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AR help - please :)

jpm

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So this is now an sbr on an a2 stock with rifle buffer? I'm assuming this is now a carbine or mid length gas system now. You need a
carbine buffer and spring

I don't think that's going to cause the problem he describes which seems to happen when things heat up and abate when things cool down.
 

enbloc

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Ah. I thought you gave him the lower that he was building the upper for... which you should have.
 

Sweeney

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Pull the upper off and see if the bolt/carrier binds. If it does pull the bolt out of the carrier and test only the carrier. If the carrier moves freely test the engagement of the bolt in the barrel extension. If that passes test the fit of the bolt in the carrier. Also test if the charging handle binds in its track. If that all seems good it could be a head space problem.
 

enbloc

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Pull the upper off and see if the bolt/carrier binds. If it does pull the bolt out of the carrier and test only the carrier. If the carrier moves freely test the engagement of the bolt in the barrel extension. If that passes test the fit of the bolt in the carrier. Also test if the charging handle binds in its track. If that all seems good it could be a head space problem.
This could explain heat-binding of the bolt rotation in the Carrier, making it hard to pull BCG rearward to disengage the lugs.

Davey, have you done a full disassembly of the BCG? If not, you should. And pay attention to how hard it is to remove the bolt from the carrier and inspect the gas rings.

1605922038047.png
^ These are bad, by the way...
 
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daveyburt

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Ah. I thought you gave him the lower that he was building the upper for... which you should have.

I did. He had it all.

BUT - i think i found the culprit!

After spending some quality time with both BCG's after dinner, I noticed the original colt bolt has chamfers on three sides of the lug teeth (closest to carrier). The new bolt is chamfered only on the tops, and the sides (where the other chamfers should be) are very sharp.
MPI brand lug (bolt?) - no idea, haven't look them up yet.

Otherwise, after thorough cleaning/drying the only wear i see on the new BCG is very light scuffing on the sides of the base of the gas tube fitting - even on both sides. The old BCG is the same. -assuming there's wiggle room here.

As the seizure occurred real close to full lockup - much (almost exactly) like Picton describes, I'm thinking those hard edges just don't slide nice against the face of the lugs in the barrel.
I'd assume, if i could see the fronts of those barrel lugs, i'd see a little marring...if in fact this is the issue. But, I see no shavings or filings.

Seeing as enbloc brought up power tools ;), I'm not sure I see any harm in stroking a super fine flat diamond hone on those sharp edges to ease them out?

It also seems as though the channel that supports the cam pin could use a little breaking in compared to the old one but, my finger still moves the lug in easily.

Thoughts?

ETA: made some edits as I pick up the terminologies.
 
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daveyburt

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Pull the upper off and see if the bolt/carrier binds. If it does pull the bolt out of the carrier and test only the carrier. If the carrier moves freely test the engagement of the bolt in the barrel extension. If that passes test the fit of the bolt in the carrier. Also test if the charging handle binds in its track. If that all seems good it could be a head space problem.

-just seeing this - thank you!
....though I'm not sure of your terminologies. (will investigate)
:)
...which is why I had it built for me.
lol

thx!
 

sschevy

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I’m betting $5 buck it’s the offset pin

if the pin is too tight And slightly off center it will cause misalignment in the upper and lower causing the BCG to run into interference with the receiver extension

that said if the cam is loose it will move around and do the same thing

if you take a screwdriver and turn the cam pin back and forth with the rear takedown pin pulled out you will physically see what I’m talking about , I’ve had a lot of these come through the shop lately
 

enbloc

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I did. He had it all.

BUT - i think i found the culprit!

After spending some quality time with both BCG's after dinner, I noticed the original colt bolt has chamfers on three sides of the lug teeth (closest to carrier). The new bolt is chamfered only on the tops, and the sides (where the other chamfers should be) are very sharp.
MPI brand lug (bolt?) - no idea, haven't look them up yet.

