Help diagnose this CMMG

dvdcrr · April 16, 2020, 01:57 PM

ok my buddy bought a 18" stainless bull free float CMMG upper in about 2012. Complete factory free float varmint upper When he first got it it was short stroking. We took it down and it appeared that the gas block was not centered on the gas port when the sxrew was cinched into the dimple. Do you think it is likely that the dimple was indexed improperly? That is kind of what we thought but now looking back, they had to have used a jig right? That makes me wonder if maybe it was just a misalignmmet of the block.
How woild you go about fixing this?

DnPRK · April 17, 2020, 02:14 AM

The pilot hole for the gas block set screw is also the hole for the gas passage. Loosen the set screws, pull the gas block forward and measure the distance from the barrel gas port to the barrel shoulder. Compare that measurement to the distance between the set screw hole and edge of the gas block.

Also use a sharpie to mark a line on the barrel shoulder that points to the gas port. When you reinstall the gas block, the gas tube should be centered over that line.

Check the bolt carrier key for gas leaks or looseness.

Check the bolt gas rings to make sure they aren't loose inside the bolt carrier.

Personally, I don't like set screw gas blocks and use clamp-on or pinned gas blocks.

dvdcrr · April 18, 2020, 01:14 PM

will do thanks

HiBC · April 19, 2020, 09:57 AM

There are other choices,but Yankee Hill makes a good clamp on steel gas block.

April 16, 2020, 01:57 PM	#1
dvdcrr Senior Member Join Date: July 5, 2010 Posts: 665	Help diagnose this CMMG ok my buddy bought a 18" stainless bull free float CMMG upper in about 2012. Complete factory free float varmint upper When he first got it it was short stroking. We took it down and it appeared that the gas block was not centered on the gas port when the sxrew was cinched into the dimple. Do you think it is likely that the dimple was indexed improperly? That is kind of what we thought but now looking back, they had to have used a jig right? That makes me wonder if maybe it was just a misalignmmet of the block. How woild you go about fixing this? __________________ "All warfare is based on deception. Hence when we are able to attack we must seem unable....when using our forces we must appear inactive. When we are near we must make the enemy believe we are far away."Sun Tzu The Art of War.

April 18, 2020, 01:14 PM	#3
dvdcrr Senior Member Join Date: July 5, 2010 Posts: 665	will do thanks __________________ "All warfare is based on deception. Hence when we are able to attack we must seem unable....when using our forces we must appear inactive. When we are near we must make the enemy believe we are far away."Sun Tzu The Art of War.

April 17, 2020, 02:14 AM	#2
DnPRK Senior Member Join Date: August 24, 2001 Location: LC, Ca Posts: 1,917	The pilot hole for the gas block set screw is also the hole for the gas passage. Loosen the set screws, pull the gas block forward and measure the distance from the barrel gas port to the barrel shoulder. Compare that measurement to the distance between the set screw hole and edge of the gas block. Also use a sharpie to mark a line on the barrel shoulder that points to the gas port. When you reinstall the gas block, the gas tube should be centered over that line. Check the bolt carrier key for gas leaks or looseness. Check the bolt gas rings to make sure they aren't loose inside the bolt carrier. Personally, I don't like set screw gas blocks and use clamp-on or pinned gas blocks.

April 19, 2020, 09:57 AM	#4
HiBC Senior Member Join Date: November 13, 2006 Posts: 8,288	There are other choices,but Yankee Hill makes a good clamp on steel gas block.