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1/25/2018 7:38:29 AM
Posted: 2/24/2006 3:07:13 PM EST
When installing a low-profile LaRue gas block on an LMT M4 profile barrel, I noticed that when aligned with the circle of parkerizing on the bottom of the barrel that was left by the removed FSB the gas block did not index off the barrel shoulder.

Basically I'm lining up the new gas block by centering the hole for the rear set screw on the bottom of the lo-pro gas block with the circle of parkerizing residue on the bottom of the barrel that was left when the barrel was parkerized with the standard FSB in place.

Is this correct?
Link Posted: 2/24/2006 3:56:53 PM EST
Here's about all you can do:

1. Make a mark back on top of the barrel shoulder (where the original handguard cap seated) to designate top dead center.

2. Measure the distance from the back of the gas block to the center of the gas hole inside the gas block.

3. Measure the distance from the barrel shoulder to the center of the gas port.

The difference between #2 and #3 measurements will tell you how much gap (if any) to leave between the gas block and the barrel shoulder.
Link Posted: 2/25/2006 1:52:50 AM EST
[Last Edit: 2/25/2006 1:58:02 AM EST by dennysguns]
Link Posted: 2/25/2006 5:06:05 PM EST

Originally Posted By dennysguns:
On a GI type barrel just move the block forward the same amount as the thickness as the front handguard retaining sheet metal. To center the gas block on a GI type barrel do the following.
1. Get your barrel installed with the barrel nut in the proper position.
2. Install the gas tube in the LaRue gas block.
3. Slide the gas block onto the barrel.
4. Line up the gas block with the barrel pin. Tighten one set screw.
5. Do the bolt drop test. That is remove your bolt from the carrier and with the ejection port cover open, drop the bolt carrier in the barrel. If the bolt carrier drops right in you got it straight.
6. Red Loctite the other screw. Remove the first screw and red Loctite it. Done

On a match barrel there will be a centering mark on the bottom of the barrel. Line up the cap screw hole with that mark and you are golden. If the barrel only has one centering mark line up on the mark and install the front set screw and then red locktite the rear set screw. Remove the front set screw and drll a dimple in the barrel around 1/16" deep. Clean out the thread and with red locktite install the front set screw. Done hr


This is exactly what I wound up doing with the addition of adding a tiny (the tip of a 5/32 drill bit) dimple to the barrel for each set screw, and it worked perfectly. Got it all assembled and dumped three mags through it this afternoon and it ran like a top.


Link Posted: 2/26/2006 4:37:05 AM EST
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