Otherwise, after thorough cleaning/drying the only wear i see on the new BCG is very light scuffing on the sides of the base of the gas tube fitting - even on both sides. The old BCG is the same. -assuming there's wiggle room here.

As the seizure occurred real close to full lockup - much (almost exactly) like Picton describes, I'm thinking those hard edges just don't slide nice against the face of the lugs in the barrel.
I'd assume, if i could see the fronts of those barrel lugs, i'd see a little marring...if in fact this is the issue. But, I see no shavings or filings.

Seeing as enbloc brought up power tools ;), I'm not sure I see any harm in stroking a super fine flat diamond hone on those sharp edges to ease them out?

It also seems as though the channel that supports the cam pin could use a little breaking in compared to the old one but, my finger still moves the lug in easily.

Thoughts?

ETA: made some edits as I pick up the terminologies.
Keep in mind...
That if the build came with a new BCG, and you intend to have your Guy repair it, any file-work may not allow him to return it if it is indeed bad, for warranty.

I'd take the new build back to the range with the old BCG and do a full function test...
~Enbloc
 

daveyburt

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I’m betting $5 buck it’s the offset pin

if the pin is too tight And slightly off center it will cause misalignment in the upper and lower causing the BCG to run into interference with the receiver extension

that said if the cam is loose it will move around and do the same thing

if you take a screwdriver and turn the cam pin back and forth with the rear takedown pin pulled out you will physically see what I’m talking about , I’ve had a lot of these come through the shop lately

Ya know...when I close the upper against the lower, there's a little tension just as it's closing. I questioned that and though it odd. I had one of those accuwedges in my old upper - which had a similar effect when closing but, I knew that was the wedge.

I'm glad you mentioned that!
It was so minor, and similar to how it was with the wedge, I didn't put it high on the 'culprit' list.

thank you!!
 

sschevy

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Ya know...when I close the upper against the lower, there's a little tension just as it's closing. I questioned that and though it odd. I had one of those accuwedges in my old upper - which had a similar effect when closing but, I knew that was the wedge.

I'm glad you mentioned that!
It was so minor, and similar to how it was with the wedge, I didn't put it high on the 'culprit' list.

thank you!!
As you close it up and get to that tension point use your screwdriver and find the sweet spot where it smooths out , at that point lock the screw against itself and give it a go

you will definitely feel it loosen up as you close it when you rock the screw slightly back and forth
 

daveyburt

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Keep in mind...
That if the build came with a new BCG, and you intend to have your Guy repair it, any file-work may not allow him to return it if it is indeed bad, for warranty.

I'd take the new build back to the range with the old BCG and to a full function test...
~Enbloc

Yes good point - i was planning to wait till i chatted with him but, I did locate my hones.
;)

I'm hoping to get back to the club withing the next couple days with the old BCG in the new upper.
But, based on the cam pin thought, I may take the old upper and new BCG.

dunno...but, hope the Smith calls me soon to square it up.
 

daveyburt

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As you close it up and get to that tension point use your screwdriver and find the sweet spot where it smooths out , at that point lock the screw against itself and give it a go

you will definitely feel it loosen up as you close it when you rock the screw slightly back and forth

:) thank you!
this would be the simplest - fingers crossed.
 

enbloc

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Check your BCG protrusion on rear of Up-Rec with bolt closed and locked.
should not be excessive and you can check it against known good assembled AR.
1605923078344.png
may also indicate poor lug lockup.
 
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I've seen some butt-stocks interfering with charging handle rearward movement. Any signs of light scratches on the area where the CH rides back over the butt-stock?
The new upper may be sitting a little low for your butt-stock configuration. Are you running an A2-style stock or a collapsible style stock?

Photo of configuration would help a lot...

Here is a gross representation of what I mean.

View attachment 413483

That would cause the opposite of what the OP described. He said it sticks initially then goes like a bat out of hell once it moves. In your example the bolt would move easily at first then get tougher to move later in the stroke as the handle started riding up the stock.

