Modern Sporter - Build Discussions

[UR2X3M4ME]

Guys at Brownell's says buffer length versus gas system length is a myth, they are totally independent and not related. Youtube: https://youtu.be/fvINsqlQMp8

Not happy with that. My understanding is that gas system length has a direct impact on timing of the action. It does make sense the buffer system may need an adjustment in either length, spring tension and/or buffer weight to support variations of gas system length.

Rather than re-inventing the wheel, it would be much simpler and practical for more knowledgeable and experiences gunners to chime in share the gospel.

[twinge6805]

Hello Everyone! I have been working on my Modern Sporter build this fall and have run into an issue I am unable to solve. I am not even sure what step would be appropriate to try next to troubleshoot the issue. Despite research via Google I am not finding anything so I am hoping maybe someone else here may recognize my issue and be able to let me know how to rectify it.

I will start by listing all my parts:
-MS Lower (mid 300’s serial)
-MS FA Upper
-CMMG Lower parks kit (pretty much mil-spec Fire Control, Mag Catch, Buffer Catch and Bolt Catch -Parts)
-CMMG Upper parts kit (FA Plunger and dust cover)
-Buffer Tube Kit from TNA (Carbine Buffer tube, Carbine Buffer (3oz), Carbine spring)
-Faxxon Gunner 18.6” .223 Wylde Barrel – Rifle Length Gas
-Faxxon BCG Group – Pre-assembled and Bolt headspace checked to Barrel by TNA
-Rifle Length Gas Tube and Aluminum Gas Block from TNA
-TNA Butterfly Ambi Charging Handle
-Cheapie eBay handguard (ohhunt)

The issue is that the rifle very rarely cycles. Less than one in ten trigger pulls results in the rifle being left in a condition where it can fire a second shot. Usually the rifle will simply fail to pickup a second round and the bolt will go back forward into battery, lock up correctly but the chamber will be empty. Sometimes I will get a feeding error as shown in the attached image where the bolt is over top of the next cartridge as if it failed to catch the base of the cartridge but then pushed it forward from the side of the cartridge and then lost traction on the feed.


If I put just one round in the magazine it will almost never lock open, sometimes it will lock open on the front of the bolt carrier but not on the bolt face like it should so the next round wont feed when the bolt is released since it is too far forward.

I have tried the following ammo/mags in all combos with no changes in behavior with any combo:
- 2 types of ammo (Federal AE .223 Rem 55G FMJBT and PMC X-Tac 62G 5.56 Green Tips)
- 4 types of Mags (2xMFT Pistol, 2x C-Products Pistol, 1x 20 round Gen3 Pmag, 2x5/10 .458 Socom Mags)

Initially I thought it was an issue with the bolt not going far enough back due to the MS deign having less space for travel. The bolt face only went a few mm behind the bolt catch so I took some rubber off the back of the buffer, maybe ¼ to 1/6 of an inch. Enough to allow more travel not so much that the gas key could make contact with the top of the buffer tube. I also took one ring off the buffer spring. This made no difference.

Next I thought I may be the BCG issue so I checked that when air is blown into the gas key it isn’t leaking out, it is but just barely so I don’t think that is the issue. Additionally the gas seals on the bolt are tight enough to support the weight of the carrier if I place the BCG bolt-face down on a flat surface. The internet said this these were decent tests of a proper gas seal on the BCG. I also bought this BCG pre-assembled.

I then disassembled the gas block on the barrel. The hole in the gas block is much larger than the one in the barrel but it was still almost perfectly centered as the carbon buildup shows. There was some extra carbon that escaped around the barrel but it doesn’t seem to get out from under the gas block as it doesn’t go far enough back or forward on the barrel to escape (see attached image). It seems like the seal is good enough but I’d love some input on that.



A bit of carbon is visible on the gas tube indicating some may be leaking out where the gas tube goes into the gas block but it is a very small amount and it seems based on my internet searched this is somewhat normal? (see attached image) Don’t think it is possible to align the gas tube wrong in the block as the roll pin prevents it being installed incorrectly.



The Gas tube going into the upper receiver is not binding on the gas key or causing friction. If I remove the bolt from the BCG and push it all the way forward into the upper receiver and onto the gas tube it takes only 20-30 degree angle and the carrier will slide backwards and free of the gas tube from gravity.

