New .308 Build with Fitment / Firing Issues

[daveblack]

First post here. A member here suggested I ask the knowledgeable members here about my build. I am not 100% sure what is going wrong with my build.

Built a .308 with these parts:
-Aero upper
-Right to Bear Arms 80% lower
-Faxon Firearms Low-Profile 0.750" Gas block (fixed)
-Heavy Rifle Buffer (9oz)
-Armalite Buffer Spring
-Fixed rifle stock
-Luth-AR 308 Bolt Catch
-20" Ballistic Advantage barrel

There is a gap when I connect the upper and lower. You can see where the Aero upper hole does not align with the rear takedown slot. I can get the rear takedown pin in only by hammering it in. Picture is with the front takedown pin only.

When I fire it, the round ejects, but the bolt only goes back halfway and the next round doesn't feed right. Can pull bolt all the way back with the charging handle to reload manually.

I have this same problem with an identical upper/lower combo. On that build, the bolt gets caught when I pull the handle all the way back. It slams forward when I hammer out the rear takedown pin.

I have also tried a different upper and have the same fitment issue. It looks like the slope of the lower doesn't start early enough at the back.

Any ideas how to fix this?

[jtallen83]

Buy Aero lowers. Sorry, all I got.

[DustBuster]

I’m not an expert but I read somewhere here of someone else’s upper being out of spec “misaligned” and it caused the bolt to contact the inside of the buffer tube.  Sounds like your upper is not quite aligned azimuthally because of the fit.  The top of your upper should be perfectly parallel to the top of the lower.... maybe some filing and or reaming could help... but I’m not sure really

[Thirstydg323]

5D Tactical 80% lower will fit an Aero upper

[Matt.Cross]

We've seen this so many times I've lost count. The best way to build a large AR is to source your upper and lower from the same manufacturer so as to guarantee a match. You're not the first or last person to be bit by the lack of a "standard" for large ARs. I'd recommend getting an Aero lower.

[98Z5V]

18 hours ago, daveblack said:

-Heavy Rifle Buffer (9oz)
 

What's the length on this rifle buffer?  It's either 5.200" long or it's 5.900" long.

What's your gas port diameter on this barrel. 

[daveblack]

On 4/24/2021 at 10:29 PM, DustBuster said:

I’m not an expert but I read somewhere here of someone else’s upper being out of spec “misaligned” and it caused the bolt to contact the inside of the buffer tube.  Sounds like your upper is not quite aligned azimuthally because of the fit.  The top of your upper should be perfectly parallel to the top of the lower.... maybe some filing and or reaming could help... but I’m not sure really

Hmm... that makes sense. Not opposed to doing some machining on it - what do I have to lose? Looking at this again, it looks like the takedown holes are off - the rear looks like it should be centered. If I grind out about a millimeter on the left of the front/rear, it will lay flat, but it will leave a gap at the back. Someone else did this on another lower with fitment issues. Anyone think that is a good idea?

On 4/24/2021 at 10:45 PM, Thirstydg323 said:

5D Tactical 80% lower will fit an Aero upper

Thanks - I will keep that in mind. Used a 5D Tactical rig to mill this. Went with Right to Bear Arms 80% lower, because wanted the durability of forged, but looks like that was a mistake.

On 4/24/2021 at 11:01 PM, Matt.Cross said:

We've seen this so many times I've lost count. The best way to build a large AR is to source your upper and lower from the same manufacturer so as to guarantee a match. You're not the first or last person to be bit by the lack of a "standard" for large ARs. I'd recommend getting an Aero lower.

Thanks for the advice. I should have looked harder for a matched set. Wanted to go with an 80% lower, because of current political climate. Difficult to find 80% matched sets. Daytona Tactical only company that I can find that has anything right now, but their reviews are pretty abysmal. Might give them a shot anyway.

19 hours ago, 98Z5V said:

What's the length on this rifle buffer?  It's either 5.200" long or it's 5.900" long.

What's your gas port diameter on this barrel. 

5.2" buffer.

I don't know. Is there a way to measure? This is the barrel: https://www.ballisticadvantage.com/20-308-heavy-profile-rifle-length-ar-308-barrel-modern-series.html

 

Edited April 26, 2021 by daveblack

[98Z5V]

31 minutes ago, daveblack said:

5.2" buffer.

I don't know. Is there a way to measure? This is the barrel: https://www.ballisticadvantage.com/20-308-heavy-profile-rifle-length-ar-308-barrel-modern-series.html

That's the right buffer, then, as far as length goes.

Measure the gas port diameter with numbered drill bits, until one doesn't fit - the next one down that fit, is the gas port diameter.  A small drill index set of numbered drill bits is cheaper than buying machinist's pin gauges.

Gas port diameter on that barrel profile need to be 0.093"~0.096".

I'd start with a standard weight rifle buffer that's 5.4oz - if your gas port is too small, you're compounding the problem with that heavy buffer.

