How much of a gap between upper and lower is TOO much?

[Tack14]

Hi everyone,

This may not really be a good question to ask without having pics ready to show, I can get a couple.

But in general terms I am wondering how concerned I should be about a gap between fit of upper and lower receiver in the rear of the action?  There is a bit of  daylight you can see between upper and lower.  At the rear where the gap is horizontal rather than vertical (due to curvature of the shape of the upper receiver at the top back face) it is less than .100".

The takedown pin goes in no problem, it's actually a little on the loose side I noticed.  Not crazy loose but before I got the detents etc. in if I leaned over to the right the rear pin would drop out if they were oiled.

Almost looks like something is standing the upper receiver off just a tiny bit, despite the takedown pin fitting through the takedown pin lug and holes in the lower just fine.

Parts: KAK Industry LR-308 upper receiver, AMT (american made tactical) DPMS pattern lower.  KAK 308 pivot and takedown pins.

May just be OCD but if there is a problem I want to deal with it.  These lowers don't have a gap/fit adjustment screw etc.

Thanks!

Tack14

 

[edgecrusher]

Well…. Pics? Also is there a build thread related to this gun? Has it been shot? How familiar are you to AR’s? I have had duty issued POS that I could slide a poker hand through… M4 but still

[DNP]

As long as the carrier doesn’t bind when going into the buffer tube, it’ll probably just be cosmetic. This is one of the reasons we stress keeping the upper an lower from the same manufacturer. With no specs, you’ll likely get a better fit if both pieces are from the same shop using their own dimensions and specs. 

[Tack14]

Thanks.  Yep, will get a couple pics.  Should I open a thread just for this AR?  If so, maybe I'll just hold off and post there.

I am relatively familiar with AR's but no expert, that's for sure.  Familiar enough that I've seen AR15's over the years that had some wobbles and or daylight between upper and lower too including factory built AR's, and know that some worry about it and some don't.

Haven't tried to shoot it yet. But getting pretty close now.

I tend to analyze and over-analyze, but along lines of what DNP was saying, it was the ramifications of how BCG cycles and mates with the buffer tube if it's too much play that I was concerned about. 

Will investigate that further too, exactly how smooth or "bindy" the travel of the BCG is.  It feels pretty smooth but i have not tried to go really "ride it slowly" to see if I can feel binding, but it makes sense I should try to figure out of there is any.  Overall it seems fine, seriously, the rear takedown pin doesn't wobble around in the hole or anything like that,.

DNP I hear you  -- I'd definitely go for some matched upper/lower sets if I could have at the time.  When I started researching building 308 AR's I had some difficulty finding that many brands that make both halves, but did find a couple.  Grey Ghost Precision for example.  Just haven't seen them around anywhere (unobtanium) anytime/anywhere recently except for the SLR ones, which do seem pretty nice, albeit expensive - although with the kind of fit these are said to have with each other I would agree its worth paying more for.

Building some 308 AR's was something I always wanted to do but, well frankly I saw that as something in jeopardy early this year with the general swing of things.

I looked at what I could get my hands on, and yeah, I know that is a really bad idea sometimes.  But it doesn't seem like I really did that bad.  Will post a couple pics when I can get some good ones with the some good lighting etc.  or wait to see whether people think I ought to open a thread for this 308 AR or not. 

 

 

 

Edited October 5, 2021 by Tack14

[DustBuster]

My upper and lower purchased from the same company were pretty snug but with a hairline gap with the rear takedown  pin in.  Now that I’ve shot it more than 300 times, there is a little more gap near the back and even more sideways wiggle from the front pivot pin.  I suppose the Cerakote is slowly wearing.  I am not too worried about it since the scope is rock solid with the barrel and the upper. I suppose eventually I will try to shim the top part of the takedown pin hole on the lower with some thin epoxy or something to force the pin lower.  Or a dab of epoxy on the lower side of the hole in the upper.

