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Posted: 11/27/2004 12:09:31 PM EDT
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I just had a rifle put together from various parts: *barrel: Bushmaster 16" 1/9 hbar (came as an assembly with gas tube, etc...) The FSB came installed on the barrel, and is the standard A2 FSB *upper: RRA upper from Eaglefirearms.net *bolt carrier/bolt/charging handle: RRA "enhanced" from Eaglefirearms *lower: Stag Arms, came complete from Eagle with M4 stock and all internals. The hammer is the kind without the notch on the face. Came with an "h" marked buffer. *ammo: PCA polymer cased ammo (which works fine in my other three rifles), and PMC .223 *magazines: 6 different USGI mags that are known to function perfectly I took it to the range today for the first time, chambered the first round, fired, and it failed to eject. I hand cycled it, it ejected the first round, chambered a second round, and fired the second round. This time, and almost all subsequent shots, the rifle would eject but fail to feed a new round into the chamber. Basically as it is now, it's a straight-pull bolt action rifle. When firing the PCA ammo, it was easy to pull the charging handle to eject and chamber a fresh round. When firing the PMC, about half of the time the bolt would bind up and it was very difficult to clear the spent brass. When the rifle is empty, the bolt carrier slides freely in the buffer tube when pulling the charging handle, but is a litter rougher than I'm used to. My first thought was that there was some kind of blockage in the gas system, maybe where the FSB covers the gas hole in the barrel. To check this, I plugged the chamber end with my pinky and blew through the muzzle end. (it was disassembled from the lower, of course) It seemed to pass air as freely as my other uppers. This was a long shot anyway because most of the cases would eject properly. The carrier key lines up perfectly with the end of the gas tube as far as I can tell. The next thing I checked was the gas ring alignment. They were staggered properly. Another thought was that the "enhanced" RRA bolt carrier, combined with the hammer (without a notch on the face), was somehow preventing the bolt carrier from going back far enough. I don't think this is the case, because I can hand-cycle just fine. This should also rule out any possibility of the mag catch being in the wrong place or bad magazines. Maybe it's possible that the "h" buffer in combination with the heavier RRA bolt carrier is causing the carrier to short-stroke. There's no way for me to tell until I get back to the range and use parts from other rifles to rule this out. The only other thing I can think of is that the buffer tube is slightly out of alignment. Like I said above, it seems to hand-cycle freely when empty, with a very small amount of binding. I can rule this out by slapping the upper on another lower and test firing. Another thing to mention that might make a difference is that the buffer tube is screwed in 1 full turn farther than is necessary to retain the buffer stop pin. The buffer stop pin does move freely, though. If I back the buffer tube out 1 turn, there's just barely enough of a lip on the buffer tube to retain the pin, which is why I turned it in as far as I did. I don't know how far in the buffer tube was screwed in when it came from the factory, and I removed it only to install a burnsed loop for a sling. Can anybody else think of another thing to check in order to get to the bottom of this problem? I'll be going back to the range tomorrow or Sunday with Q3131A and various parts from my other three rifles to see what's what. |
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By the numbers, Lock the bolt back by hand (with the extension set the one turn forward). If the bolt catch locks the bolt back (on the face of the bolt), then leave the receiver extension alone. Next, make sure that the rifle is clean, and you have lubed all the upper bearing surfaces (including the buffer and extension) with a fresh coat of CLP. While you have the buffer out, check the roll pin that holds the bumper in place. If it protrudes past the buffer surface, then use a file and take the pin down to be flush wit the sides of the buffer. Also, with the file and recoil spring in hand, knock the end of coil burs off the spring to prevent them from scraping down the buffer/receiver extension. Then, check the top/back upper surface of the disconnector for signs that the hammer tail has been making contact with it (indent or white line just behind the hook). If you find such marks, take .040 off the tail of the hammer to allow the carrier to lower the hammer down and the hammer tail to not bind up on the back of the disconnector. Now to top it off, make sure that the carrier key screws are tight/staked (35 inch lbs), and you have indexed the barrel extension true with the upper receiver (bottom lug between the feed ramps is centered with the front take down lug). Last as you have figured out, is run either Q3131a or XM-197 ammo threw the rifle to confirm that the other ammo is not the problem in regards to breaking in the New/Tight rifle. Bottom line is that the rifle/upper is still tight/new, and will need to break in (parts mate together, and enough time to allow the parts to knock off the factory milling burs threw live fire). Because of this, it is advised to keep all the upper bearing surfaces lubed with a light coat CLP (the only lube you