a gas issue. Swap a new BCG into this build. If it works fine, then your BCG was bad. If it doesn't work, then it's in the gas system of the upper. Double-double check the lower parts, hammer spring installed correctly and disconnector spring installed correctly (wide end down). I'm sure about this - but what buffer spring do you have in the lower? You said a 14.5" upper. Not sure a rifle spring will fit in there but maybe that's the case? And it's too much force for the BCG to overcome? That's a wild ass guess. Try the first part first though. Can't just randomly swap parts around or a differential diagnosis is impossible. One this at a time, a know quantity. Just because your BCG works in another rifle doesn't mean it will work in yours. I did swap out the BCG with one from another rifle that functions just fine. I know this may not mean much since like you say a BCG may work in one rifle but not in the other. My buffer spring is a carbine spring, just about 11" wrong like the T/S guide calls for. It does seem slightly stiffer than the spring in my other carbine which is about 1/3" shorter but both are still within specs for the carbine spring according to the T/S guide. I am going to order a BCG from Bravo Company next week, I just have to wait because I am in the middle of moving out of state right now and don't have a permanent address yet. Now I am going to go over the lower parts again, maybe I am missing something. |
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All the parts installed correctly?
Barrel fully cleaned from taken the rifle out to be fired the first time? Upper bearing areas correctly CLP lubed to break in the rifle? Ammo, are you using factory new brass cased ammo to break in the rifle, so we don't need to guess on the reloads/ if the rifle will every function with steel cased or surplus ammo that is being used? With the upper shotgunned open, stick your finger into the center back of the carrier and pull the B/C all the way back until it maxes out on the charging handle, then push it all the way back in (do this a few times). Does the carrier move like it's gliding on greased glass, or is the carrier key binding in the upper slot/ the gas tube not aligned with the carrier key and nothing smooth about the B/C moving in the upper receiver? Upper locked back on the lower, pull the charging handle back slowly. Should take a slight more effect to start the B/C back since the bolt is unlocking against the barrel extension lugs and the hammer being cocked backwards, but once you get past such (about a 1/3 way back), the tension needed to pull the CH the last part back should be constant in pressure (read last few inches of the charging handle pull should not be greater in tension, hence either the recoil spring wound bound out or the hammer wedging between the disc and bottom of the carrier to cause the increase in tension. No mention of the sights, but check flow from the end of the tube through the barrel port (blow on the back of the tube), and if you have a gas block, confirm that the tube is installed into the block correctly, and the gas block passage is centered on the barrel gas port at well (above blow test will tell if everything is install and aligned correctly). Lastly, when you test the rifle again, single round into the mag, that mag insert into the rifle and the round charged, followed by that single round being fired, with the correct action the bolt locking back on the bolt catch. We need to know this from the start, which tells us if the rifle is full stroking or not to begin with, then we can resolve other problems quickly from such knowledge. |
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