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View Full Version : Sun Devil Upper/Lower Problem, Whats wrong?


what2be
07-19-2008, 7:44 PM
Well, my AR build could be the longest in history, going on 8 months now, but the biggest hangup was the Les Baer Barrel, bolt carrier assembly. That alone took 3 months almost. I think them moving in the middle of my order hung things up to be honest. Anyway, now that I got that and assembled my AR, im having a big problem... which Ive never heard of before. Maybe someone has experienced something similar?

When pulling the charge handle, the bolt carrier gets stuck partially back in the buffer tube. Sometimes tapping releases it, other times you have to pull out the rear pin. When you leave both pins out, and hold upper/lower by hand tightly, it does the same thing, but when you give it a little gap,
it will charge, release fine. The charging handle is a very tight fit in the upper, it takes considerable force to move it back and forth, but its improving. the small side extruded metal on the charging handle near the front of the handle (where it catches the bolt carrier nose) is wearing in and becoming shiny. The charging handle is a Badger ordinance tactical gen II, and the upper/lower by sundevil is a billet, teflon/nickel. Its almost like the tolerances are too tight. Now, when I remove charging handle, put a firing pin in one of the holes on the outside of the bolt carrier and try to charge the carrier by hand, it gets stuck as well, which tells me its nothing to do with the charging handle (although i could be looking at 2 different problems). I also checked to make sure buffer tube and recoil tube were functioning correctly (perfect) as well as that the bolt carrier was making contact on the buffer recoil tube correctly(perfect).

At first I thought it was because I had the tension screw too tight in the lower, but after backing it way off, it was the same. I then took my complete upper, put it on my friends AR, and it functioned perfectly. So now im perplexed as to whats going on. I think i can rule out my upper, but maybe my lower is wacky? I never tried putting his upper on my lower, with the pins in them (it did work fine with no pins in the lower and his upper though).

Any ideas?

paladin4415
07-19-2008, 7:58 PM
You already had the right idea. Put a different upper on that lower and see what happens. If you have the same problem with an upper that you know works, than it's the lower.

adamsreeftank
07-19-2008, 8:04 PM
Try removing the trigger and hammer and see if the problem goes away. Maybe it is binding on the bolt or carrier.

jacques
07-19-2008, 8:15 PM
SOunds like something is OOS on the lower. If you can complete the process by putting your lower on his and pin it, that may tell you something. Do a function check with your lower on his upper.

what2be
07-19-2008, 8:24 PM
Try removing the trigger and hammer and see if the problem goes away. Maybe it is binding on the bolt or carrier.

Excellent idea. I never thought that maybe the trigger assembly is binding up as the bolt is traveling back. Hmmmm...ill check that out and post my findings tomorrow.

I bought this receiver bare and have LITERALLy built this thing from scratch, every spring, detent, pin, etc have been thought out by me and assembled. No kits at all, every part was ordered individually, so it shouldnt be a surprise something aint meshing right.

Thanks!

eviioiive
07-19-2008, 9:54 PM
i have the same upper/lower and my build works fine, the only oos area i observed was the pistol grip area....

ar15barrels
07-20-2008, 12:01 AM
Get rid of the oversized Les Baer N/M bolt carrier.
It was designed to run in a standard receiver.
The typical clearance of a bolt carrier to the receiver is 0.007" or more.
The N/M carriers are made oversized to reduce this clearance.
Billet upper receivers are also made smaller than normal to take up most of that 0.007" clearance.
What you have is an undersized receiver bore and an oversized carrier.
Each is to accomodate other standard parts being used, but you are using two non-standard parts.

A standard carrier will run fine in that upper.
Otherwise, you will need to take lapping compound and lap out the bore of the receiver with the oversized bolt carrier.
That will make them work, but it also cuts through the anodizing which is the hard surface protecting the softer aluminum underneath.

I prefer to leave the anodizing intact.
That's why a standard carrier is the best solution.

NeoWeird
07-20-2008, 3:54 AM
While what Randall said is good to know, I don't think that is what's going on here. Going to a smaller carrier may make the problem go away, it does not solve it. If it were in fact an oversized carrier trying to go into an undersizer receiver than he would have trouble putting it in the upper period, let alone charging it. Plus he mentioned that he put it on a friend's lower and it worked fine.

