Christensen barrelled upper short stroking

[mumzer]

Yeah...that again...brand new upper built as follows.

Gibbz side charger
WELL USED - JP QPQ full mass carrier
NEW - JP Bolt with gapless rings
20" Barrel - SAAMI Spec
MLGS - yeah i know its weird
fixed gas block
.067 gas port
JP Captured spring recoil system

port alignment is perfect. Port isn't plugged. No maching burr or trash in it.

test ammo is Hornady factory amax load.
test weather is 90*, baro at 29.99

symptom - short stroking regularly, but not every round. Perhaps 1 out of 3 rounds will result in a F2F. All brass dribbles out gently beside the rifle and will stay on the bench in my shooting stall. I'm against a wall but they lack the velocity to go bouncing off the table.

Bolt will not lock back on an empty mag. Mags are ASC

Seems pretty obviously undergassed to me and port seems way too small. I'm thinking i need a .078"

any alternative suggestions?

thanks

[Jtmbtech]

Lighter spring on SCS?

[A5BLASTER]

Maybe try a heavery spring on the SC, but from what I understand your gasport does sound like it is cut small maybe opening it up and going with a adjustable gas block might help it.

This is just some thoughts I had, a member with more knowledge then me should be along shortly im sure.

[ricsmall]

I've no experience with an MLGS on 20" pipe but think somewhere around .080" would b close. Try lrrpf52. He has lots of knowledge and understanding of the dwell time and all that. Most 20" pipes with rifle length will have a .093" port.

Richard

[mumzer]

thanks guys...this is consistent with my math...port is too small. thanks.

[mumzer]

Lighter spring on SCS?

if i didnt run an m4 upper on it as well, this is just what i would do next.

[lawndartmike]

I swapped a light weight carrier from my Grendel into my short stroking in my 7.62x39 and that cured it. But while swapping the carrier I got the bolts mixed up so that was not the permanent answer to the short stroking problem. I'm not a fan of drilling out the gas port so.....

I bought one of these light weight buffers and it now cycles perfectly with the standard bolt carrier: http://www.tacticallink.com/Taccom-Ultra...-AR15.html

I swapped a light weight carrier from my Grendel and that also cured it.

[mumzer]

thanks. useful comps

[LRRPF52]

I would consider a .078" gas port (5/64"), which you can choke or buffer and spring if needed.

For a gas-operated firearm to run reliably, it has to be over-gassed to some extent in order for the BCG to travel far enough to the rear to allow stripping of the next cartridge from the feed device, across a reasonable temperature and conditions range.

[mumzer]

Went to a .785 (wire size #47). Runs like a champ now. Ejection is relatively gentle and shells fall around 4 o'clock.