View Full Version : Twisted AR upper?

11-29-2011, 8:56 PM
Hey all,

Don't post much on here but was wondering if I could get an opinion from some of you AR builders out there.

I recently put together an AR upper (16") and upon taking it to the range over the weekend, found it shooting a good 6-8 inches left with both the flip up irons and a low powered scope at 25 yards. Both the irons and the scope were initially set to their mechanical zero and I found I had to crank the windage over on both of them quite a bit to get center target (about 3/4 of a full turn on the rear iron sight and about 1/2 a full turn on the scope). I've never had to do that on any of my previous "out the box" ARs and I'm a little concerned I may have "twisted" the upper receiver when torquing down the barrel nut, thus causing a less than perfect alignment between barrel and receiver.

Essentially what happened was I had a brain fart while installing the barrel and neglected to put the front pin in the action block. I torqued the barrel nut down to about 55 ft lbs and upon going to remove the upper from the block, had the "oh ****" moment and realized the front pin wasn't installed. I immediately checked the fit with the lower and it seemed fine and the BCG wasn't binding or anything. Nevetheless, that way left shooting at the range has me concerned.

So my question is, is it even possible to "twist" an upper given the situation described? I had kind of assumed an upper would simply crack/break before it would take on a permanent twist.

As a side note, all parts were new and of "decent" quality (Nothing fancy: DPMS upper receiver & YHM barrel). Also, I did have the guy next to me shoot the rifle to rule out shooter error and he was having the same problem.


Coyote Brown
11-30-2011, 5:49 AM
BUISs??? Take them off and reinstall .

11-30-2011, 6:22 AM
remount the sights and scope.

11-30-2011, 6:29 AM
Is the front flip up sight mounted on a FF rail or a railed gas block?


11-30-2011, 12:07 PM
if you twisted the upper, it wouldnt fit onto the lower

11-30-2011, 6:11 PM
Thanks for replies.

Don't think it's an issue with the mounting of the sights or the scope. The front sight is mounted on a free floated quad rail which I know can result in less than optimum accuracy but the fact that the scope was shooting to about the same POI as the irons makes me think it's something a little more.

Anyway, I broke down the entire upper earlier this afternoon to check the fit with all parts and make sure the indexing pin on the barrel didn't sheer off of something. All seemed fine and the upper went back together fine.

Will get it out on a proper range over the weekend and see if the issues persist.

11-30-2011, 9:09 PM
if your problem still exists check the muzzle attachment and barrel crown. some flash hiders brakes have an up and a down.

11-30-2011, 9:09 PM
if your problem still exists check the muzzle attachment and barrel crown. some flash hiders/brakes have an up and a down.

12-01-2011, 5:03 AM
who made the barrel?

12-01-2011, 5:13 AM
I would agree that the sights might not be properly mounted to the rail. I had a similar problem when mounting my scope with a Nikon mount. The mount would shift slightly to the right when the screws were torqued down. Take a look at the alignmnet of the scope or sight mount to see if they are completely square. You just might see an offset. Just a thought.

If you want to test the "squareness" of the lower then just separate the lower, place on a flat table and put a builder's square up against the side of the lower. That should give you some indication.

12-01-2011, 7:20 AM
I just cant trust those action blocks that insert into the bottom of the upper and fix with pins. The take-down pin holes see alot of the torque with those things and they certainly aren't designed for that kind of force. Any barrel work involving torque and I use the action block that clamps OVER the upper.

12-01-2011, 8:50 AM
Seems like a major coincidence however I would remount the irons and then sand down the scope mounts and verify that everything is straight and level. Midway sells a kit with all the parts you need, But you can do this with a plumb level, 9-12 inch level, and a dowel rod with sand paper.

Basically level the gun out in a vise, then hang the plumb level so it splits the barrel in half (string goes down the middle of the barrel, make sure the buttstock is perpendicular to the board you hang the plumb on.) When you look through the scope you should see the line almost perfectly on the crosshairs. What I like to do is then move it back to 30 or so yards and verify that the vertical line matches the plumb line. If its not, you want to start smoothing out those scope mounts. Also, some mounts get tweaked when tightened down too hard, I have seen this a few times with friends guns, they ended up bending/destroying a few simmons scopes. oops.

I did this process on my .17hmr and was hitting shotgun shell primers dead nuts at 75 yards, coolest day shooting in a long time ha ha ha I still needed a few turns on the elevation nobs, but the left/right was spot on.