M4-ish build

OK. AR15.com suggests carefully inspecting the finger/leg of your bolt catch, to ensure that it is long enough to be reliably raised into the locking position by the mag follower.

I would also consider removing the bolt catch and checking it for burrs or wear--and stoning/relieving any possible high spots which might make it drag against the lower inletting under recoil.

Given the tiny amount of time the blot catch has to perform its function, any drag at all could be just enough to inhibit function.

I would also consider making sure that the roll pin fits loosely in the bolt catch hole, so that the bolt catch is perfectly free to pivot so it is not having to overcome any resistance as the mag follower lifts it into position.

Hope you get it sussed out.
 
OK. AR15.com suggests carefully inspecting the finger/leg of your bolt catch, to ensure that it is long enough to be reliably raised into the locking position by the mag follower.

I would also consider removing the bolt catch and checking it for burrs or wear--and stoning/relieving any possible high spots which might make it drag against the lower inletting under recoil.

Given the tiny amount of time the blot catch has to perform its function, any drag at all could be just enough to inhibit function.

I would also consider making sure that the roll pin fits loosely in the bolt catch hole, so that the bolt catch is perfectly free to pivot so it is not having to overcome any resistance as the mag follower lifts it into position.

Hope you get it sussed out.
I hope to, I'm sure I will 🤞

So its funny you mentioned the bolt catch, the first one I had in the gun, the leg was a tad shorter than ones in my other guns so I replaced it. If you build enough of these your bound to get a troublemaker eventually.
 
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Episode 3......the saga continues!

Headed back to the range today just to troubleshoot this rifle further, besides extra parts I also brought another rifle of similar build. We'll call the rifle having the trouble rifle "A" and the other rifle "B" Ammo used today was Federal Lake City 62gr green tip.

Rifle B is a 16" 300 blk, both guns have Anderson lowers and the same stocks, Magpul fixed carbine. both have carbine buffers but rifle B has a spring that has blue paint on it, I remember buying it but just don't remember from who or what was special about it. Edit, spring is a Sprinco enhanced (blue) buffer spring..

After a lot of internet surfing the first thing I tried is firing rifle A with one round and my finger putting pressure on the bolt catch, did this several times and each time the bolt locked open.

Next thing I did was take the lower from rifle B and put it on A's upper, ran multiple rounds and except for once, the bolt locked back each time.

Next thing I tried was taking the buffer spring and buffer from rifle B and putting in in Rifle A's lower, no-go still will not lock back.

I put everything back the way it was originally, I had several extra springs with Me and tried each of them in rifle A with no change.

As I'm writing this I just realized I should have tried rifle A's lower on rifle B and see if I get the same malfunction....DAMN IT!!!!!

Rifle A is still ejecting at around 1-2 o'clock but the gas port measured within spec so overgassing is probably not the issue although that ejection pattern suggests it is?????

Finally is an act of frustration I lubed the ever loving shit out of the BCG, I mean dripping oil wet. after doing this It actually locked back a few times but not consistently.
Again tried different buffers but anything heavier than a "H" the gun starts getting sluggish.

So after all this it's pointing to the lower, I have already swapped out the bolt catch and buffer tube, spring and buffer......What the f*** am I missing here!!!
 
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What the f*** am I missing here!!!
amount of friction from the brand new bolt to the parts it is in contact with - top of the trigger, lower, dunno.
you should look at it all to see what is rubbing.

as with lower B you have larger tolerances, so it worked. applying pressure to lower A you also increased tolerances, so it worked. if you would have an aero m4e1 lower - it has a bolt there under the grip you could adjust to raise upper a bit to make it work. but you did not buy an aero?

what to do - really, to lube it, and shoot more, a 50-100 shots if tolerance is minimal - it should just make it work. if not - you need to find the culprit, by rubbing marks.

also, no one shoots a brand new AR if it is not a dripping wet with lube.
 
amount of friction from the brand new bolt to the parts it is in contact with - top of the trigger, lower, dunno.
you should look at it all to see what is rubbing.

as with lower B you have larger tolerances, so it worked. applying pressure to lower A you also increased tolerances, so it worked. if you would have an aero m4e1 lower - it has a bolt there under the grip you could adjust to raise upper a bit to make it work. but you did not buy an aero?

what to do - really, to lube it, and shoot more, a 50-100 shots if tolerance is minimal - it should just make it work. if not - you need to find the culprit, by rubbing marks.

also, no one shoots a brand new AR if it is not a dripping wet with lube.
After today this gun probably has 300-400 rounds on it, enough to break it in. No it's not an Aero lower, a lot of people don't agree with what I'm about to say but a lower is a lower. I have used some Aero's and they're nice, the take up screw is kind of a novelty if you ask me but thats just my opinion. In regards to the dripping wet with oil , that hasn't been my experience until now, in over 2 dozen builds I've never had to soak a BCG like I did today.
 
i guess it is a true luck if in 2 dozen builds you never had to deal with any parts to be out of spec.
try to swap out a trigger or a bolt - see if that will help.
True, not to say that there haven't been issues with other builds but nothing like this annoying problem, I have a lot of parts squirreled away so if I have an issue I usually have the parts on hand to swap out. I swapped out the BCG last time I took it to the range, maybe the time before that....but it didn't work either.
 