From memory I bet my gap if spread open is at most 1/32”=.03125”  at the rearmost.  That’s way less than your .1 max.  I will confirm tonight for the heck of it.  

The thing that bugs me about things loosening up is the sideways rotation on the front pivot pin, which causes the rear hole of the upper to sometimes miss the slot as I’m closing the halves after having them pivoted open.  I have to not just blindly close the halves when the gun is on its side. Not that big of a deal for now.

[DustBuster]

I took some pictures, one side’s gap is a little more than the other.  The biggest gaps are at my radius points, but then on the verticals it is back to nearly snug.

[98Z5V]

5 hours ago, Tack14 said:

Thanks.  Yep, will get a couple pics.  Should I open a thread just for this AR?  If so, maybe I'll just hold off and post there.

 

Nah, man - this is the thread.  Just post the whole build here, the thread is already running. 

Here's my opinion on the upper and lower fitment, and "slop" in the receiver fitment...  Lot's of people freak out about this, but hear me out...

It doesn't matter.  The only way it's gonna affect accuracy, is if the upper is actually flopping back and forth, left and right, in the lower.  Once you slam a magazine in there, and rounds are pushing up on the bottom of that bolt carrier - even "sloppy receiver fitment" doesn't move.  It can't.  But it drives people crazy, thinking that they're losing accuracy, from those "sloppy fitment" receivers.  It's rubbish.

The accuracy of your gun is the barrel quality, the fitment of the barrel to the upper, the fact that the BCG headspaces, and it locks up tight (and isn't falling all over the place inside the upper receiver - hotdog down the hallway-thing)...   and the ammo that you're shooting.  That's it for upper parts, for accuracy.  The only part in the lower that can affect accuracy is the trigger.  Example - you WILL get better accuracy, in the exact same gun, with a Geissele Hi-Speed National Match trigger, than you will with some gritty, run of the mill mass produced mil-spec trigger with an 8lb pull weight.  That's a given.

It doesn't matter if the receiver fitment sucks, and you can wiggle it at home, in the living room, without a loaded mag locked in place.  That doesn't matter.  The upper parts, and their fitment matter, and that good trigger hitting that firing pin in the upper (in the BCG) matters. 

If you have to change receivers because the accuracy sucks, and those receivers had a little play in them - unloaded - then you've reached a great point in life.  You have outshot your equipment, and need to upgrade your equipment in order to shoot better.  In reality, no matter what you build, that doesn't happen often.  With receiver play being the issue - I've never seen anyone improve their shooting, with measurable results, by changing everything over to "tighter receivers..." 

My $0.02...  

Edited October 6, 2021 by 98Z5V

[98Z5V]

Damn.  Double-Tap...  

Edited October 6, 2021 by 98Z5V

[Tack14]

Thanks everyone for the replies. Really appreciate all the advice, experience and insight.  I'll probably be the paranoid long tailed cat in a room full of rocking chairs n00b until I have

@DustBusterlooks like we are in similar range.  .100" was definitely an overestimate (didn't have a feeler gauge or other implement immediately handy)

@98Z5V thanks a lot.  I was thinking it might be like that.  It is a fairly good fit once put together.  Only sound I hear is the buffer weights and upper/lower don't seem to be moving separately from each other.

Here are a few pics.  They're not super great but I am a little mentally challenged this evening, just got off work. 😣  I put one of my flashy's up pointing up at an angle behind to try to get some light in the gap/crack.

 

 

 

 

[Tack14]

whoops didnt finish first paragraph.  ...until I have successfully test  fired, verified gas/cycling etc. and have gas adjusted and the other basic stuff.  Basic sights zeroed etc.

Edited October 7, 2021 by Tack14

[Tack14]

Anyway that is how the KAK LR-308 upper and AMT DPMS 308 (they call it an "AR-10 DPMS Gen 1") lower fit together.

Another build I'm workin on with same upper and lower receivers has same gap/fit but is also as snug assembled.