View Full Version : New build. BCG won't retract fully. Gets stuck when about 2/3 retracted.
ClangClang
11-11-17, 23:11
I have a situation I am unable to diagnose on a new build. I have a new lower pistol build and I am attempting to attach an upper. The upper fails basic function testing. I can't pull the charging handle all the way back without forcing it. If I pull past the point of resistance then the BCG gets stuck and doesn't return forward. I have to use the forward assist to send the BCG back into battery. When I try the same upper on another lower, everything works fine. When I try a known good upper on the problem lower, I experience the same issue. So the issue is definitely with the new lower. Any ideas? I don't see any new wear patterns or drag marks on anything. Any ideas on where I can start troubleshooting? I'm kind of stumped.
Check to see if your receiver extension is too far in or too far out. The bolt carrier may be binding up against the buffer retaining pin.
I assume you have a standard pistol buffer tube and a standard carbine length spring and buffer.
First, give it a good overall check for straightness by holding the lower (with upper removed) at arms length and sight down the top of the buffer tube across the receiver. You should be able to notice any cast left or right by doing this.
With the upper still removed, and the buufer and spring in place, slowly push the rear of the BCG against the buffer, pushing the BCG into the extension. It should go in and out smoothly nearly to the carrier key without noticeable drag, Take into account the resistance/upward tilt as the bottom of the BCG goes over the hammer. Since the upper receiver is not holding the BCG in alignment with the receiver extension the BCG is somewhat free to move left or right. If you get any resistance here, it is mostly likely a defect in the tube.
Take the buffer and spring out, reattach the upper, reinsert the BCG, then run the action. It should allow you to cycle all the way to the rear. The BCG should fall forward when you hold the muzzle down, if it doesn't, give a sharp downward movement to snap it forward. We are looking to see it there is any rubbing/extra effort indicating the BCG is rubbing against the tube as you bring the action to the rear.
If that is the case, it indicates that the bore of the upper receiver and the bore of the receiver extension/buffer tube are not aligned.
From what you describe, I would guess that the receiver is slightly out of spec where they threaded for the receiver extension.
The hammer may not be fully cocking, causing drag on the bottom of the carrier.
More details would certainly help. Things like parts used, etc..
Does the boltlock function test ok?
Is the hammer function test ok?
What’s up with the buffer retention pin & spring?
Is there a burr or damage to the charging handle or groove it slides in?
Does a different BCG give the same results?
If you can't tell/see what is wrong then eliminate variables. Try removing the action spring/buffer. Try removing the hammer.
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So Clang Clang, what did you figure out?
ClangClang
11-16-17, 02:03
Hey guys, thanks so much for the suggestions and info. Apologies for not checking back in sooner. I'm a .gov contractor and have been getting crushed at work this week. It's 0300 and I'm just finishing up now. Gotta be back at 0800. I haven't even been into my safe since I first posted. I'll finally have some time to trouble shoot, post some photos, and respond to everyone over the weekend. Scouts honor. Thanks fellas!
ClangClang
12-04-17, 19:35
Welp, I finally found some time to get back into the safe and trouble shoot tonight. Life has been BUUUUSY.
So it looks like the BCG is definitely rubbing on the hammer, as Todd K alluded to. I'm pretty sure the hammer is cocking fully though. It is a KAC carrier on an 80% lower, with a Rise Armament RA-535 drop in trigger pack. I used some crayola water soluble marker to color on the bolt face like so:
https://i.imgur.com/3oPZEBSl.jpg
After cycling the action once, I reopened the rifle and this was the result:
https://i.imgur.com/RbPSQ7Ql.jpg
So it's definitely rubbing. The question is, how much rub is OK?
EDIT: The RA has two tension screws that protrude downward from the trigger housing and push down against the floor of the trigger pocket. I suspected I might have slightly overtightened them (which forces the entire trigger unit upwards from the lower), so after a quick loosening (1/2 turn of each screw), the BCG cycles much, much more cleanly. It's not quite as slick as a factory KAC or BCM, but it's pretty good.
Should I gently relieve the leading edge of the bottom of my carrier with some emery paper? Just to put a subtle round on that edge so that it slides over the hammer better? Not sure if that's a wise idea or not. I'm asking because I'm almost positive that doing any work to the hammer is definitely a bad idea.
It is a KAC carrier on an 80% lower....
Verify the hammer pin in in the correct position.... The center of the hammer pin should be .625 inch from the top shelf of the lower.
If it's not, get a new lower.
Find a print online and take caliper to check the dimmensions.
Does the bcg go all the way back and forward when the trigger is fully pressed and the disconnecter is holding the hammer back.
