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Centerfire Rifles - Semiautomatic or Gas Operated Centerfire rifles, carbines and other gas operated rifles.

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  #1  
Old 02-25-2021, 2:40 AM
fishgoh0nk fishgoh0nk is offline
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Default Lapping BCM upper, thoughts?

Recently installed a BA 10.5 on a Bravo Company upper, 9inch Geissele MK14 with a Seekins adjustable gasblock. Gas port's aligned, everything's dandy until I mounted an aimpoint t2 and the POA didn't match the bore laser POI, it was actually quite off.

Upon further inspection, looks like the barrel skews a bit to the right, there's just barely enough clearance between the gasblock and the ID of the rail. I'd like to center this up.

Now, lots of people say "BCM uppers are true". Before taking everything off and doing an inspection before lapping, anyone have any insights they could share before I start here?

Thanks in advance - hopefully I can get some good discussion here before I head over to ARF
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  #2  
Old 02-25-2021, 5:12 AM
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Did you try adjusting the red dot?
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Old 02-25-2021, 6:06 AM
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I would first check the travel adj. On the red dot.
Make sure both adj. Are centered.

Install again and check. Those bore laser only get you close. They are not precise..
If your barrel still looks off. Lay.a.straight edge on top of the rail on the receiver. A one foot straight edge will be able to see if the barrel is off to one side.
If its off. Then I would suspect the upper receiver bore may not be straight.

Lapping the receiver won't help if the barrel points to one side.
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  #4  
Old 02-25-2021, 10:04 AM
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Quote:
Originally Posted by m16 View Post
Did you try adjusting the red dot?
yea, it was definitely was a lot more windage to adjust out of the box than most i've done.


Quote:
Originally Posted by baih777 View Post
I would first check the travel adj. On the red dot.
Make sure both adj. Are centered.

Install again and check. Those bore laser only get you close. They are not precise..
If your barrel still looks off. Lay.a.straight edge on top of the rail on the receiver. A one foot straight edge will be able to see if the barrel is off to one side.
If its off. Then I would suspect the upper receiver bore may not be straight.

Lapping the receiver won't help if the barrel points to one side.
Why won't lapping the receiver help with barrel square? if the receiver is not true, too much material on the left, I feel like it would cause the barrel to point to the right when torqued down.

It's not too far off, to the naked eye it looks alright, but if you were to press 3-5lbs on either side of the barrel, there's more give going to the way it's sided than the other (cocked to the right, pushing the barrel yields more play in relation to the handguard than if pushing the barrel to the left)

This is what i'm trying to solve, the GB just barely touches the handguard, I can slide some cardstock between the two, but the clearance is under 1.25mm.




The handguards are pretty much deadon with the receiver, I can run my hand down either side and it's smooth




Naked eye straightness seems good



Right side clearance


Left side clearance

Last edited by fishgoh0nk; 02-25-2021 at 10:21 AM..
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  #5  
Old 02-25-2021, 10:17 AM
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I have not been able to fit my PTG receiver facing tool into any BCM upper receivers, the tolerances just don't allow it to fit. While you may have some variance, I don't think you'll have an easy time correcting it at the receiver face unless you have an under sized tool for the BCM receiver.

Honestly, your asymmetrical gas block looks like it's having trouble fitting under the super slim profile handguard regardless of your barrel alignment. I'd try to find a lower profile (non adjustable?) gas block to see if that fits any better. Or try to twist the gas block a couple degrees away from the contact point to get better clearance - they usually have a large gas port opening for some extra wiggle room that still allows for normal function. I'd be much more worried about the gas block contacting that rail than the perceived misalignment of the barrel.
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  #6  
Old 02-25-2021, 10:31 AM
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Quote:
Originally Posted by DrewTheBrave View Post
I have not been able to fit my PTG receiver facing tool into any BCM upper receivers, the tolerances just don't allow it to fit. While you may have some variance, I don't think you'll have an easy time correcting it at the receiver face unless you have an under sized tool for the BCM receiver.
Hmmm that's good to know - BCM always recommended heating the receiver face to fit the barrels for tight tolerances, I guess we'll see - going to try it with a brownells lapping device.

