Sticky chamber

Chazp89 · Sep 2, 2017

I'm sure it's been covered before, but I have an early dpms ar with a 20" mystery heavy barrel (gun was bought used and I can't find any markings, I'm sure they are hidden under the float tube) its in .223. basically the bolt gets stuck closed and it takes a healthy thump on the butt with pressure down on the charging handle to get the bolt open. At one point I ran wolf steel case ammo but only ran a few rounds before I pawned it off on someone else after I found my rifle doesn't like it. Mostly I run American eagle out of it now. My friend recently encountered the same issue with his rock river arms 16" ar in .223 and hasn't resolved the problem in his.

I think I may have gotten the issue with mine figured out after a thorough chamber cleaning and light polish. It seems to function alright with dummies but I still need to take it out and test fire.

Has anyone else encountered the same thing? Any other suggestions to try before I take it out?

The one with the issue is the scoped one in the front

mopar92 · Sep 2, 2017

1st you need to figure out exactly what your rifle is chambered in.
Until you KNOW for sure I would only use brass cased .223
Federal sells a bulk pack of .223 at Walmart for a decent price.
Using 5.56 in a .223 chambered gun can lead to the pressure fluctuations that can and will lead to the problem you describe.

It can also be caused by incorrect assembly or a misaligned gas block/tube/port. A simple way to check for this problem is to remove the scope and any sights and hang the rifle vertically then ducttape a string to the buttstock with a bit of weight (I use fishing weights) on the end. This will let you see if everything is physically in alignment but you could still have a misdrilled port or gas block or drilled the wrong size.

If all that checks out then use a magnet to check if your gas block is aluminum. If it is should be replaced as they always develop leaks.

If all that checks out then I'd recommend pinging Chad at SOTAR to have him evaluate it. Or just ping him rightaway if you want the easy button approach.

Chazp89 · Sep 2, 2017

mopar92 said:
1st you need to figure out exactly what your rifle is chambered in.
Until you KNOW for sure I would only use brass cased .223
Federal sells a bulk pack of .223 at Walmart for a decent price.
Using 5.56 in a .223 chambered gun can lead to the pressure fluctuations that can and will lead to the problem you describe.

It can also be caused by incorrect assembly or a misaligned gas block/tube/port. A simple way to check for this problem is to remove the scope and any sights and hang the rifle vertically then ducttape a string to the buttstock with a bit of weight (I use fishing weights) on the end. This will let you see if everything is physically in alignment but you could still have a misdrilled port or gas block or drilled the wrong size.

If all that checks out then use a magnet to check if your gas block is aluminum. If it is should be replaced as they always develop leaks.

If all that checks out then I'd recommend pinging Chad at SOTAR to have him evaluate it. Or just ping him rightaway if you want the easy button approach.

Thanks for the advice, it has shown the same problems with Winchester brass .233 but I will pull the free float tube off to check just to be safe. It does have a steel gas block.

Chazp89 · Sep 2, 2017

Update:

Pulled the float tube off and the barrel is void of markings, the barrel nut/tube assembly was only hand tight.... NOTE: I did not put this one together

Chazp89 · Sep 2, 2017

It appears there is no separate barrel nut, no seam or anything, its built into the tube

Chazp89 · Sep 2, 2017

Chazp89 said:
It appears there is no separate barrel nut, no seam or anything, its built into the tube

Never mind... its just stuck and dirty

Chazp89 · Sep 2, 2017

Looks like when the gas block was installed, the holes for the pins that retain it are oversized on the barrel, allowing the block to move a bit in all directions.

dist1646 · Sep 2, 2017

Does the bolt stick when function checked without ammo?
The float tube is a standard DPMS unit and the barrel nut is the part with the holes in it. It should separate from the rest of the tube. It may be locktited together and require heat to separate from the tube.

dist1646 · Sep 2, 2017

Chazp89 said:
Looks like when the gas block was installed, the holes for the pins that retain it are oversized on the barrel, allowing the block to move a bit in all directions.

That is a problem. Follow Mopar's advice, take the easy button and let Chad take a look at it.

3paul10 · Sep 2, 2017

Nicely done fellas.

Chazp89 · Sep 2, 2017

dist1646 said:
Does the bolt stick when function checked without ammo?
The float tube is a standard DPMS unit and the barrel nut is the part with the holes in it. It should separate from the rest of the tube. It may be locktited together and require heat to separate from the tube.

Testing it dry it functions fine. When I tested with dummies it would stick but after I gave the chamber a light polish it doesn't stick with the dummies

Yeah that was my mistake, it is a separate part, just has the loctite holding it I guess.

Chazp89 · Sep 2, 2017

Did dpms ever sell a rifle with a heavy 20" barrel that would be completely unmarked? Or would you say that's a no name aftermarket barrel?

It's going on the rebuild list because I was never totally satisfied with it anyway

John from MD · Sep 2, 2017

The DPMS barrels that I have owned are marked with MP, caliber and twist on top just behind the muzzle.

d33rhunt3r · Sep 26, 2017

Chazp89 said:
Did dpms ever sell a rifle with a heavy 20" barrel that would be completely unmarked? Or would you say that's a no name aftermarket barrel?

It's going on the rebuild list because I was never totally satisfied with it anyway

DPMS Bull 20 has a 20" heavy barrel

Pinecone · Sep 26, 2017

If it stuck with dummies, it might be a very tight chamber.

I have one Colt that will do this with anything that will not drop FULLY into the case gauge.

When I did a carbine class, they would do drills where they would load the mag with some dummy rounds. We needed a steel rod to get the bolt open/the dummy out.

Sticky chamber

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