Short Stroking Fun!!

[localfiend]

I was directed here for more info, the gun in question isn't a carbine (20") but I hope you can forgive me. Here's a thread on another site for reference but I'll post a summary here.

http://forums.second-amendment.org/index.php?topic=3784.0

I have a 20" Stag upper (yes, I know, a yucky stag) that has not functioned properly from day 1. The gun will function fine for the first magazine, as it ggets warmer and into a second mag the gun will start short stroking. The first few malfunctions start with the bolt barely catching the back end of a case, and as it gets even warmer the bolt wont even come back far enough to grab another round and I'm stuck with a strait pull bolt action.

Here's a list of all the things I have tested/tried to diagnose the problem:

1. Swapped out for known functioning magazines

2. Tried multiple types of ammo (LC M193, PRVI M193, Barnaul 55g, Some LC 62g, the Aim US Man. stuff, and even a little wolf) - The upper does not discriminate, they all malfunction it just doesn't take as long with the wolf.

3. Tried multiple lowers( from multiple manufacturers), some with an A2 stock setup and others with a Carbine setup.

4. Tried a massive dose of CLP instead of my standard light coat with no change.

5. Checked the gas rings and gas key for leaks, wear, looseness. They are all fine even when hot.

6. To further rule out a bolt/carrier problem I have tried multiple sets with no change.

The only thing I can think of is that the gas system is leaking somewhere once it gets hot - or something in the upper is out of spec enough to rub on the carrier when it warms up.

Knocked off the FSB last night to check things out. Here are a few pics:

Gas Port:
http://i59.photobucket.com/albums/g295/localfiend/M4/gasport.jpg
(As best as I can measure with a set of calibers the port ID is right around .093-.094 which according to some info I found on arfcom is correct for a 20" barrel with an OD of .75. You can also see that the gas block lines up properly from the soot marks and it also seals as well, I can't notice any gas leaking from the FSB on this upper)

Gas Block:
http://i59.photobucket.com/albums/g295/localfiend/M4/gasblock.jpg

Gas Tube:
As you can see, there is a bit of soot buildup even outside of where the tube plugs in to the gas block, it just doesn't seem like enough to be causing this kind of problem as my other uppers seem to be about the same.
http://i59.photobucket.com/albums/g295/localfiend/M4/gastube.jpg
(Gas tube OD is ~.071 and the end towards the carrier key is ~.076, it lines up with the carrier fine in the upper though the soot pattern is not quite even)

Gas Tube Plugin thingy(err whatever its called):
http://i59.photobucket.com/albums/g295/localfiend/M4/Gastubeplugin.jpg
(This is the only place I can determine a leak, as there is soot on the tube where it plugs in, however, it doesn't seem to be any worse than any of my other uppers)

Upper Receiver:
http://i59.photobucket.com/albums/g295/localfiend/M4/upper.jpg
(This looks a bit dirty, but mostly the light combined with a bunch of solvent as I had just given it a pretty good scrub with a chamber brush, either way it looks to me like the ramps and everything are fine)




I just want to get as many opinions as I can before I put this thing back together and figure out what else to try. Anybody have any ideas for me to check out?

[Robb Jensen]

I predict your problem is at the carrier key.....

[localfiend]

I predict your problem is at the carrier key.....

That is one of the things that it could be, but I should have ruled that out as I have tried 2 different bolt/carrier combos that function properly in other weapons. All I can say is that this has been a very confusing problem.

[theJanitor]

gas tube partially plugged? does your BCG move freely in the upper?

[localfiend]

gas tube partially plugged?

Nope, gas tube was clean. I'm hoping its something simple like that though.

[localfiend]

does your BCG move freely in the upper?

Missed the BCG question. Yes, it moves freely in the upper. On another forum a member suggested that I should try dropping the BCG into the upper to see if it would lock all the way in just from gravity and it passed the test with more than one group. I still need to check it when hot, but right now am exploring other options while the FSB is still off.