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Originally Posted by DrewTheBrave View Post
Honestly, your asymmetrical gas block looks like it's having trouble fitting under the super slim profile handguard regardless of your barrel alignment. I'd try to find a lower profile (non adjustable?) gas block to see if that fits any better. Or try to twist the gas block a couple degrees away from the contact point to get better clearance - they usually have a large gas port opening for some extra wiggle room that still allows for normal function. I'd be much more worried about the gas block contacting that rail than the perceived misalignment of the barrel.
I might give this a shot, unfortunately I already dimpled the barrel with how it's mounted here. I do have a geissele super gas block in my stash. Guess I'll have to figure out another option when I move to a free state and go suppressed.
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  #7  
Old 02-25-2021, 12:14 PM
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Quote:
Originally Posted by fishgoh0nk View Post


I might give this a shot, unfortunately I already dimpled the barrel with how it's mounted here. I do have a geissele super gas block in my stash. Guess I'll have to figure out another option when I move to a free state and go suppressed.
I prefer clamp on gas blocks since they don't need dimpling. Wojtek makes great ones with standard and adjustable options.
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  #8  
Old 02-25-2021, 12:45 PM
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Get an slr. Much slimmer and works great. Probably will have a set screw of proper distance

Why running adjustable when (i assume) unsuppressed?
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Old 02-25-2021, 1:58 PM
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Originally Posted by dousan View Post
Get an slr. Much slimmer and works great. Probably will have a set screw of proper distance

Why running adjustable when (i assume) unsuppressed?

Yea, I was looking at that as an option, unfortunately the steel ones are all OOS or backordered; the titanium version is too $$. There's no real reason other than I'm running 10.5 and wanted to fine tune the recoil and ejection behavior and possibly suppressed some day

No one seems to be a proponent of receiver lapping? Can't seem to get a consensus anywhere, probably because lack of data to yield an concrete benefit from lapping, then it's just a matter of how anal one can get about accuracy. Given this is a 10.5 BA, it normally wouldn't be worth the effort, but it's covid season and I got plenty of time to fiddle.
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Old 02-25-2021, 2:03 PM
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Id say take it shooting at various distances and see how it does. May be a non issue.

If you has lasers n stuff it might end up being more of an issue but doubt it

I use a 10.3" g urgi with the g gas block (dd port is like .069/.070 i think) and with an rc2 and a geiselle h3 super thingie the ejection is fine as well as everything else.
Slap on lower and go blast away.

Minimal gas huffing.
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  #11  
Old 02-25-2021, 2:06 PM
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Can you slide a piece of paper between the gas block and the handguns?
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Old 02-25-2021, 2:10 PM
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In order to.see.if lapping would help. You need to pull the barrel and see if the barrel seated correctly.
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  #13  
Old 02-25-2021, 3:21 PM
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Before lapping anything, take a good look at the barrel extension. It could be off instead of the receiver.
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  #14  
Old 02-25-2021, 3:24 PM
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Quote:
Originally Posted by baih777 View Post
Can you slide a piece of paper between the gas block and the handguns?
yup, card stock, slightly thicker than paper can be slipped between the gasblock and the handguards, so they're not touching.

Quote:
Originally Posted by dousan View Post
Id say take it shooting at various distances and see how it does. May be a non issue.
Yea, I guess we'll find out; just wanted to keep smithing at a minimum when at the range

I think easiest remedy is just to slap on the G gasblock and tune from there.

I'll sleep on it; maybe i'll wake up and say "eff it, lets do some lapping" Taking the handguards and removing the gblock is already half the work, wouldn't be too bad to go all the way and "accurize" it.

Thanks for all the feedback!

Last edited by fishgoh0nk; 02-25-2021 at 3:31 PM..
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  #15  
Old 02-26-2021, 4:17 PM
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Ok, Found it - there's a small gap on the right side where the receiver meets the barrel extension.



I used another BCM upper I had, fresh out of the box, like the prior, but this one definitely had a tighter extension bore, had to thermal fit it a bit. yields a smaller gap...




Very odd that the first BCM upper had zero resistance; and the second one, though looser than normal installations, had some resistance. I can definitely say the BA barrel extension might be a tad out of spec, smaller, but I might have an out of spec BCM upper (first trial). Bought this upper years ago, think BCM would honor this?