That alone let's you know the problem is within the lower. What it sounds like to me is misaligned threads in the lower, which is VERY easy to overlook in quality control because the hole will in fact be centered and the threads won't show that they are out of spec unless something is threaded into them.

What I think is happening is since the threads are misaligned, the tube is canted off at an angle. The result is as the carrier comes back the buffer tube continues to cant until the carrier makes contact, at which point it pushes on the carrier until the carrier binds against the receiver's opposite wall. The carrier can't bend much at all, so it locks in place. Giving it a good smack in the opposite direction of the bind will jar it over enough to remove the tension of the bind and spring tension sends the carrier home into battery.

My suggestion is take the upper off the lower as well as the stock. Set the lower on a flat surface and take a straight edge of some type, it doesn't have to be a ruler just something you know for sure is straight, and lay it across the receiver body so it passes along the buffer tube. Leaving a small gap helps as you can see down the straight edge as it gets larger or smaller. Rotate the receiver around and check at about 8 evenly spaced locations at minimum as the threads can cause the cant to go in any direction.

If there is a problem, the gap will change either smaller or larger. Keep in mind that a human hair is about .002" thick, so that .007" tolerance that Randall talked about could be the difference of a handful of hairs and that's it. It may not be a country mile as .010" is all it will take for a bind like that. That said, if you have them or access to a pair, a caliper might help if you see no difference. Of course holding the straight edge straight and measuring is somewhat difficult with only two hands so you might need help or some painter's tape to hold the straight edge.

Good luck diagnosing and fixing the problem.

ar15barrels
07-20-2008, 7:59 AM
he mentioned that he put it on a friend's lower and it worked fine.

I missed that.

Check hammer height.

http://ar15barrels.com/tech/hammer-height.jpg

If the hammer sticks up too much, it will bind on the bottom of the carrier.

Clearance as necessary like this:

http://ar15barrels.com/tech/hammer-mod.jpg

ar15barrels
07-20-2008, 8:02 AM
If it were in fact an oversized carrier trying to go into an undersizer receiver than he would have trouble putting it in the upper period, let alone charging it.

The amount of clearance required to simply assemble the parts statically is smaller than the amount of clearance necessary for the parts to move dynamically.
When the clearances are tight, the carrier will bounce back and forth within the receover bore and lock up rather than sliding.
Lube helps, but does not cure this problem.
Lapping the fit out larger or replacing the carrier is the cure.

I have seen two uppers with N/M carriers where the receiver bore had to be opened up.

what2be
07-21-2008, 12:48 PM
Well, I tried my upper on my friends lower, and it worked fine.
I tried HIS upper on my lower, it worked fine.
I tried my upper on my lower, and the bolt carrier would only travel back a few inches before it siezed up.

The problem was the DPMS Bolt Release, as well as my Les Baer National Match Bolt carrier. Both must be just a bit oversize, causing my problem. Pics are attached.

Also notice that I had to file the edge of the thumb release on the DPMS bolt release, as it would fit into the recessed area on the forged upper. Im thinkint the DPMS bolt release is a POS. Anyway, hope this helps anyone else that may have this combination of parts.

BTW, if you look at the pics close, you can see the small horizontal line on the pad of the latch release where its STILL making (light) contact with the bolt carrier. I need to dissasemble one more time and file on it a little bit.
Thanks for all the replies, they got me thinking and helped me narrow the problem.
Ill also make a new post today on my build, since its my first complete AR build from the ground up.


http://i295.photobucket.com/albums/mm154/what2be/P1000031.jpg
http://i295.photobucket.com/albums/mm154/what2be/P1000030.jpg
http://i295.photobucket.com/albums/mm154/what2be/P1000026.jpg
http://i295.photobucket.com/albums/mm154/what2be/P1000025.jpg

ar15barrels
07-21-2008, 12:53 PM
Also notice that I had to file the edge of the thumb release on the DPMS bolt release, as it would fit into the recessed area on the forged upper.

That's not a forged upper.
It's either extruded or billet.

CSACANNONEER
07-21-2008, 12:55 PM
Darn, you figured it out. I put together my ultra cheap (less than $425 total) black rifle this weekend and had a similar problem. My bolt release was sticking a bit. A little spray lube and a few rounds through it seemed to fix it though.

what2be
07-21-2008, 12:57 PM
That's not a forged upper.
It's either extruded or billet.

Your correct, its a billet upper/lower.