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militarymoron
12-04-17, 22:55
So it's definitely rubbing. The question is, how much rub is OK?
Should I gently relieve the leading edge of the bottom of my carrier with some emery paper? Just to put a subtle round on that edge so that it slides over the hammer better? Not sure if that's a wise idea or not. I'm asking because I'm almost positive that doing any work to the hammer is definitely a bad idea.
It's supposed to rub - that's how the hammer is cocked - by the carrier. But it's not supposed to restrict it once it's cocked. Do NOT alter the carrier; it's not the leading edge that's causing any issues. Honestly, I'd swap your drop-in trigger with a USGI/standard one to test out, since you already found out that it's the one causing the issue.
There should be no need to polish the carrier for function. The hammer does ride against the carrier, but it should not be felt much after the initial force to cock the hammer.
Your hammer is not able to move far enough past cocked, and / or the trigger pin holes are off.
My vote is for another out of spec 80% lower. That's just a SWAG. #MARRGA
My vote is for another out of spec 80% lower. That's just a SWAG. #MARRGA
He probably should have mentioned it is an 80% before post #10.
He probably should have mentioned it is an 80% before post #10.
My vote is for another out of spec 80% lower. That's just a SWAG. #MARRGAI wanted to say, thats what you get when you buy the cheapest stuff. ..followed by a you get what you pay for rant.
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Not the cheapest when you factor in your time...
There looks to be a lot of metal shavings in your picture. I would fully disassemble the lower and clean it out with acetone or alcohol and then as militarymoron mentioned put in a USGI trigger and see if you still have issues.
The OP has been nothing but courteous in his posts.
As far as 'the cheapest stuff,' I may be mistaken but I always thought KAC components were held in pretty high esteem most places. Likewise, the Rise RA-535 may not be my cup o' tea, but it generally has good reviews and isn't cheap at around 230ish.
Obviously the OP has a reason for building an 80% lower, maybe for the challenge of constructing his own, or maybe to be completely off the books, I don't know, but in a world of readily available stripped lowers priced 150.00 and below, I doubt that 'being cheap' was the priority.
The OP came here asking for ideas/help, not a hard time.
I don't think anyone has been rude. 80% lowers are what they are, only as good as the person doing the finishing. Out of spec trigger parts or lower are the obvious as previously mentioned. Matter of elimination at this point.
I wish people would spend more time reading some of the troubleshooting stuff here and applying some of the basics rather than worrying about the latest Uber-light unobtanium gas block or super duper whizbang.
I don't think anyone has been rude. 80% lowers are what they are, only as good as the person doing the finishing. Out of spec trigger parts or lower are the obvious as previously mentioned. Matter of elimination at this point.
But I built it with my Dremel.
Sorry. Couldn’t help myself.
ClangClang
12-06-17, 10:55
He probably should have mentioned it is an 80% before post #10.
If I posted that it was an 80%, it's virtually certain all the responses would have been useless "it's your lower that's out of spec dude" without any other information to help the troubleshooting process.
I wanted to say, thats what you get when you buy the cheapest stuff. ..followed by a you get what you pay for rant.
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Going 80% is VERY far from the cheapest option.
There looks to be a lot of metal shavings in your picture. I would fully disassemble the lower and clean it out with acetone or alcohol and then as militarymoron mentioned put in a USGI trigger and see if you still have issues.
Those aren't metal shavings, just slight wear to the anodization which shows up looking fuzzy in my low-rez phone camera. The lower is completely clean.
ClangClang
12-06-17, 11:01
It's also not my first rodeo when it comes to basic machine work. I am fairly confident that the lower was machined correctly and within spec, hence my original question. If you refer back to my "edit" on my last post, reducing the tension on the trigger set screws actually solved the issue about 95% of the way. I actually believe the trigger to be the culprit, but I don't have any spare other triggers lying around to test it. I've got some more Geissele triggers coming whenever the Black Friday orders finally show up (probably another couple weeks, according to the disclaimer at checkout) and will re-test the lower then.
I appreciate everyone's input thus far, even the non-helpful posts. I certainly knew to expect some of that when posting about an 80% lower. I wanted to try an 80% for the challenge of it. It's certainly not ever going to be pressed into use as a duty or SD/HD platform.
If the trigger did fix your issue 95% then ignore what i wrote regarding your build.
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I had something similar happen with an LMT upper that had an improperly timed barrel nut causing the gas tube to be canted. Easy to check for this by dropping the bcg into the upper. You should be able to remove it fairly easily.
I also had a "premium" PSA bcg where the bolt was stiff in the BC. After 30 round or so, it loosened up.
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