Just finished my first AR15 build and need some help dealing with short stroking. I have done a fair amount of online research and have done extensive troubleshooting including switching ammo, switching mags, realignment of the gas block and swapping upper and lower with a buddies AR.  I have narrowed the issue down to being in the upper and not an issue in the BCG.  I finally have gotten to the point where 9 of 10 times it will feed the next round, but it still only engages the bolt catch on the first round out of a clean and freshly lubed action.

The fun is a bit of a Fraken-rifle.  Here's a list of the components I used:
- bushmaster lower (stripped), upper and BCG
- carbine buffer tube, spring and buffer
- Voodoo Innovations EVO ultralight 16" barrel, 5.56, mid-length
- VLTOR .625 low profile clamp on gas block
- Spikes Tactical gas tube

MORE IN NEXT POST...

The most recent change was to move the gas block .030" toward the muzzle. This was because I found that the distance from the back of the gas block was .030" shorter than the distance from the barrel shoulder to the gas port.  Prior to this change the gun had only fed a second round once in 64 rounds fired.

One thing I noticed is the hole in the bottom of the gas tube is a fair amount smaller than the matching hole in the gas block leaving a lip in that opening. Also there is evidence of gas leakage between the gas block and the gas tube.

Another thing I noticed was that the BCG (with bolt removed) was riding against the top side of the gas tube add it moved into battery.  At the time I moved the gas block forward I tried bending this just slightly.  The last time I took the gas block and tube off I noticed non-uniform fouling on the back end of the gas tube.

While the gun is now feeding the next round, it still only locks open that first time and did jam 1 out the 10 rounds I fired. This makes me wonder if the chamber is rough and the issue is excessive extraction force.

Here's my question, which of the following should I try?
- drill out the opening in the gas tube to as close to the size of the hole in the gas block as I'm able to get it.
- tweak the gas tube a little more till there is no perceivable drag
- polish the chamber
- just keeping shooting it to break it in
- something else entirely?

Thanks in advance,
Brian

I had a factory built Sig with short stroking issues.
Turned out the gas port was too small per specs I found on this and one other board.
Have you checked gas port size?
16" w\ carbine gas system and .750 gas block diameter should have min of .070" gas port and as big as up to .087".
I drilled mine to .070" and is has run fine even on low powered ammo.
I posted the specs for all barrels listed in the trouble shooting section.

Looks like your specific barrel type isn't listed but it is something to work from.

http://www.ar15.com/forums/t_3_66/663944_Sig_M400_short_stroking_fixed.html

I didn't have pin gauges to accurately measure the gas port, but eyeballing it with my calipers, I estimated the port size at .063".  My gas block Dia is .625" so the port size would be right at the min per the link you posted.  I'll get some pin gauges and check it.

Thanks!

Any other thoughts?

I would first try a new gas tube to remedy the leak at the gas block. A large enough leak would cause the carrier to not cycle back enough to strip the next round. Also, is the carrier the full auto or semi auto type? I ask because if its not a gas supply issue, it could be that the bcg is cycling faster than the magazine can keep up with. I've read on multiple occassions that over-gassed guns are far more common than under-gassed. Couple that with a semi auto carrier, and/or too light of a buffer, and/or too light or heavy of a buffer spring will result in the action outrunning the magazine.

OP, most likely you need to adjust the gas tube so it doesn't rub on the carrier key. It will cause short stroking. Take a Sharpe and mark the bulged part of the gas tube and slide the carrier back in, remove and check where the color is removed from the tube. Bend the tube and repeat until there is no contact with the tube.

Strip the BCG completely and clean well and oil it up wet. A friend had the cycle his new upper a couple hundred times before the gas seals seated and the bolt slid easy in the carrier. His short stroking went away with just that.  

Check the inside of the upper for signs that the cam pin is rubbing. The anodizing will be rubbed off if it is.

Make sure you have a std buffer, not a heavy buffer.

The leakage around the gas tube at the GB is common. You can always use a little blue Loctite to seal it. Turn the weapon upside down and dab it on the outside of the tube. It has to be free from oil first so use some acetone or isopropyl  to clean the tube and GB first.

Most likely it is mainly your tube alignment. Very common issue.

Thanks for all the input. I do have a semi-auto BCG but swapped in a friend's that is broken in and is a full auto. Still had feeding issues.

I'll check the gas tube alignment.

Any thoughts on drilling out the hole in the gas tube to more closely match the gas block?

Is there an easy way to know if it is possibly over gassed?

Thanks!

