Lining up a gas block

[Mark A]

Installing a low profile gas block on my M&P 10 to facilitate the rail I want. I've never installed one before but so far it seems pretty straight forward. My question is in lining up the hole in the barrel and the hole in the gas block. It appears that every thing is straight up and down. But of course I can't see it and I hate to take some things on faith. Is there some kind of test?

 

Is the worst thing that will happen if mis-aligned that the action won't cycle?

 

thanks, Mark

[mikegideon]

Mark the barrel and the gas block with a pencil while you CAN see the holes. Be real persnickety and accurate. 

[KKing]

I use a scale (think precise ruler for machinists).
Measure from the center of the hole in the gas block to the end of the gas block. Then take that measurement and use it to go from the center of the hole in the barrel out however far it was, mark it with a pencil. I then take the scale and put the edge over half the hole in the barrel and draw a straight line back to the other mark I made. I also draw a line up the side of the gas block that is a indicator for where the hole is in the gas block. Now you have a line that indicates where both holes are (gas block and barrel) and a line that tells you where your gas block should stop.

Sounds involved and complicated, but I promise it's not, just hard to put into words. Pretty fool proof if you do it this way

[mikegideon]

I use a scale (think precise ruler for machinists).
Measure from the center of the hole in the gas block to the end of the gas block. Then take that measurement and use it to go from the center of the hole in the barrel out however far it was, mark it with a pencil. I then take the scale and put the edge over half the hole in the barrel and draw a straight line back to the other mark I made. I also draw a line up the side of the gas block that is a indicator for where the hole is in the gas block. Now you have a line that indicates where both holes are (gas block and barrel) and a line that tells you where your gas block should stop.

Sounds involved and complicated, but I promise it's not, just hard to put into words. Pretty fool proof if you do it this way

 

You did a lot better than me. What he ^ said.

[Dolomite_supafly]

Also, take into account for the handguard retainer. The gas block should not sit flush against the barrel shoulder because of this. The handguard retainer is .040" thick so the gas block will sit .040" farther back if you do not take that into account. 10 sheets of paper is about the right thickness.

 

You will need to center it both side to side and front to back.

[KKing]

[quote name="Dolomite_supafly" post="1047183" timestamp="1381533817"] Also, take into account for the handguard retainer. The gas block should not sit flush against the barrel shoulder because of this. The handguard retainer is .040" thick so the gas block will sit .040" farther back if you do not take that into account. 10 sheets of paper is about the right thickness. You will need to center it both side to side and front to back.[/quote]Alright, I've built several AR's, and I do mean several....but that term is new to me, handguard retainer I mean. What you're referring to only applies to non free floats correct? Like GI M4 handguards? That's all I can think of, since all I've ever used are free float handguards. I will reinforce your point though, the gas block should not ever sit against the shoulder of the barrel Edited October 11, 2013 by KKing

[TripleDigitRide]

Has anyone ever used something like a large paperclip, with an "L" shape bend in it? Slide that into the barrel, sticking the end through the barrel hole and into the gas block? Is something like that even possible?

[mikegideon]

Alright, I've built several AR's, and I do mean several....but that term is new to me, handguard retainer I mean. What you're referring to only applies to non free floats correct? Like GI M4 handguards? That's all I can think of, since all I've ever used are free float handguards. I will reinforce your point though, the gas block should not ever sit against the shoulder of the barrel

 

Not sure that's true for lo profile gas blocks, that are never used with that retainer. You need to measure.

[KKing]

Not sure that's true for lo profile gas blocks, that are never used with that retainer. You need to measure.

That's kinda what I thought, or all I could figure anyway.

[KKing]

Has anyone ever used something like a large paperclip, with an "L" shape bend in it? Slide that into the barrel, sticking the end through the barrel hole and into the gas block? Is something like that even possible?