You put finger pressure on the bolt catch and the rifle worked, so the issue is related to bolt catch travel, how far it travels and how much upward pressure is being exerted on the bolt catch by the spring.

Maybe check that the bolt catch recess isn't drilled too deep, maybe polish the bolt catch and corresponding receiver recess, and make sure that that the bolt catch pivots easily on the roll pin.
 
You put finger pressure on the bolt catch and the rifle worked, so the issue is related to bolt catch travel, how far it travels and how much upward pressure is being exerted on the bolt catch by the spring.
But it locks back when you manually retract the BCG with an empty mag in place, this tells me its doing its job no?
Maybe check that the bolt catch recess isn't drilled too deep, maybe polish the bolt catch and corresponding receiver recess, and make sure that that the bolt catch pivots easily on the roll pin. The catch pivots easily on the roll pin, I'll have to check the recess and compare it to the other rifle. This is why this is driving me nuts, its exhibiting signs of overgassing and short stroking at the same time!! [crying]
 
i think it is a fraction of a mm diff you not gonna be able to measure. i would setup it on a lower it likes best, really, and try to see what is it that gets contact.
most usual thing affected by a wrong height is the belly of the bolt on the top of the trigger. but it should have left some visible marks.

you can use some black permanent marker on all those mating surfaces and check after some shots of where it rubs, as it has to, somewhere.
 
So messing with my Iphone I realized the camera has a slow motion setting. I'm gonna set the rifle up in my rest and the phone on a tripod and try to get a good closeup of the BCG cylcing on slow motion, maybe that will reveal something?
 
Tried the slow motion thing and while it helped the frame rate on the Iphone just isn't fast enough to slow it down to get good picture but from what I could see it looked the the bcg was not going back far enough, I did get one lock back in about a dozen attempts.

Gonna try one more thing before I toss this in the river (just kidding Andrea). I'm going to try a reduced power buffer spring and if that don't work I'm done!
 
Well friends, I'm beaten! as they say on "The 100" My fight is over! Today I put the reduced power recoil spring in, same shit. Brought the gun back home threw another bolt catch in (third one now, and spring is correct), back to the range, same shit.

When I got back home from the second range trip I had a shot of Jameson, relaxed a bit and went in for a deeper look around, I had the calipers and micrometer out comparing this lower to other ones, nothing really out of whack and then I spot something. I noticed the buffer was protruding just a bit from the tube, on the other lower the buffer was pretty much flush with the tube, and then I see it, the hole for the buffer retainer pin is drilled just a bit too far forward, I confirmed with two other lowers. On the good lowers there was about a thread and a half of space between the hole and the internal cut out of the receiver, on the troubled receiver that space was barely half a thread so this is allowing the buffer to stick out farther and tilt downwards a bit more than on the other lowers.

Now I'm still trying to figure how this could cause the bolt to not stay open but it has me convinced now that the lower is f***ed because if this hole is not correct, what else is wrong, maybe the lower wasn't indexed properly in a jig? IDK as I really don't know how they machine these things. So, I have one more virgin Aero M4E1 receiver, I'm gonna swap everything over to that receiver and see what happens.
 
Well....swapped everything over to the Aero lower and took it to the range yesterday...Same.......f***ing........thing! No lock back on the last round. I'm just gonna put this away for a while in a dark corner of the safe and move on to something else because I literally don't know what else to do.
 
OK. There is no way you have 2 receivers that are out of spec.

Rifle B cycled and locked, and the issue seems to have something to do with bolt travel.

Time to take a look at the buffer tube itself.

Look down inside with a bright flashlight. Is there a screw from the butt-pad of the stock protruding into the bottom of the buffer tube? That might limit bolt travel.

If not--maybe the buffer tube itself is out of spec.

Measure the inside dimension/depth of the buffer tube from the bottom wall to the receiver extension ring.

Compare that dimension with the rifle (B) that cycled and locked.

If it is shorter, swap out the buffer tube.
 
OK. There is no way you have 2 receivers that are out of spec.

Rifle B cycled and locked, and the issue seems to have something to do with bolt travel.

Time to take a look at the buffer tube itself.

Look down inside with a bright flashlight. Is there a screw from the butt-pad of the stock protruding into the bottom of the buffer tube? That might limit bolt travel.

If not--maybe the buffer tube itself is out of spec.

Measure the inside dimension/depth of the buffer tube from the bottom wall to the receiver extension ring.

Compare that dimension with the rifle (B) that cycled and locked.

If it is shorter, swap out the buffer tub
Except it locks back manually no problem every time, I would think if that situation existed I wouldn't be able to do that, no?
 
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