You didn't mention whose buffer tube you used.  Manually retract the charging handle and see how much clearance you have between the bolt and the bolt catch.  If you're not 3/16 of an inch or bigger your buffer tube might not be to spec.  I've had that happen.

Where did you find those numbers?
That's pretty big for a carbine gas system on a 16" barrel.
.087" is way overgassed.

Not only that, the OP has a mid-length gas system on a 16" barrel.
All of my BCM and DD 16" mid-lengths have a .0625" gas port.
Regardless of gas block journal diameter.

The buffer tube was from Joe Bob Outfitters.  Not sure of the brand. Checked the clearance behind the bolt catch and it's about .2" which is about the 3/16" mentioned.

I have another friend who has a heavier buffer I could try to see if it is cycling too fast.

Checked the BCG without the bolt installed and it's making very slight contact now that I have tweaked it.

Still seeing a little bit of gas leakage between the gas tube and the gas block. Maybe I'll try a little loctite on it to see if it will seal up.

One thing I noticed when I just removed the BCG was that the two leading edges of the top wear surfaces show more wear than any other surfaces. Is this normal?

What ammo were you shooting?

I have mostly been using American Eagle .223.  I have also tried Winchester 5.56, DRT and some local reloads that shot well in my buddies AR.  After the last adjustment I made by offsetting the gas block .030" forward the American Eagle shoots fairly well with only one FTF in 10 rounds.  Still trying to figure out why it won't consistently lock open with an empty mag.

I found the specs on this board and on the M4carbine board.
Then I confirmed the specs with a local smith.
Details are in the thread I linked to in the above post.
I realized op's barrel is mid length, I specifically stated his barrel type was not on the list I provided.

Still need more HELP!

Here's an update on my troubleshooting
- Bought a wolff reduced power spring = FTF on almost every round, usually with the bcg wedging against the top of the round
- went back to standard spring
- Tried some winchester 69gr. BTHP match ammo = cycles every round proprely and when gun had been freshly cleaned bolt locked open on empty mag
- Tried an "H" buffer = multiple feeding issues including FTE on rounds that were feeding correctly
- Tried "H" buffer with reduced power spring = same problems
- returned to standard spring and buffer
- Tweaked the buffer tube to minimal contact with gas key = no change

As the winchester match ammo was a fair amount slower than the others, my theory is the increased time between when the bullet passes the gas port and exits the muzzle allows more gas into the gas tube. If this is the case, the only remaining option I see is to enlarge the gas port in the barrel.  Any thoughts?  I have been hesitant to do this just because it's not something that can be undone (at least not easily).

Thanks in advance for any suggestions!

It would probably be a much less headache if you took the rifle to a qualified gunsmith. In the meantime I suggest dropping a bore gauge down the barrel.

I have that Voodoo barrel and it has a .078 dia gas port just like my 2 other 16" middy pencil barrels do.
Measured with pin gages.

If the holes in the gas tube, block, and barrel are not lined up then obviously that would be a problem.
Do not enlarge the gas port more than one drill size at a time, then test fire.
The hole in the gas tube is smaller than the hole in the block in every one I have looked in.
If in doubt gas tubes are cheap get another.
It does not hurt to have spares.
ETA: in more than thirty years I have only seen one undersized gas port.

^^^ This

Thanks everyone for the input. I'm going to get some pin gauges or even drill bits and try to get an accurate measurement on the gas port.  I am certain just by eyeballing it with my calipers that it isn't .078".  Before I drill it out I'll talk with a friend of mine who is a gunsmith.  I sent an email to voodoo and got no response.  Guess I'll try again to find a phone # for customer service.

Good news is the barrel shoots very well. Shot 4 shots from the bench in under 0.6" @ 100yds.  3 out of 4 were under 0.4"!

Figured it out!!!

Pulled the gun apart (again!) and used a drill b bit as a pin gauge. A .063" bit will fit snugly into the gas port, but stops short of entering the bore. If I flip the bit over and put it into the gas port, I can just see barley see the very tip in the bore. Looks like Voodoo stopped just a little short.

Now the question of what to do. Try to get Voodoo to fix it or fix it myself. I'm tempted to buy a nice new .063" drill bit and drill it out by hand using my tap handle.

Thoughts on the best way to fix this?

Thanks!

Use a stop of some kind so you physically cannot drill too far.

If you do not have the right tools send it back! I use a drill press and I watch through the bore so I can see the bit come through. Do not even attempt freehand.
Lately I've had several companies that tell me to just keep the defective part and send me a new one. If VooDoo is a legit company they need to know that the monkies in their shop F'ed up and be given a chance to fix it.