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Posted: 11/29/2002 2:55:36 PM EDT
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I recently purchased a Bushmaster 16" upper to go with an Armalite lower. It was at a local gun show that the purchase was made, the upper included the bolt carrier group.When I got around to the range there were a couple of problems I encountered. The front sight was out of alignment. How do you adjust the front sight laterally? My Armalite upper had allen screws in which to make adjustments, this upper has some sort of pins, are they removeable? The second and more concerning problem was the fact that when I chambered a round and shot, it would eject the spent casing, but would not chamber the next round. My first thought was gas tube blockage or leakage. I went back to the house and took a closer look at the bolt, carrier and I compared them with the carrier group that came with my Armalite and guess what? I think they sold me an M-16 carrier.This is illegal that I'm aware of, but could this be the source of my feed problems? Or could the fact that I went from a 20" upper to a 16" upper with a common lower, the buffer spring tension exerting to much force. Are buffer springs from a rifle that had a 20" upper the same tension as a spring from a 16" upper? any help is greatly appreciated. |
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The lower should work equally well for any length barrel. If the next round is being left in the magazine, or if the bolt smashes it halfway between the chamber and the magazine, then your upper may be "short stroking". Does the bolt lock back on the last round? The rear sight should center the point of impact within 12 clicks. There are lots of things that cause sight alignment problems. Simple rotation or clocking of the receiver, barrel not in alignment with the receiver (face of the receiver not perpendicular), bent barrel, or bore not at center. If it is a Bushmaster upper they may fix the problem for you. |
| In reference to the short cycling I took the hand guards off and noticed alot of carbon on the upper handguard near the gas tube entry into the sight post. I checked the gas rings on the bolt for stagger and tightness. I checked the allen screws on the . The ammo used is proven to work S.A. surplus. The magazines are proven to work . It just seems like the bolt and carrier do not travel far enough back in the reciever for the bolt to catch the next round out of the magazine. I even checked the buffer spring for any binding and found no obstructions. Could it be the fact that the carrier Ibought at the gun show assuming it was for an Ar and not an M-16 be a factor.I looked at the bottom of the carrier I bought with my original Armalite with the one he sold me at the gun show and it appears to be an M-16 carrier. Regardless if I ever hear from these people again though I have emailed, but could this be what is causing the problem with the short cycling. Can an M-16 carrier work properly in an AR-15? |
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M-16 carrier should work just fine. If you are concerned about legality, you can modify it on a bench grinder so that the autosear trip notch is like on a semi-auto version, and it can never work with full auto trigger parts, thus becoming legal. Short stroke... Lots and lots of advice back through the threads. Look 'em over. Polish the chamber, makes sure the ammo is not showing signs of excess pressure, shoot it enough to break it in, change the gas tube, and when all else fails, we can talk about bumping the gas port size, but it is always the last resort. |
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Thank you guys for your advice. I've disassembled the gas tubeleaks and thoroughly cleaned. I've cleaned path from barrel through barrel orifice for gas tube. Is there some sort of a non-hardening sealant to put on the gas tube entry into the sight post that helps to retain a seal until the carbon build up seals any leaks with repeated shooting? I've read somewhere that maybe loctite or a silicone of some sort, but maybe I,m wrong. I've yet to try at the range I hope this works. The only other possible things, the ammo, which worked in my other upper/bolt combo or the magazines which worked in the other as well. Could the other possibility be a loose barrel. This was a used upper after all that by my judgement seems to be in good condition. |
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Leaks at the front sight housing are not a big deal. NRA Service Rifle shooters take that dark smudge on the barrel as a badge of office. Loctite is to glue the sight housing to the barrel after we have the sight squared up with only a couple clicks windage. I would leave that end alone. The big leakers are: Bolt Rings - The bolt should stay put against gravity with the rings alone. When it slips by tipping the carrier, replace the rings; Carrier Key - If you can find powder gas residues around the joint between the key and the carrier, you might need to lap the surfaces together, but you should definitely buy two new screws and install them per the TM; Gas Tube to Carrier Key - Check that the knob turned on the end of the tube is not worn and the key mates up with it nicely; Bolt Tail in the Carrier - Dial calipers will tell you the diameter of the tail and of the hole in the carrier. They should be within 0.006" of each other. Fix this by selecting parts. The test I have used - Clean the muzzle/flash suppressor; Chamber a fired case; Blow into the muzzle and note the leaks. AR's should leak a little all over, but no big leaks anywhere. |
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