should be using in the rifle). Other areas on the rifle, such as the bolt to barrel extension lugs can be hand polished/lapped, but live fire will do the same thing. Also, depending on how tight/rough the parts are, you may have to apply a few coats of CLP during the break in session to keep the rifle from jamming (just pull the carrier and give it another shot of CLP). Once you get to around the 400 round mark, you can back off on the CLP, but still keep a thin coat of CLP on the parts to prevent rusting. P.S. Don't rub in that you get to go shoot this weekend. Was planning on making it out today, but is way too cold to play here in CO (been in SF/Vegas, just flew back for turkey day and have not re-acclimated to the cold yet). As for Sunday, I get to spend the day in flight to Hawaii (8 hour on a plane, bored out of my mind since my laptop battery will die before I even hit the coast). I'll be there for three weeks, and don't get to take any of the toys. If anyone is a local (Oahu), IM me and find it in your heart to take me out shooting while I'm there. |
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I checked the carrier key, and it does wobble slightly side to side and up and down. It's a brand new carrier (RRA enhanced). The key is staked, and I'm not sure if I'm gonna be able to tighten it. Should I send it back to Eagle, or is it simple enough to tighten it myself and use red locktite? I'm not even sure if this would cause the problem, but it's a start. |
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A word of caution; Make sure that you have the torque wrench set at 35 inch lbs, and stop when it clicks. The problem may be that the screws were over tightened/over staked from the start, and on there way out (over stretched, and are about to snap). The screws are tempered, but when over tightened, they tend to stress hairline crack. Add heat and the questionable screws easily tend to stretch, and may be the case with the current screws/key being loose. If you have the time to wait, the better option would be to get two new screws for the re-install. This would save you from having to dig out the old screws if they do go south/snap. Also, give Chris a call and let him know that you need two replacement screws for the carrier that you bought from him. Chances are he can get them out Monday, and have you back in action before next weekend (or at least allow you to replace the old ones after shooting this Sunday). Note: The olds screws may hold for a few shootings, but it's best to get new ones installed ASAP. Also, I'm a big fan on Blue loctite on the key/carrier, but not to hold the screws. I use it in between the key and carrier surfaces to form a thin gasket between the two (it saves me time not having to lap the two together if they/the mating surfaces are suspect from the start). |
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I agree, new screws! And ditto the locktite on mating surfaces. Also, this from Armalite: Technical Note 47: Increased Carrier Key Torque For years, veteran ArmaLite® assemblers have tightened carrier keys using a simple Allen wrench. This process has been successful. As we added employees we decided to use calibrated torque wrenches to assure that we secure the key screws to the proper level. The government standard for carrier key torque is 35 to 40 inch pounds, and we secured torque wrenches set to that value. We then noticed an INCREASE in loose carrier keys. It turns out that we had tightened carrier key screws better without the wrench than with it. The problem is that the government torque values shown in maintenance manuals are too low. We have examined the engineering data related to the screw itself, and have increased torque to 55 inch pounds to improve carrier key tightness. We recommend that all AR owners make sure that their carrier key screws are secured to this level, especially if their rifles sometimes shows signs of weak or slow cycling. If your AR “short strokes,” clean the underside of the carrier key and tighten both screws to 55 to 60 inch pounds. Tighten the front screw first. If your rifle isn’t short stroking, just make sure that the screws are tight and restake if needed. ---- The Armalite Tech Notes (in the Library on their site, www.armalite.com) are an invaluable tool. I will add, how to "stake" the screws after tightening: Have someone hold the carrier horizontally, and on the side with the key supported on a hard surface, such as the flat "anvil" portion of a bench vise. Then with an ordinary nail set or a steel punch, held nearly vertically, the end on the side of the key on the edge right next to the screw head, tap the punch or nail set with a hammer. This will push metal over onto the screw head. |
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Thanks to all who took the time to reply! I took the rifle to the range today, and used the bolt/carrier from one of my Bushmasters. It fed and functioned perfectly with the Bushy carrier. Just for shits and giggles I put the RRA carrier in my Bushmaster to see what would happen, and for some reason it functions perfectly in the Bushmaster. I then put the RRA carrier back in the malfunctioning rifle, and it short-stroked. I also noticed that there was substantially less recoil than with the good bold carrier. I will order a new key and two new screws this week and replace the whole key assembly. Thanks again for the help. |
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