Edited April 26, 2021 by 98Z5V

[DustBuster]

Again, I stress, I’m very new to these.  File and or ream only if your sure about any benefits.  Rule out other buffer size/gas port/spring probs first.  Hammering in your rear takedown pin makes me cringe... but you did it and it has fired like that, and you’ve been able to pull the pin back out, so that fitment issue might not be the full culprit.  I had mag follower issues on magpul 20s and 25s, and a Lancer 30 rndr.  All of those issues were the follower from the mag came up too high and forced my bolt stop too high too soon, stopping my bolt before it could get back far enough to stop behind it on an empty mag.  I guess that problem wouldn’t effect the bolt if rounds were on top of the follower in the mag, but who knows, maybe your bolt stop is too high to begin with in its down position?  You could remove the bolt stop and try firing it, I think, to rule out that being an issue.... but you’d have to hammer the takedown pin back in to test and pretty  soon your hole will be 3/8” oval!  Rule out what u can before grinding away on upper or lower I’d say

[98Z5V]

11 minutes ago, DustBuster said:

 Rule out other buffer size/gas port/spring probs first. 

There's not a problem with his recoil spring, at all. 

[DustBuster]

On 4/24/2021 at 7:57 AM, daveblack said:

I have this same problem with an identical upper/lower combo. On that build, the bolt gets caught when I pull the handle all the way back. It slams forward when I hammer out the rear takedown pin.

I have also tried a different upper and have the same fitment issue. It looks like the slope of the lower doesn't start early enough at the back.

Any ideas how to fix this?

Yeah I’m sure you are right about that...spring not being the problem... DaveBlack mentioned the above earlier and that made me think about other problems besides gas port since he experienced the bolt getting caught even with the charging handle pull.  I’ve got my own issue with my gun now, which is minor and probably will require a drill bit to the gas port, even though mine is already 

.093" +/- .001". for a Faxon rifle length barrel.

I will step aside and let the true Gunsmithers solve this riddle

[daveblack]

On 4/26/2021 at 12:32 AM, 98Z5V said:

That's the right buffer, then, as far as length goes.

Measure the gas port diameter with numbered drill bits, until one doesn't fit - the next one down that fit, is the gas port diameter.  A small drill index set of numbered drill bits is cheaper than buying machinist's pin gauges.

Gas port diameter on that barrel profile need to be 0.093"~0.096".

I'd start with a standard weight rifle buffer that's 5.4oz - if your gas port is too small, you're compounding the problem with that heavy buffer.

Thanks - that's good advice. I will give this a shot, if I still have problems.

 

On 4/26/2021 at 12:35 AM, DustBuster said:

You could remove the bolt stop and try firing it, I think, to rule out that being an issue.... but you’d have to hammer the takedown pin back in to test and pretty  soon your hole will be 3/8” oval!  Rule out what u can before grinding away on upper or lower I’d say

I think you may have helped me find the major problem. The Luth-AR 308 Bolt Catch seems to stick out too far/high. I took it out, put in the front takedown pin and everything almost perfectly lines up. The charging handle pulls back clean now. The upper was see sawing around that bolt catch when I had the front takedown pin in before.

Ordered some Aero bolt catch replacements. Fingers crossed. I'll report back if this fixes the problem.

That's not the only problem. The lower/upper mating is still a little off in the rear takedown pin. I'm going to have to either shave a hair off the front of the lower or rear lug of the upper.

Edited April 29, 2021 by daveblack

[DustBuster]

Those pesky bolt catches... I had to add a shim to one type of magazine and remove some material from the lancer to keep the bolt catch from prematurely stopping my bolt from fully opening on an empty magazine.  Slightly different problem as yours, but maybe the same.  The pmag 10 for me was problem free.  I’m sure if I wouldn’t have fiddled with the gun before firing it for the first time I would have had bolts getting stuck and failure to feed. Hope it works for you, my rank is Zero and maybe I’ll be bumped up a smidgeon haha

[daveblack]

 

On 4/29/2021 at 12:24 AM, DustBuster said:

Those pesky bolt catches... I had to add a shim to one type of magazine and remove some material from the lancer to keep the bolt catch from prematurely stopping my bolt from fully opening on an empty magazine.  Slightly different problem as yours, but maybe the same.  The pmag 10 for me was problem free.  I’m sure if I wouldn’t have fiddled with the gun before firing it for the first time I would have had bolts getting stuck and failure to feed. Hope it works for you, my rank is Zero and maybe I’ll be bumped up a smidgeon haha

Points for helping people here? I like that.

Well the Aero bolt catch didn't completely fix the problem. Dimensions are about the same. It looks stronger. I filed down the top of the bolt catch. It helped. Also removed a hair off the back of the upper. Here is what that looks like:

On 4/26/2021 at 12:32 AM, 98Z5V said:

Measure the gas port diameter with numbered drill bits, until one doesn't fit - the next one down that fit, is the gas port diameter.  A small drill index set of numbered drill bits is cheaper than buying machinist's pin gauges.

