I'm in the planning stage of assembling an AR. I don't need another one. I just want one that's been assembled to my own standards.

I pretty much know what's involved. My only quandry is how to know if the port in the gas block is perfectly aligned with the gas hole in the barrel.

In a perfect world all gas block ports from every manufacturer would line up with the gas hole in barrels from every manufacturer.

What is the slick trick to insure that both are lined up right?

I've watched a few videos and know what's involved in order to just affix the gas block to the barrel. But it seems to me that you just have to trust that the port in the block and the hole in the barrel are both exactly where they're supposed to be. I've done enough machining to know that that's not always the case.

So how do you make sure that they align?

The hole should be TDC and the gas block hole is bigger than the gas port. Don’t butt it up against the shoulder, there’s a space there for the end cap that’s no longer used much.

Measure and use an air hose to verify. It’s not that difficult but it can make you check and recheck.

Thanks.

I’ve never tried it but saw a video on YouTube called something like, “QuickTip: gas block spaghetti“ that might help.

All the blocks I have put on we’re dimpled and have been fine..

I use one of these to take the guesswork out

https://www.midwayusa.com/product/1019522258

I changed the forend to a free float on a Windham M-4 I have. Needed to use a low profile gas block to allow the the hand guard to fit. The new gas block wasn’t built to butt up against the shoulder on my barrel like the factory gas block/front sight did. I had to use calipers to get measurements for the gap needed to align the gas port to the block. I also marked TDC on the barrel and block. Surprisingly my math skills worked the first try and got things in line.

Not OP, but how far out should the gas block be seated to maintained the end cap spacing, if that's even needed since the block has an oversized hole?

Not relevant in B's scenario due to (I'm assuming) him not having one, but I use a borescope.

As an aside, I've found the pre-dimpled barrels to pretty much be on the spot.

1 - Boroscope
2 - The Jig that Kenlguy recommended
3 - Gas Block Genie https://gasblockgenie.com/gas-block-genie/

Guess I'm just a lucky so-and-so.
Of th ef 15 (+/-) AR's I've assembled, the only "gas block" failure I had was me not properly seating the gas block. Backed up and properly seated the gas block and it worked like a champ.

I won't sit here and argue the adjustable gas block (AGB) isn't a useful tool, but I've never installed one and the only 3F (feed, fire, function) failures I've had, I traced back to wrong parts or improper installation. 👍

I have to laugh, as I suggested using a small rubber hose a couple years ago and got some good laughs. Hey, that's how I do it. It makes sense and it works. I've never been one that likes to buy fancy gadgets, when something cheap that is laying around works just fine. A piece of vacuum line works good for this.

For the most part, it’s probably not needed but I just like to be as precise as I can be and some barrels are cut differently. The thin stamped end cap can’t be more than the thickness of a business card.

Lots of guys use an 0.025" feeler gauge with good results.

My Anderson came with a high profile gas block which listed a tad to the left. It's a $400 AR and that's the only problem it had, so I ain't bitching.



Being OCD about irons, I had to fix that; so I leveled the gas block to the top rail. I miscalculated the set-back however and created a hi-cap single shot.

I set the block back against the cap with just enough tension to hold it in place, which realigned it with the gas port. It has run fine ever since.

Anderson had also dimpled the barrel in the out-of-square location. I pulled that screw, ground the tip flat and reinstalled it just deep enough for contact with the barrel. I used the opposite screw to lock the gas block square with the rail. Both screws got degreased, a dose of blue loctite and 'ample' torque to tighten them. I doubt they move but if they do, I'll add a dimple where it needs to go.

I measure to the center on the gas port to the GB shoulder & from the rear edge of the GB to the center of the hole & place it accordingly.............gets it very close.

You will usually end up around .020" off the GB shoulder on the barrel.

If the GB is a set screw type & the barrel dimpled, I still verify the location............never seen one off, but I use only decent quality barrels from reliable manufacturers.

MM

I have put the gas tube into the gas block and when you slide the tube into the upper make sure it is in the middle of the upper and TDC of the barrel. Leave the gas block .025" away from the shoulder and everything should line up. It hasn't failed me yet.

kwg

What you're describing is what I meant when I said that I had done enough machining to know that not everything ends up where it's supposed to be.

But one suggestion. Since you've now got it in the correct position, you could pin it in place and it's there for good.

Thanks Bristoe. I've straightened the front sight bases on various AKs/SKSs so I'm familiar with the process.

Boy, I do not miss straightening sight towers! Get them within 2 MOA (.012") and you might as well give up.

I have adjustable gas blocks on most of mine, but I use suppressors (they are nice to have to be able to reduce gas flow given the longer period of back pressure) - that’s the only reason I have for spending the extra $ on them, as I’ve never needed one for function either.

I have used the SLR adjustable gas blocks and have their jigs for dimpling the barrel (pretty fool proof for proper installation of their gas blocks).

Pretty good article on installing the various types of gas blocks - https://www.firearmsnews.com/editorial/how-to-install-ar-gas-blocks/382939 . I’m sure someone here will (nit)pick it apart as always…

SLR's AGB's are by far the best I've used, just expensive; ditto for their handguards. Outstanding company, IMHO.

MM

Another site I visit has a BUNCH of AR "assemblers" on it.
The AGB is the first thing they run to when something goes wrong. They even drill out the barrel port to get more gas.
Some of them use blue Loc-Tite to seal the gas block to the barrel! 🤪 ....etc, etc, etc....