MK 1 Cycling issues

[Old Ironside]

Good morning, I'm working on a semi auto Mk1 right now and i'm having cycling issues. the gun will not cycle back after it fires.
- the gun is a mk1 upper receiver mounted on a mk2 lower
- I'm using MK 7 military ammo to test the gun
- I've tried adjusting the gas regulator to all 4 positions - no change
- 1 thing have noticed is that the piston, when fully forward, is only halfway the front gas vents on the gas cylinder.

Any idea what could cause the issue? thanks

Information

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[SNAFU22]

Hi Old Ironsides, on my Mk3, with the piston half way through the vent holes the slide still has about 7/8" to go and the bolt has not started into lock up.

[SNAFU22]

OK, lazy Sunday. The piston on my Mk3 is 6 7/8" from the front of the slide. Please provide a little more information. Did the gun ever work? What happened just before it quit working? Just trying to play psychic here, and not that good at it. Hope this helps

[Old Ironside]

Thanks for the schematic Snafu. the piston is half way across the front row of gas port when the bolt is fully forward. The gun is a rebuilt gun. The upper receiver was not cut up, and I scrounged the rest of the parts. 1 thing I did noticed when I did some comparison with my buddy's mk2 is that in my case there is a lot of play between the bipod sleeve and the gas cylinder. Starting to wonder if the gas cylinder outer diameter is just worn out to much that its letting to much gas escape? I read somewhere that this was a big issue with early MK1s and that's why the gas cylinder what modified.

[WallyG.]

If your gas cylinder has two rows of ports... a rear set and a set that is under the bipod sleeve when it is mounted... you have most likely answered your own question as to why your gun is not fully cycling. Also... is the barrel and gas nut you are using the ones previously used in a semi auto or are they a new addition? If a new addition and they are unmodified... depending upon the semi internals that you are using... the barrel's gas port and the nut's channels might need to be enlarged to allow more gas impulse to drive the greater mass of the semi-auto internals. However, a sloppy fit between the bipod sleeve and a type I gas piston will certainly cause issues. If the gas nut is threaded in (not held in by a cross pin)... it may be REALLY hard to remove without damaging the gun. Caution here. But only two ways to repair... a new gas cylinder or sleeving a bipod to close up the slop/gap.

[SNAFU22]

Could you post a picture of your Mk1.

[AlexRod85]

Hmm, Descent MK1 first 24hours impressions and issues..i can't turn cycling dynamics off and on..