Last edited by fishgoh0nk; 02-26-2021 at 4:18 PM.. Reason: warranty
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  #16  
Old 02-26-2021, 4:34 PM
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Quote:
Originally Posted by dousan View Post
Get an slr. Much slimmer and works great. Probably will have a set screw of proper distance

Why running adjustable when (i assume) unsuppressed?
This. What is the inside diameter of your handrail? This is an important dimension to know when putting together an upper. It is not always made available but usually can be found with much Googling. Slim rails have few choices. Even as slim as the SLR adjustable block is, it does not fit under some rails and a non-adjustable SLR block is needed. That Syrac or Seekins block (I think) is definitely not slim and will hit/rub when fired if it's close now. That would be too little clearance for my OCD, even if you trued up the receiver face, if tat is indeed an issue.
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Old 02-26-2021, 4:51 PM
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Looks like to me it could be your barrel extension. Not the best extension I've ever seen...

How does it make sense that the receiver is the problem? If it was straight before and the only change was a new barrel, why would you think the receiver is the problem? Do you get me?

It makes way more sense to set the optic. Take it to the range and shoot it before you make modifications to METAL.

Have you lapped a receiver successfully in the past? I've lapped 3. Two were successful, BUT made NO difference. The third I went over 70% of the surface lapped and caused the extension to run slightly past the feed ramps, just enough to create a mismatch. There is no way to fix that properly without tools and knowledge I don't have. Had to trash the receiver.



ALSO, I have one barrel that seats slightly to one side too. My OCD got to me. So, I tried the barrel in another receiver and got the same results. I just went ahead and sighted it in. It turned out to be above average accuracy. But it is slightly misaligned.


BTW - My experience with laser bore sighters have been dismal, I was lucky to get within 8 MOA with those things. I tried 3 different types.
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  #18  
Old 02-26-2021, 5:38 PM
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Quote:
Originally Posted by AregularGuy View Post
This. What is the inside diameter of your handrail? This is an important dimension to know when putting together an upper. It is not always made available but usually can be found with much Googling. Slim rails have few choices. Even as slim as the SLR adjustable block is, it does not fit under some rails and a non-adjustable SLR block is needed. That Syrac or Seekins block (I think) is definitely not slim and will hit/rub when fired if it's close now. That would be too little clearance for my OCD, even if you trued up the receiver face, if tat is indeed an issue.
The ID is 1.27" - I think I can clock the gasblock a tad right to give it more clearance without affecting the gasport. Yea, I think I'll go to the range and try out the gas regulation; if it's wide open, i'll replace it with the Geissele block.

Quote:
Originally Posted by ScottsBad View Post
Looks like to me it could be your barrel extension. Not the best extension I've ever seen...

How does it make sense that the receiver is the problem? If it was straight before and the only change was a new barrel, why would you think the receiver is the problem? Do you get me?
Oh, this is a brand new upper; basically 2 strangers meeting in the dark, a BA/BCM say hello.

Quote:
Originally Posted by ScottsBad View Post
It makes way more sense to set the optic. Take it to the range and shoot it before you make modifications to METAL.

Have you lapped a receiver successfully in the past? I've lapped 3. Two were successful, BUT made NO difference. The third I went over 70% of the surface lapped and caused the extension to run slightly past the feed ramps, just enough to create a mismatch. There is no way to fix that properly without tools and knowledge I don't have. Had to trash the receiver.



ALSO, I have one barrel that seats slightly to one side too. My OCD got to me. So, I tried the barrel in another receiver and got the same results. I just went ahead and sighted it in. It turned out to be above average accuracy. But it is slightly misaligned.


BTW - My experience with laser bore sighters have been dismal, I was lucky to get within 8 MOA with those things. I tried 3 different types.
Good to know, I guess I'm just super anal here, never was, but then again, I've never had to go more than 10 moa adjustment on windage with a brand new optic. The bore laser could be bad, but I've had an used this for over 10 years; guess we'll have to test fire it to know for sure.

