View Full Version : Bolt not locking to the rear when mag runs dry
My weapon has had this issue the past couple of times I have gone to the range. Sometimes the bolt locks back on a dry weapon, and sometimes it doesn't. I lube up the BCG and and the buffer assembly generously and then it solves the problem until about 3 or 4 mags later, then it starts happening again. I'm thinking maybe it has something to do with the low profile gas block that I installed to replace the front sight gas block when I got a longer free-float rail, as the weapon didn't have this problem prior to the change. I suppose it might also have something to do with the buffer, as I changed those out some time ago too. But it's an H buffer and I have always been under the impression that that's the best buffer for a 16 inch barrel with a carbine length gas system. Does anyone have any suggestions about what might be causing this malfunction. Any ideas would be very much appreciated, as I know that there are folks on here that know infinitely more than I do about trouble shooting the AR platform. Thanks in advance.
Belmont31R
03-07-10, 19:50
Probably is the gas or lack thereof.
Who did the gas block install? Check to see if its aligned properly.
H buffer should not be an issue with a carbine.
Check the gas tube to make sure its installed properly.
Try locking the bolt to the rear several times with your empty mags. If the mag spring is weak it should do it manually too. I had a mag do this to me last range trip. It lost enough strength it wouldn't push the bolt catch up anymore. I could repeat it cycling the bolt manually.
But anytime you change something, and then encounter problems that did not exist before 99% of the time its something you changes. Either a out of spec part or a bad install. Start there...
Buffer selection is not a simple match up to barrel length and gas system. It depends on the gas port size too.
Buffer selection may or may not be part of the issue.
lethal dose
03-07-10, 19:57
Could be a bent bolt catch roll pin. When I assembled my dd lower, I bent mine slightly, resulting in spotty reliability. Like belmont said, try replicating it manually. Do this with the upper off and an empty mag seated and watch everything.
I was having same problem with my MP-15 with the H buffer. put the carbine back in and had no problems
check for alignment, as belmont said, on the gas block- also check for carbon leakage at the gas block and gas tube. if you're leaking bad enough to short-stroke, you'll probably have a pretty good size deposit of black shit on your barrel and/or on the tube.
have you checked your gas key? make sure your screws are tight and staked
Thomas M-4
03-07-10, 20:02
What magazines are you using??
check for alignment, as belmont said, on the gas block- also check for carbon leakage at the gas block and gas tube. if you're leaking bad enough to short-stroke, you'll probably have a pretty good size deposit of black shit on your barrel and/or on the tube.
have you checked your gas key? make sure your screws are tight and staked
I'll check it out ASAP. I'll also check to see where my rounds are ejecting. Thanks guys.
What magazines are you using??
All PMAGs
Thomas M-4
03-07-10, 20:23
All PMAGs
Roger that you should be G2G in that department.
Where is your ejection?
The rounds eject to my 4 oclock.
Probably is the gas or lack thereof.
Who did the gas block install? Check to see if its aligned properly.
H buffer should not be an issue with a carbine.
Check the gas tube to make sure its installed properly.
Try locking the bolt to the rear several times with your empty mags. If the mag spring is weak it should do it manually too. I had a mag do this to me last range trip. It lost enough strength it wouldn't push the bolt catch up anymore. I could repeat it cycling the bolt manually.
But anytime you change something, and then encounter problems that did not exist before 99% of the time its something you changes. Either a out of spec part or a bad install. Start there...
I really do get the feeling that the problem is it's not transfering as much gas as it should be. I'm getting ready to check the gas tube right now.
cgcorea
I'm thinking maybe it has something to do with the low profile gas block that I installed to replace the front sight gas block when I got a longer free-float rail, as the weapon didn't have this problem prior to the change.
Theirs your problem... right there.... That's why I never mess with my Gas blocks and only use ones that are pinned...
check for alignment, as belmont said, on the gas block- also check for carbon leakage at the gas block and gas tube. if you're leaking bad enough to short-stroke, you'll probably have a pretty good size deposit of black shit on your barrel and/or on the tube.
have you checked your gas key? make sure your screws are tight and staked
Is this the type of carbon build up you are talking about? There's some pretty good uild up here where the tube meets the gas block as well as a little bit of build up near where the tube enters the upper. I'm trying to take a pic of the latter but my flash isn't cooperating.
http://i3.photobucket.com/albums/y78/regulator1775/DSCN1159.jpg
Belmont31R
03-07-10, 21:13
That barrel profile is setup to have a hand guard cap which puts some space between the "step" of the barrel and the gas block. Just butting the gas block up against the step 'can' put the gas block too far back.
Some gas leakage like in the pic isn't bad. The carbon will actually build up and help seal things off. Doesn't look bad to me. Its not blown gas all over the place which would be a major leak.
Most gas blocks position the rear hole for the bolts so when you remove the rear bolt the gas hole is directly above it. So what you do (if this is the case) is install the gas block upside down, and you center the threaded rear hole over the gas port in the barrel. Take a measurement if you can of how far off the step the gas block is. Then turn it 180 degrees, and make sure the gas block is still the same distance away from the step. If you look on the bottom of your original FSB you will see a little hole where the sling loop would normally go, and that is the same thing I am talking about. Its there because when they made the gas block they had to machine up through there for the hole the gas goes up into the gas tube through. Also works as I described to make sure that hole will be centered above the gas port in the barrel....;)
The rounds eject to my 4 oclock.
There goes my idea. Good luck!
Ok. upon further inspection of the gas system I found that the gas block was installed incorrectly and the roll pin that secured the tube into the gas block damaged the gas tube. That's the last time I trust "ANYONE" with "ANY" kind of work done to my weapon unless it's someone with an established reputation for good work, and knowing his shit. Otherwise I'm just gonna do it myself which is what I usually do but in this particular case I didn't have the tools with me. So I took it to a gun shop here in NOVA (won't name names as to not **** with anyone's koolaid.) to have them do it. Big mistake on my part. I was actually wanting to take it to Rob over at VA Arms co( I've read nothing but good things about him on here which tells me he's probably a kick-ass armorer) but they said he was out of town on business. Anyways, I ordered a whole new gas block and gas tube and got it 2 days ago and installed everything myself. Today I went to the range and put her to work and am happy to report that everything is running 100 percent. Thanks for all the help and suggestions guys. I really appreciate it.
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