Seeking input: Rifle length gas system - short stroking

[American123]

I've already spent a good amount of time looking this up online. Have come away with a mountain of "things to consider" to see which ones might fix the issue. Thought I would also ask here:

With a PSA 18" barrel upper, and a rifle length gas tube. I was experiencing failure to eject, and short stroking issues round after round. It did it with multiple Pmag magazines and also the D60 drum. It was failing to extract from the chamber.

I realized that the BCG which I had heavily lubed (who knows how long ago) had gotten pretty sticky, even into the firing pin channel, I removed all the lube and tried again. After that it was extracting but not loading the next round. After going through a mag like that, I was able to get it to go through another mag without having failures, but when I tried the D60 again it would have failures to feed. I tested the mags I used the same ammo with a 10.5 inch pistol upper and all the mags, including the D60 worked without a hitch which led me to believe it wasn't the mags or ammo and was more related to a gas issue.

Online research said it could be anything:
- too small gas port in the barrel
- tolerance stacking - try a different BCG
- misaligned gas block and gas port
- some type of metal shaving obstructing the gas port or tube.
- lower buffer weight needed, or lighter spring

I was using a carbine lower. I've had really positive experience with other PSA 18" rifle length uppers. If others have experienced similar issues, what did you identify to be the problem?

I can definitely use deductive tests to try to narrow this down further, I was hopeful after what I already did try with it that it was cycling well through 1 mag, but definitely don't trust it enough for it to be anything but a range tester until I can do more testing with it. I'm happy to offer it to the minds here for what they hypothesize.

Edit to add: I need to put a lot more rounds through it, but I found with the D60, it would fail to feed when the D60 was full, but after limping through half the mag, it would load the next ones. The D60 did not have this problem with the 10.5 pistol.

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[Geardo]

My 18" rifle length PSA did the same thing when I first bought it.

The fix for me turned out to be the NiB coating on the BCG was too thick, and the BCG was stiff. Just took a little polishing and it worked great after that.

Sounds like you are on the right track with your probable issues.

I would also suggest looking at ammo. If it Will cycle full power 5.56 ammo, but not .223 or steel case I would leave it as is.

 

[American123]

My 18" rifle length PSA did the same thing when I first bought it.

The fix for me turned out to be the NiB coating on the BCG was too thick, and the BCG was stiff. Just took a little polishing and it worked great after that.

Sounds like you are on the right track with your probable issues.

I would also suggest looking at ammo. If it Will cycle full power 5.56 ammo, but not .223 or steel case I would leave it as is.

That's good input. One thing I noticed was that when I pulled the upper from the lower, holding the upper straight up and down, if I pulled the BCG out a ways using the CH and then let it drop, it was a lot slower in falling than any other uppers I have experienced.

That would definitely match with what you said.

Logically speaking - if the BCG was a hair oversized wear and tear might minimally reduce dimensions enough to matter, even the smallest amount of extra roughness that could make the difference.

Ammo is something I can continue to check, but I was shooting my reloads which I pretty much shoot exclusively and other PSA 18" uppers with the exact same parts run them without issue - they are loaded rather hot.

First thing I will try when I get back out is another BCG - should have just swapped with the pistol right then and there, but I didn't think about it.

Thanks!

 

[titsonritz]

Short stroking almost always has something to do with gas system. As @Taco_lean mentions use full power ammo for diagnostics.

- too small gas port in the barrel - A good port size would be .0995 for a 18" rifle length gas system. Normally I'd be surprised if a PSA gas port size is too small, they are often too large, but an 18" w/ rifle length gas tube is a different animal and they need a pretty good size hole to make up for the short dwell time.

- tolerance stacking - try a different BCG - Possible, but I'd be looking at the carrier key as well

- misaligned gas block and gas port - Being PSA they most likely did not dimple the barrel or loctite the set screws in the LPGB so this has been a historical issue.