I used the slow-motion mode on my phone to record the cycle of the rifle. Slow motion only goes at about 200 Frame Per Second so I recorded it three times and this attached image
shows a grab of the furthest rearward bolt travel captured on any of the videos. It appears to not being bottoming out on the back of the buffer tube as I can pull the bolt back further with the charging handle than is shown in the image. It is worth noting though that it could be a bit further back since 200 FPS capture is not getting everything, this is just the furthest back of the three videos I captured.



Brass ejection is at about 4-5 o’clock. It is somewhat consistent, never going more than 5 or less than 4. No brass has ever failed to eject with either type of ammo or any magazine. It never hits the brass deflector, I am sure of this because I put a piece of electrical tape on the brass deflector to keep it new looking and the tape has no dents or indication of anything making contact with it. The brass is falling about 4 feet from my feet on the ground in this direction. Cannot be 100% sure on the distance because the brass could be bouncing on impact and I don’t see that since I am keeping my eyes forward to maintain muzzle control. The pile of brass is quite small though, couple feet in diameter. No indication it is being rough on the brass when ejecting, extractor marks are minimal as is the ejector mark. No signs of overpressure I can see. I have all the brass separate and can post images if it helps.

I would really appreciate any input as I am not sure what to try next. I do not have access to other parts to swap out so things like trying a new BCG other part will require a whole new purchase, something I am not keen to do unless there is a high level of confidence it is the root cause of the issue. If there are any other pictures I could take that would provide more useful info please let me know. Thanks!

[PinaKaleada]

It’s highly likely due to undergas OR buffer too heavy OR spring too strong.

I kind of think your gas block is a little leaky judging by how the carbon build up looks like at the port.

I also would not use aluminum gas blocks

[DILLIGAF]

what size is your gasport ? Sounds undergased .

[PinaKaleada]

what size is your gasport ? Sounds undergased .

Gas port should be ok for the 18.6” gunner from Faxon. I have two and they’re made slightly larger than their 20” gunner ones.

Undergas seems like a good call, considering it didn’t pick up the second round like that.

OP: Does it lock back on the last round? If not, then it’s definitely undergassed

[DILLIGAF]

Gas port should be ok for the 18.6” gunner from Faxon. I have two and they’re made slightly larger than their 20” gunner ones.

Nice

[JayCarver]

I would replace the gas block with a high quality one and start there.

[45C]

I think I seen this issue ,maybe last year.
Thought it might have something to do with the buffer tube,being screwed in to far,and not allowing the bolt to travel back far enough.
Maybe, I'm not 100% certain though.

I think the users name was Rohab

[twinge6805]

It’s highly likely due to undergas OR buffer too heavy OR spring too strong.

I kind of think your gas block is a little leaky judging by how the carbon build up looks like at the port.

I also would not use aluminum gas blocks

Ok so the carbon going around the barrel a bit is not normal? The carbon trail doesn't seem to go out from under the gas block but maybe the gas does? I don't think it can be an over heavy buffer as the 3oz carbine seems to be the lightest available.

Gas port should be ok for the 18.6” gunner from Faxon. I have two and they’re made slightly larger than their 20” gunner ones.

Undergas seems like a good call, considering it didn’t pick up the second round like that.

OP: Does it lock back on the last round? If not, then it’s definitely undergassed

As per my original post it never locks back. Sometimes it will on the front of the carrier but never on the front of the bolt where it should. Usually it just doesnt lock at all. Good to hear your 2 barrels had correct gas ports, means it is unlikely mine is the wrong size.

I think I seen this issue ,maybe last year.
Thought it might have something to do with the buffer tube,being screwed in to far,and not allowing the bolt to travel back far enough.
Maybe, I'm not 100% certain though.

I think the users name was Rohab

If I screw the buffer tube out any further it doesnt capture the detent that holds in the buffer. As I said in my OP I already took off about 1/4" from the back of the buffer to allow even more travel without any effect. If I allow the BCG to go back any further the gas key on the top of the BCG will impact with the back of the receiver which I think is really bad. Is it possible I still need more travel? I am talking about when I manually pull it back with the charging handle mind you, which is much further than it appears to be going back from the rifle cycling.

[greg olmstead]

Pretty sure its under gassed. Brass should land between 3 and 4 o'clock. Forward of 3 is over gassed and past 4 is undergassed. Measure your gas port. It should be close to .096". Weigh your buffer. Some of the stuff from tna lacks quality control. You could have got a h2 instead of a carbine.
How does the underside of your charging handle look? The gas key could be scraping on it as I see you bought an El cheapo one. Does your gas key wiggle at all?