AR Sponsor
Posted: 4/3/2004 2:29:09 PM EDT
|
I have a PWA lower that usually fails to lock back the bolt on an empty mag. -USGI mags in excellent condition. -Ammo is Georgia Arms factory reloads that function perfectly in my other ARs. -I can pull the charging lever back manually and it travels back smoothly and locks with no problem, but it fails to lock on the last round 9 times out of 10. -I'm 99.9% sure that it's not a problem with the upper because this lower behaves exactly the same with 3 different uppers (all of which function perfectly on other lowers). -I don't see any scratching inside the buffer tube. Trying to solve this problem, I replaced the old-style "CAR" telestock with a new-style 6 position telestock from Bushmaster (complete with new buffer and spring). Still no binding or scratching that I can see, no change in the symptoms. -The lower has a new Armalite parts kit. When I first installed the lower parts kit, the rifle was short-stroking so badly that it would sometimes fail to pick up a new round. It turned out that the hammer "tail" was peening the disconnector "tail". I took maybe .050 off of the hammer tail, and it started picking up new rounds, but failed to lock back on the last round. I then knocked off another .050 or so, but it didn't seem to help. I was curious as to whether the hammer tail was still hitting the disconnector tail, so I placed a small sticker on the disconnector tail. After a few magazines, I can see that the hammer tail does make contact, but it's not hitting hard enough to cut the sticker, it did wear off the ink though. My questions: 1) What does the hammer "tail" do (other than add mass to the hammer)? 2) Is the hammer tail supposed to strike the disconnector tail? 3) Any ideas as to what else could be causing this lower to short stroke? |
|
Short stroke check is outlined in the Troubleshooting Checklist. Skip to the part where you fire while pushing in on the bolt catch. If the bolt still doesn't lock back then the problem is most likely in the upper. The "tail" is where the upper hammer hook sits on the full auto version. Most ARs have hammers that strike the top of the disco. |
|
Short stroke check is outlined in the Troubleshooting Checklist. Skip to the part where you fire while pushing in on the bolt catch. I read that (as well as all the FAQs and short stroke threads I could find). I did that test and it does not lock open with me pressing on the bottom of the bolt catch (like I would do to lock the bolt back with no magazine). If the bolt still doesn't lock back then the problem is most likely in the upper. All three of my uppers short stroke [i]only[/i] on this one lower. All three work flawlessly on my other 2 lowers. :( The "tail" is where the upper hammer hook sits on the full auto version. Cool, thanks for the info. That sounds like removing a little too much from the tail of the hammer isn't likely to cause other problems. |
|
j3_ is right on. My pre-ban worked flawlessly up to round 2000 when the bolt catch quit. I'm not sure what I'll do to fix it but here's the problem as best as I can tell: under recoil the follower (green, USGI) moves foreward in the mag. The mag (purchased NIW, USGI, both Wolff +10% and USGI springs) moves foreward in the mag well, and the bolt catch barely misses the follower. I should note that I can insert empty mags all day long and with rare exception they engage the catch just fine. The sad thing is that even my post-ban catch nearly misses the follower. It's impossible to measure but it looks like engagement is no more than a few thousandths when I push everything foreward to simulate an extreme. I have a DPMS VLD mag catch (LR-11-LRP) on order even though they said it won't work. I'll post the results when I get it. |
|
Are you using the same carrier with each of the uppers or does each upper have a dedicated carrier? Each upper has it's own bolt and carrier, that's why I find it hard to beleive that it's a gas system problem... I'm starting to fear that it's something fundamentally wrong with the lower; something terrible like the buffer tube threads being off center (although I don't see evidence of binding). |
|
Griz, Lets just start with the lower, First off, make sure that the roll pin in the buffer is flush with the sides of the buffer surface. If needed, use a file and flush the pin’s ends. On the carbine buffer spring, measure it and confirm that it’s free length is around 10.5 “(carbine length spring for the shorter carbine buffer/tube). If you come up with 11.75”, then you have standard length spring, and should be used with a full-length/standard buffer only. When you reinsert the buffer/spring, make sure that they are lightly lubed with CLP, including the inside of the receiver extension. Next is the opening of the receiver extension. Make sure that the taper entry section is in fact tapered, and is not catching the carrier as it starts to move back. If needed, take any high spots down to the correct tapering, and re-color the tube with Birch-Casey aluminum black. Then lets move onto the buffer retaining pin. Make sure that the pin tip is not grinding down the bottom of the carrier as it moves back. A worn tip is a sure sign that it making contact, and the tip will need to be shortened. Moving on from there, the bolt catch should move freely in the receiver. If it is tight/binding, then lube the catch and work it in/loosen it up by hand until it moves freely (top to bottom) without binding. If you find that the catch has difficulty moving to the up most top of the stroke, you will need to pull the catch and clean out the crap in the detent/spring hole. During machining, burs/scrap can be left in the hole, then sealed/bonded into the skin of the anodizing. If needed, clean the hole and reassemble the catch (drill bit spun by hand makes short work of removing imperfections/crap from the detent hole). ___ Now with any one of the uppers on the lower, draw the B/C as far back as it can go until the buffer bottom out on the back of the receiver extension. The front of the bolt should be just in front of the back of ejection port, yet behind the bolt catch around 3/8”. This room allows the needed time for the catch to rise as the buffer** is stalling the BC on the rearward stroke If you find that the front of the bolt is not coming back far enough for the stall and lock, you will need check the back of the receiver extension for foreign objects or need to relieve the buffer bumper to gain the needed room. If you do need to relieve the buffer bumper, do not remove more material that necessary or the bolt will be stopped behind the back of the ejection port, which changes the ejection path, and badly dings up the brass on ejection. Generally, I time/adjust the buffer bumper to stop the bolt ¼” in front of the back of ejection port. This allows the bumper room to compress, yet still maintain the bolt in front of the ejection port for a clean spent casing eject. **Regarding stall, make sure that the buffer it a tungsten steel stacked type, and not the cheap BB filled type. The BB type lacks any real dead blow effect, and the needed stall time to allow the mag to raise the catch to catch the bolt. Last item to cover is all of the bearing surfaces of the upper receiver should be lightly lubed in CLP. This will prevent upper binding, and add in breaking in the lower/uppers as they mate. Even is the uppers are worn in, keep them lubed until the receiver extension and spring have matted/set. Added information: On the buffer spring, use a file and clean the burs from the end of coils. This will prevent the spring end burs from scraping down the receiver extension and slowing the buffers travel. On the hammer tail. If you are using a Modified M-16 hammer (not L cut on the firing pin contact surface top) then removing portions of the tail may not be enough, you may have to roll file/mill the top edge of the firing pin contact surface. Granted that shorting the tail may keep it from contacting the tail of the disconnector, the hammer may still be binding up on the top of disconnector hook as it is being lowered. If needed, take a look at the difference between the two hammer types, and relieve the top edge (don’t need to make the L cut, just remove the metal where the L cut is made from point to point). [url] http://www.biggerhammer.net/ar15/hammers/[/url] |
AR Sponsor