I think it would prove to be more of a chore than a help, I could be wrong though

[mikegideon]

I have installed a couple of lo profiles. I just don't remember for sure

[Dolomite_supafly]

I will say that 90% of the the time the gas port on the gas block is larger than the port so it wouldn't matter but I have seen a couple that were small enough that a misalignment could affect flow. Best thing to do is compare the gas ports. I normally measure from the edge of the gas block to the center of the port on the gas block. Then compare that to the distance from the shoulder to the center of the gas port on the barrel. If there is a difference then you need to figure out of the port on the gas block is big enough that is doesn't matter. If it does then you need to offset for the thickness of the handguard retainer, .040".

[LngRngShtr]

measure and mark as above and a quick and dirty test to see if it is at least somewhat aligned is a quick shot of gun scrubber down the gas tube from the receiver end, while holding the upper muzzle lower than receiver,  if it runs out the muzzle you have a passageway if not you have an alignment issue

 

John

[mikegideon]

I shot some CLP down a gas tube once. Interesting effect when I shot the gun. It lubed everything :)

[KKing]

I shot some CLP down a gas tube once. Interesting effect when I shot the gun. It lubed everything :)

Big cloud of smoke?

[mikegideon]

Big cloud of smoke?

 

Not a big cloud, but it rolled out of every crack in the receiver

[Dolomite_supafly]

You get the same effect with whatever you squirt in your suppressor. I use hand sanitizer as suppressor coolant. After the first shot the action is soaked in it. Same thing if I shoot the suppressor with WD-40, water or wire pulling gel even though they don't work as well as the hand sanitizer.

[Mark A]

Thanks for the replies. I did have the up and down part scribed but I made an assumption ( there is that word again ) on the front to back part.

Anyone wanna guess that I am probably .040 off???

Oh well, off it comes again in the morning. I take solace in the fact that for every dumb question I ask and every mistake I make there is at least one TGO'er that will learn from that for their upcoming builds.

Tis a good thing I got over being embarrassed a long time ago...

Mark

[mikegideon]

Thanks for the replies. I did have the up and down part scribed but I made an assumption ( there is that word again ) on the front to back part.

Anyone wanna guess that I am probably .040 off???

Oh well, off it comes again in the morning. I take solace in the fact that for every dumb question I ask and every mistake I make there is at least one TGO'er that will learn from that for their upcoming builds.

Tis a good thing I got over being embarrassed a long time ago...

Mark

 

Before you take it off, see if you can see a gap in this video...

 

http://www.youtube.com/watch?v=yiouuNqPZpg

[Clod Stomper]

Don't sweat it TOO much. It isn't rocket surgery. My first couple, I did the precise measuring, scribing, etc. Now, I just eyeball it over the receiver rail. It has worked out just as well.

It is a good idea to measure the distance from the shoulder on the barrel and the gas port and compare it to the distance from the back of the gas block to the gas hole. While many or most gas blocks are made to be used with a handguard end cap, I have used at least one (low-profile) that was not. It lined up perfectly with the gas port when bumped up snugly against the shoulder on the barrel.

And like Dolomite said above, most gas block holes are large enough to allow for a slight misalignment, front-to-back and side-to-side. Even a couple hundredths.

[RED333]

Thanks for the replies. I did have the up and down part scribed but I made an assumption ( there is that word again ) on the front to back part.

Anyone wanna guess that I am probably .040 off???

Oh well, off it comes again in the morning. I take solace in the fact that for every dumb question I ask and every mistake I make there is at least one TGO'er that will learn from that for their upcoming builds.

Tis a good thing I got over being embarrassed a long time ago...

Mark

No such thing, the only dumb ones are the ones NOT ASKED.

And yes I do learn, hanging around here is better than collage.

[Oh Shoot]

....And yes I do learn, hanging around here is better than collage.

 

I dunno,  gun collage can be darn cool:

 

 

 

 

:)

 

- OS

Edited October 13, 2013 by Oh Shoot

[mikegideon]

I dunno,  gun collage can be darn cool:

 

 

 

 

:)

 

- OS

 

I hope you know... I left that one for you :)

[Oh Shoot]

I hope you know... I left that one for you :)

 

Only fair, being I'm semi-official Ombudsman and English Nazi, eh?

 

- OS