In the end, even after reseating the barrel onto another brand new BCM upper, the barrel offsets the same. I have a 16" DD in another BCM upper that sits fine in the same rail system (different length) and anchor to one side. I really want to know, is it the receiver or the barrel? Just so I can make a mark in my brain for anecdotes as to manufacturer quality etc. I'm now thinking 80% it's the barrel/extension; I've built 7 BCM uppers and never had a hitch and this is my first time buying BA... guess you get what you pay for...
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Old 02-26-2021, 5:48 PM
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Scratch that - minimum clearance for barrel whip should be 1/16 - I don't think 1/32 is going to cut it - switching to Geissele gasblock :\
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Old 02-26-2021, 6:06 PM
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Quote:
Originally Posted by fishgoh0nk View Post
everything's dandy until I mounted an aimpoint t2 and the POA didn't match the bore laser POI
Put your bore laser in an immovable v block and rotate it and see if the laser is actually pointing to the center of the chamber/bore.
I have seen those laser boresighters make a 10" circle at 10ft away from the laser boresighter.

That's around 70moa of error.
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Old 02-26-2021, 6:10 PM
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Yeah, slow motion yootoobs show visible whip. Even 1/16th might not be enough. Although the bullet may be gone by the time the barrel contacts anything.

Can you see any daylight just putting a metal square on the extension?
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Old 02-26-2021, 6:13 PM
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Quote:
Originally Posted by fishgoh0nk View Post
Ok, Found it - there's a small gap on the right side where the receiver meets the barrel extension.
Not after you tighten the barrel nut.
The barrel nut will suck the extension in tight to the receiver.
The two pictures you posted above showing left and right side clearances to the rail being within 1/2mm of each other shows your reciever face to be pretty darn square.

The clearance to the gas block is too tight though.
You really want to stay with 1/16" to 1/8" clearance.

Go shoot the gun with the rail removed and see if the zero shifts from removing or installing the rail.
If it does, that's evidence of the barrel and gas block striking each other during barrel vibration.
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Old 02-26-2021, 8:14 PM
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Quote:
Originally Posted by ar15barrels View Post
Put your bore laser in an immovable v block and rotate it and see if the laser is actually pointing to the center of the chamber/bore.
I have seen those laser boresighters make a 10" circle at 10ft away from the laser boresighter.

That's around 70moa of error.
That's one of the first things I did, I rotated it around in the chamber to find variance, at 10 yds in a bench, I didn't see any change.

Quote:
Originally Posted by sigstroker View Post
Yeah, slow motion yootoobs show visible whip. Even 1/16th might not be enough. Although the bullet may be gone by the time the barrel contacts anything.

Can you see any daylight just putting a metal square on the extension?
Laying a square rule on the left/right edges yields a flat edge, but even with an untrue receiver face, it would still yield a flat edge. The square edge would need to have something perpendicular/90 to it to show me an untrue edge. Since I'm on the second upper now, I can test it with the next build I do with a new barrel. I'm just really irked that the replaced upper with higher gap on the right did not need thermofitting like it normally does.

Quote:
Originally Posted by ar15barrels View Post
Not after you tighten the barrel nut.
The barrel nut will suck the extension in tight to the receiver.
The two pictures you posted above showing left and right side clearances to the rail being within 1/2mm of each other shows your reciever face to be pretty darn square.

The clearance to the gas block is too tight though.
You really want to stay with 1/16" to 1/8" clearance.

Go shoot the gun with the rail removed and see if the zero shifts from removing or installing the rail.
If it does, that's evidence of the barrel and gas block striking each other during barrel vibration.
I switched it out, this is the second time this seekins gasblock has been installed and removed; will probably never get to use it

@ar15barrels Do you know the main reason why a barrel can cock to one side on two different receivers, same rail? Is it more commonly that there's an issue with the receiver, barrel extension, barrel, rail system (nut/rail)?
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Old 02-26-2021, 8:34 PM
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You know what the REALLY crazy thing is - new upper, new gasblock, Scalarworks T2 + laser bore sight have the same POA at 50yds. I thought I had to change at least the windage. That's pretty sick! Even though off I think the barrel to receiver mating is off, at least it's consistent.
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Old 02-26-2021, 9:05 PM
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Quote:
Originally Posted by fishgoh0nk View Post
Laying a square rule on the left/right edges yields a flat edge, but even with an untrue receiver face, it would still yield a flat edge. The square edge would need to have something perpendicular/90 to it to show me an untrue edge.
Right, that's kinda the definition of a square. Lay one leg on the end of the extension. Then look at the 90 degree leg. You might not be able to see any misalignment because there's probably nothing to lay the second leg against.
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Old 02-27-2021, 7:40 PM
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Are you planning on feeding factory 77gr OTM into the rifle or are you using cheap WWB or other factorytarget ammo? It'll be a waste of time and money if the round you're using is 2-3 MOA to begin with, there's no sense upgrading the hardware....
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Old 03-02-2021, 3:48 PM
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FWIW... I bought a Wheeler Brand lapping tool, just to gauge the quality.