[Iraqgunz]

Personally if this was a new gun I would have sent it back before I started tearing it apart.


Nope, gas tube was clean. I'm hoping its something simple like that though.

[localfiend]

Personally if this was a new gun I would have sent it back before I started tearing it apart.

I would have, had I not purchased it through a dealer rather than straight from Stag. As it was, it took 7 months to get the upper after quite a few shipping/billing problems. I was fed up and was just glad to get out the situation with at least something for my money.

Besides, I might as well tear at least 1 AR down so that I can see how it all goes together. I'm also thinking of adding a railed gas block once I get this thing running, the compact ACOG I was using and FSB didn't get along.

[Gunfighter13]

If I was to guess I would say you have a .223 chamber not a 5.56.

[localfiend]

If I was to guess I would say you have a .223 chamber not a 5.56.

Now that's something I didn't think of. The barrel is stamped 5.56, and I just went and double checked the brass I fired yesterday and don't see any obvious problems. No popped primers or pressure rings, but perhaps the chamber is just tight enough to cause malfunctions without excess pressure signs?

There any way for me to determine what kind of chamber I have?

[localfiend]

Bump. Just to get back on the first page, I'm curious to see if there's a way for me to test and see what chamber I've got.

[Iraqgunz]

Well then at this point I would obtain a new quality gas tube, and FSB pins and re-install everything. I would double check everything and then refire and see what happens.

[LIL-COMMANDO]

I would check your buffer and replace your action spring. And like the other guy said I would have sent back the weapon to Stag and make them pay for the shipping. I helped run several Carbine classes and 95% of the guys were running the Stag and there was not one malfunction out of any of the weapons..

Since you have a 20" I assume its a fixed stock. I would check the buffer and action spring as I said and make sure its not worn or a carbine set up. You cant interchange a buffer and action spring from a fixed stock to a carbine stock. Hey I know it sounds crazy but since your taking things apart just checking this out is simple.

Good Luck

[oakmax]

I have a 20" Stag upper (yes, I know, a yucky stag) that has not functioned properly from day 1

I suggest contacting stag, If theyare worth their salt you should beable to have the upper replaced,orat the least repaired.
Good luck

[DocMinster]

Well then at this point I would obtain a new quality gas tube, and FSB pins and re-install everything. I would double check everything and then refire and see what happens.

I am wondering ...are the stag FSB pins straight like RRA or tapered like Colt.

Why I ask...Tapered pin help aid on providing a tight seal and it you have even a minute deviation in tolerances and HAVE straight pins that will hold a minute deviation in place. This MAY be leaking gas as the barrel heats up and the hole expands due to the increased temp.

This is just a guess of course :)

Doc

[Iraqgunz]

Good question. I would contact Stag and ask them. I also would not have dicked around with a new weapon that was malfunctioning. I would have contacted Stag explained what happened and told them to fix it ASAP or I would have returned it for a refund and bought a good AR.


I am wondering ...are the stag FSB pins straight like RRA or tapered like Colt.

Why I ask...Tapered pin help aid on providing a tight seal and it you have even a minute deviation in tolerances and HAVE straight pins that will hold a minute deviation in place. This MAY be leaking gas as the barrel heats up and the hole expands due to the increased temp.

This is just a guess of course :)

Doc

[10MMGary]

Bump. Just to get back on the first page, I'm curious to see if there's a way for me to test and see what chamber I've got.

Get a 5.56 reamer and in a turn or two you should know if you have a true 5.56 chamber. My three and a half Stags have been fed a 5.56 diet approx 70% to 223 Rem 30%. In well over 7,000 rounds between the lot of them never a malfunction with any factory loaded 5.56 or 223 Rem. Just to be redundant :D I too would have sent back to Stag Arms on their dime and let them deal with it.

[Robb Jensen]

Send it to me.........................I'm told I can make them run. ;)