Gas port diameter on that barrel profile need to be 0.093"~0.096".

I'd start with a standard weight rifle buffer that's 5.4oz - if your gas port is too small, you're compounding the problem with that heavy buffer.

I "measured" with an allen wrench. A 5/16" allen wrench nearly fits. That would be alot more than the range you listed above. Not sure how that computes. Also took a picture. Does this look normal?

Drew lines to make sure gas block was centered and tightened down more. It was blowing outside of the gas block.

I may give it another shot at the range to see how those adjustments impacted things.

Edited May 4, 2021 by daveblack

[98Z5V]

38 minutes ago, daveblack said:

 

Points for helping people here? I like that.

Well the Aero bolt catch didn't completely fix the problem. Dimensions are about the same. It looks stronger. I filed down the top of the bolt catch. It helped. Also removed a hair off the back of the upper. Here is what that looks like:

I "measured" with an allen wrench. A 5/16" allen wrench nearly fits. That would be alot more than the range you listed above. Not sure how that computes. Also took a picture. Does this look normal?

Drew lines to make sure gas block was centered and tightened down more. It was blowing outside of the gas block.

I may give it another shot at the range to see how those adjustments impacted things.

I can see the carbon ring around your gas port in the barrel in that pic, and it's perfectly centered. 

5/16" would be 0.3125", and that's not even possible for a gas port diameter.  No way, no how. 

That carbon ring on your barrel is 0.125", because that's the diameter that the port in gas blocks are drilled, because that's the ID of a gas tube.  Nothing will ever go over 0.125", because it's not gonna work, with the gas tube ID being the restriction.  Always.  If you ever see a gas port diameter over 0.125" - then that's a bad design.  The barrel manufacturer should have upped the gas block journal diameter, to cut the gas port diameter down.  0.3125" for a gas port diameter isn't what you have - I can see it in the pic you posted.  0.3125" gas port diameter is unpossible.

Edited May 4, 2021 by 98Z5V

[Cunuckgaucho]

1 hour ago, daveblack said:

 

I "measured" with an allen wrench. A 5/16" allen wrench nearly fits. That would be alot more than the range you listed above. Not sure how that computes. Also took a picture. Does this look normal?

You'll find drill bits more accurate for measuring, of course a pin gauge set is the best. Also the size of the Allen key is based on the dimension across the flats( 0.321") not across the corners(0.355")

[daveblack]

21 hours ago, 98Z5V said:

I can see the carbon ring around your gas port in the barrel in that pic, and it's perfectly centered. 

5/16" would be 0.3125", and that's not even possible for a gas port diameter.  No way, no how. 

That carbon ring on your barrel is 0.125", because that's the diameter that the port in gas blocks are drilled, because that's the ID of a gas tube.  Nothing will ever go over 0.125", because it's not gonna work, with the gas tube ID being the restriction.  Always.  If you ever see a gas port diameter over 0.125" - then that's a bad design.  The barrel manufacturer should have upped the gas block journal diameter, to cut the gas port diameter down.  0.3125" for a gas port diameter isn't what you have - I can see it in the pic you posted.  0.3125" gas port diameter is unpossible.

Hah! Totally unpossible. You are right on - it was 5/64". 1/16" fit with a little room. As Cunuckgaucho pointed out that would make it at least 0.088" measured diagonally and <0.11". 5/16" made no sense, now that I think about it. Thanks for the expert analysis.

So try with a standard weight rifle buffer? I'm trying to minimize recoil as much as possible. Would I step up weight, if it works?

Edited May 5, 2021 by daveblack

[edgecrusher]

31 minutes ago, daveblack said:

Hah! Totally unpossible. You are right on - it was 5/64". 1/16" fit with a little room. As Cunuckgaucho pointed out that would make it at least 0.088" measured diagonally and <0.11". 5/16" made no sense, now that I think about it. Thanks for the expert analysis.

So try with a standard weight rifle buffer? I'm trying to minimize recoil as much as possible. Would I step up weight, if it works?

If by standard weight you mean 5.4oz then yes

[daveblack]

Tried a standard weight rifle buffer, plus my ground down bolt catch. It fed 4 rounds before failing to feed correctly again. Basically, the round is stuck halfway up the feed ramp and the bolt has slammed into it at a 45° angle. Put a nice dent in the round. Same as last time.

Any other suggestions to save me from buying a new upper/lower?

Edited May 15, 2021 by daveblack

[shooterrex]

Is the bolt wet with lube? How many rounds has this rifle fired? This platform doesn't always run at first. Use a different brand magazine.  Just a few things to look at.

[98Z5V]

On 5/15/2021 at 7:05 AM, daveblack said:

 

Any other suggestions to save me from buying a new upper/lower?

Pictures of the malfunction would help.