And it is for sure a looser fit then any of my PTG lapping tools.... as in a sloppy fit that wouldn't help achieving a reasonably "true" surface.

However....It might fit the tighter BCM upper.

Last edited by bfoosh006; 03-02-2021 at 3:51 PM..
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Old 03-07-2021, 4:37 PM
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Lapping complete. POI (laser bore) shifted 4.5moa to the left, barrel now sits 1/64 inch to the right in relation to MK14 handguards, previously it was 1/32 inch. I'm happy

I saw a post over at Maryland Shooters where a lapped BCM upper exposed it was not square, there was more material on the right of the receiver threads, anodizing remained on the left, pretty much a symptom that would cause my barrel to cant as well. Maybe it was a bad/lower quality batch of receivers, because my older BCM uppers does not yield canting with even a 16 inch barrel.
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Old 03-07-2021, 4:51 PM
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Quote:
Originally Posted by fishgoh0nk View Post
Lapping complete. POI (laser bore) shifted 4.5moa to the left, barrel now sits 1/64 inch to the right in relation to MK14 handguards, previously it was 1/32 inch. I'm happy

I saw a post over at Maryland Shooters where a lapped BCM upper exposed it was not square, there was more material on the right of the receiver threads, anodizing remained on the left, pretty much a symptom that would cause my barrel to cant as well. Maybe it was a bad/lower quality batch of receivers, because my older BCM uppers does not yield canting with even a 16 inch barrel.

That seems fairly significant. How much work was it? As in, just a little lapping vs significant amount/effort? Any issues with tight fit of the lapping tool in the receiver?


ETA: Nice job working it out, OP!
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Old 03-07-2021, 5:37 PM
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Worked out pretty easy, used a Brownells lapping tool, wheeler compound set 220 + 320, vised the receiver vertically, barrel facing the sky, applied the 220 and started by hand, used CLP for the lapping tool body. The tool fits right in, but this BCM receiver is kind of loose compared to my others, which usually requires thermalfit for barrels, but slipped right in. The anodizing is pretty thick, so I ended up going to the drill and turning at medium speed for 8-10 seconds, made 2 passes after checking work and re-centering the compound, wiped clean and did half the time with 320. Overall took me maybe 25 min.

You can tell the improvement by 1. handguards sit closer to receiver, 2. handguards and barrel nut are square, 3. gas tube aligns better with gas key

Thanks for listening to the gripe, it's a learning experience for me.

Quote:
Originally Posted by AregularGuy View Post
That seems fairly significant. How much work was it? As in, just a little lapping vs significant amount/effort? Any issues with tight fit of the lapping tool in the receiver?


ETA: Nice job working it out, OP!

Last edited by fishgoh0nk; 03-07-2021 at 5:48 PM.. Reason: thanks
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Old 03-07-2021, 7:35 PM
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Quote:
Originally Posted by fishgoh0nk View Post
Worked out pretty easy, used a Brownells lapping tool, wheeler compound set 220 + 320, vised the receiver vertically, barrel facing the sky, applied the 220 and started by hand, used CLP for the lapping tool body. The tool fits right in, but this BCM receiver is kind of loose compared to my others, which usually requires thermalfit for barrels, but slipped right in. The anodizing is pretty thick, so I ended up going to the drill and turning at medium speed for 8-10 seconds, made 2 passes after checking work and re-centering the compound, wiped clean and did half the time with 320. Overall took me maybe 25 min.

You can tell the improvement by 1. handguards sit closer to receiver, 2. handguards and barrel nut are square, 3. gas tube aligns better with gas key

Thanks for listening to the gripe, it's a learning experience for me.

I think this is a good learning experience. You had a logical hunch based on your measurements and gave it a shot despite some of the other advice you got here. Problem solved and a better fit are very satisfying I'd imagine. I will have to keep this in mind when/if I have to troubleshoot. Thanks!
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