OP. How well does the upper and lower mate up? Is it easy to slide the rear tear-down pin or do you have to fiddle with it a bit to get it through? If the rear pin is really tight then the carrier may be hanging up on the top of the buffer tube. Once you pull hard enough to get it to align, it flies back. The pressure from a round cycling would do it in an instant even if it's barely doable by hand. I've seen things like an acuwedge or over adjusted "slop screw" in the lower screwing up the alignment. If that's your problem you would probably see some signs of rubbing on the top of the buffer tube and top of the carrier. Though, with so few rounds fired, it may not be showing any marks yet.
 
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jpm

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I’m going to respectfully disagree That it’s normal,

With the extremely thin ears of the Colt large pin lower any added tension would be a problem in my eyes and would be immediately rectified to operate without resistance

That's interesting, I would have thought you'd want it kinda tight for the sake of accuracy.
 

enbloc

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I’m going to respectfully disagree That it’s normal,

With the extremely thin ears of the Colt large pin lower any added tension would be a problem in my eyes and would be immediately rectified to operate without resistance
Could be the case, but I was feeling it sounded more like the BCG pushing back on the buffer/spring assy and "tightening" at full close-up, and not actually binding.

Diaging through the internet can be... unwieldy. [rofl2]
 

enbloc

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That would cause the opposite of what the OP described. He said it sticks initially the goes like a bat out of hell once it moves. In your example the bolt would move easily at first then get tougher to move later in the stroke as the handle started riding up the stock.
Yeah after rereading the original post, I concur.
 

daveyburt

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cam pin was/is loose as a goose.

I tweaked as sschevy described. Closed much more nicely. I left it with a slight bit of tension, 1/16" just before closing for a 'little' snugness.
Every time I open and close the upper, the pin moves and changes the adjustment.
Even with the lock nut pegged, that pin wiggles like a pollywog.

This concerns me. Did he f-up my lower?

This explains why I couldn't even open the upper when trying to clear it at the range.

:(
 

sschevy

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cam pin was/is loose as a goose.

I tweaked as sschevy described. Closed much more nicely. I left it with a slight bit of tension, 1/16" just before closing for a 'little' snugness.
Every time I open and close the upper, the pin moves and changes the adjustment.
Even with the lock nut pegged, that pin wiggles like a pollywog.

This concerns me. Did he f-up my lower?

This explains why I couldn't even open the upper when trying to clear it at the range.

:(

I doubt you goofed the upper or lower

the cam pins have been getting difficult to source the past few years and you may just have a crappy pin

a good quality can pin will cost around $30 , a lot of money for such a small item but a well machined pin will be pricy , you may want to try another pin

not sure where you are but I have a dozen in stock at my store and if you’re willing to make the trip you can go through them all and see if you get one that fits just right
 

daveyburt

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I doubt you goofed the upper or lower

the cam pins have been getting difficult to source the past few years and you may just have a crappy pin

a good quality can pin will cost around $30 , a lot of money for such a small item but a well machined pin will be pricy , you may want to try another pin

not sure where you are but I have a dozen in stock at my store and if you’re willing to make the trip you can go through them all and see if you get one that fits just right

:) good on f-ing things up.
lol

He was having a hard time getting parts (understandable). After 5 months of no movement I said, if things aren't coming in soon, I'll just take my stuff back. - don't sweat it.
Then, the rest of the parts came in - except - the pin. It sounds like he just grabbed whatever he could

Where's your shop SS?
 

sschevy

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:) good on f-ing things up.
lol

He was having a hard time getting parts (understandable). After 5 months of no movement I said, if things aren't coming in soon, I'll just take my stuff back. - don't sweat it.
Then, the rest of the parts came in - except - the pin. It sounds like he just grabbed whatever he could

Where's your shop SS?

Peabody right on Rt 1

northeast arms
 

daveyburt

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you there tomorrow?
hours?
:)

I'm in the middle of a house overhaul and just want to know this thing is functional so I can get back to work....or at least solve one of the issues. Wiggle-matic.
hahaha
 
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