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Posted: 11/3/2011 7:19:37 AM EDT
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Here is a pic of my newly assembled AR. I haven't secured everything down yet because I am waiting on headspace gauges and I am unsure about the gas block. It is a clamp-on, low-profile block from YHM and I bought the barrel stripped. I have read all I could find on aligning the gas block with the gas hole on the barrel, but I can't find anything definitive. Does anyone have a tried and true method of insuring that the alignment is correct? I have read about using smoke and just blowing through the barrel, but it all seems kinda Aggie-rigged to me (forgive my wording, Aggies). Any assistance would be appreciated.
http://i5.photobucket.com/albums/y167/mickkennedy29/MicksAR.jpg |
Eyeballing it will work 99% of the time... unless you have eyes like this  .
You can take a brass punch and softly draw a very short line/mark longitudinally inline with the gasport on the barrel (line heading towards chamber). You also will need to draw a vertical line on the gasblock inline with the gasport hole on the gasblock. When mounting the gasblock, align the two brass "lines" to form an "L". That would be the most "accurate" way of doing it, yet eyeballing the alignment will work just fine. 
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That helps with left right alignment but not fore and aft. Left right is the easiest to get lined up with the eyeball but you cannot see fore aft alignment. For that you need one of two things. If your barrel is dimpled on the bottom, that dimple will be exactly opposite of the gas port, so be it a clamp on or set screw, there will be a hole there in the gas block that you can center up with the divot and then lock it in place. A barrel with a dimple is almost idiot proof since that aligns the gas block not only fore and aft but also port to starboard. That is a simple job then.
But if your barrel is not dimpled on the bottom, I make the fore/aft alignment then by doing a little math. Measure from the barrel ledge behind the gas port to the center of the gas port. Write that down. Now measure from the back of the gas block to the center of the gas port (again, there will be a hole in the bottom of the gas block that will be in perfect alignment with the gas port on the top side). Look through that hole and get your caliper's depth gauge in the center of the gas port and write that down. Subtract the two and then use that figure to shim the gas block off the ledge behind the gas port. Then eyeball left/right alignment and lock her down. I've done this at least 75 times on my 25 builds (I am always changing something) and have yet to miss one or come even close to missing it. All the burn marks on the barrel after removing the gas block the next time show perfect centering of the gas port in the gas block hole. |
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