- some type of metal shaving obstructing the gas port or tube. - as mentioned above, the LPGB should be easy to remove to check this, while at it, find someone with a pin gauge set and measure the port.

- lower buffer weight needed, or lighter spring - If the lower is a PSA it most likely already has a carbine buffer so going lighter really isn't an option. IMO it shouldn't require that light of a buffer in the first place. My 18" SPR will runs with a rifle buffer, a A5H3 and an H2 in a standard carbine RE.

As far as rounds not extracting, I've seen a lot tight PSA chamber that leave a pile of shaving after being hit with a 5.56 chamber reamer, regardless of what is printed on the barrel.

 

[American123]

Short stroking almost always has something to do with gas system. As @Taco_lean mentions use full power ammo for diagnostics.

- too small gas port in the barrel - A good port size would be .0995 for a 18" rifle length gas system. Normally I'd be surprised if a PSA gas port size is too small, they are often to large, but an 18" w/ rifle length gas tube is a different animal and they need a pretty good size hole to make up for the short dwell time.

- tolerance stacking - try a different BCG - Possible, but I'd be looking at the carrier key as well

- misaligned gas block and gas port - Being PSA they most likely did not dimple the barrel or loctite the set screws in the LPGB so this has been a historical issue.

- some type of metal shaving obstructing the gas port or tube. - as mentioned above, the LPGB should be easy to remove to check this, while at it, find someone with a pin gauge set and measure the port.

- lower buffer weight needed, or lighter spring - If the lower is a PSA it most likely already has a carbine buffer so going lighter really isn't an option. IMO it shouldn't require that light of a buffer in the first place. My 18" SPR will runs with a rifle buffer, a A5H3 and an H2 in a standard carbine RE.

Thanks for the input. I agree, it should be running ok as it was with the lower. With the minimal testing I already did I liked that it showed some improvement. First thing I will try is just swapping the BCG - see what that does, because if I have a BCG that is a hair oversized and causing problems, I don't want that to get mixed in to another rifle down the road. After checking that I'll pull apart the gas block and check for misalignment.

 

[titsonritz]

Whose BCG are you using?

If you end up pulling it apart, be sue to measure the port, it all starts there.

 

[No_Regerts]

Whether you're strokin' short or strokin' long, it's all good as long as it goes off.

 

[American123]

Whose BCG are you using?

If you end up pulling it apart, be sue to measure the port, it all starts there.

I don't do the best job keeping track, but I only have a few BCG brands with PSA being the most likely.

I'll be sure to measure the port if I have to pull it apart. The BCG swap is just the easy test to do before going to the port - which I should have checked the BCG already, but didn't think about it.

 

[American123]

Whether you're strokin' short or strokin' long, it's all good as long as it goes off.

Ha, Ha...

Worst case scenario, I had a single shot .223 with a 30 round magazine to manually cycle from.

Wife and I were just laughing about the southpark episode about the 'shake-weight' AKA the stroker workout.

 

[Geardo]

I don't do the best job keeping track, but I only have a few BCG brands with PSA being the most likely.

I'll be sure to measure the port if I have to pull it apart. The BCG swap is just the easy test to do before going to the port - which I should have checked the BCG already, but didn't think about it.

Try toolcraft next purchase.

 

[titsonritz]

Try toolcraft next purchase.

Be careful with Toolcraft, the make a wide range of BCG some good, some not so good.

 

[arakboss]

Be careful with Toolcraft, the make a wide range of BCG some good, some not so good.

Not to hijak thread but which toolcraft bcgs are not so good?

 

[titsonritz]

Not to hijak thread but which toolcraft bcgs are not so good?

Toolcraft will provide pretty much anything you want, that is why you see $69 BCG, usually with crappy carrier key stacking and no HPT. When I see 9310 steel instead of C158 bolt, nitride or nickle boron finishes, I run away.

 

[P7M13]

- misaligned gas block and gas port

As mentioned earlier by @titsonritz , this is a known issue, and I have experienced it personally.