supposed to be oval, or .250-ish and round?
Anybody know the answer?
I would be grateful if somebody would take a moment, snap open his AR, and look at the hole in the upper’s rear lug. Is it cylindrical and just a bit bigger than .250, or is it slightly oblong or oval in cross section? If you’ve been involved with AR’s for a long time, this may seem like either a dumb question or an obvious one. If you’re like me, though, it’s neither dumb nor obvious. I’m new to all this and very curious about the design. Is there an assumption in the spec that the lower and upper will often not be a perfect match and that the rear hole in the upper needs some slop in it? Or did somebody machining my upper wiggle the work and make an oval hole by mistake?
checked a couple of mine for you- little bigger than .25 and circular.
On both of my Bushmasters, the takedown pin hole seems to be perfectly round. The pin itself is .250 and does have a slightly flattened side (towards the receiver extension) on it with a groove where the detent rides. This detent keeps the pin in place. If your takedown pin hole is visibly oval in shape, you may have a problem. I’m no expert but none of my five AR’s has an oval hole for the takedown pin.
My brand new ArmaLite upper - perfect 0.25" circle all the way through the take down cylinder, also a very tight fit between the upper and S&W lower.
Can you post a macro shot of yours? Maybe someone can pinpoint the issue…
The upper is a brand new piece from a highly respected manufacturer. The hole in the front pin lug is cylindrical and just big enough for the a .250 pin (I used a piece of .250 drill rod) to slide easily but not sloppily. The hole in the rear lug is much bigger (the .250 drill rod can wave around quite a bit in there) and is visually a slight, horizontal oval (elongated front to back)
Sorry, I don’t have a camera with enough resolution to show you this.
The purpose of this thread is to find out if this rear hole is SUPPOSED to be larger than the front one. If it is, no problem. If it isn’t, I’ll contact the folks who made it and I’m sure they’ll take care of it.
Some I’ve seen are round and on others I’ve seen, they’re ovular (if only by a few thou). Just a SWAG here, but I’m guessing that it depends on how the Mfg. has them set up to run on the CNC. The RRA upper I’ve got laying around has just the tiniest bit of a flat I can see.
Personally speaking, so long as the gun fit together and ran well, I wouldn’t lose sleep over it.
how’s fit? unless you’ve got some huge gap, i’d probably assume it’s the way it’s supposed to be. this is actually nagging at something in the back of my mind- it’s quite possible some manufacturers make them that way deliberately.
The rear take down hole in the upper has to act as a horizontal slot. Otherwise if you had two perfectly circular and tight holes, you’d never get it to fit just any lower. You’d have to hand fit it due to tolerance stacks. However, the lower take down holes and the front take down hole on the upper are all circular.
The rear take down pin doesn’t take any recoil load, it just fixes the rotation of the upper about the front take down pin (which takes all of the recoil loads).
Does the upper and lower fit together? Does it shoot? If yes, you are worrying about nothing.
The upper and lower fit together just fine.
I haven’t shot it yet–couldn’t get to the range today.
I’m not worried. Just curious.
If these blueprints are authentic, I have my answer.
http://www.scribd.com/doc/14939212/Ar15-A3-style-Upper-receiver-blueprints
The hole in the rear lug is not a cylinder. It consists of two half circles with .125 radiuses connected by upper and lower flats .024 long. In other words, the hole is .250 top to bottom and .274 left to right.
This blueprint exactly describes my new upper.