How to align front sight base?

[nrakid88]

Well, I just bought a YHM flip up front sight/gas block a while ago. I already have it installed and it shoots XM193 fine. Not a single hiccup. With wolf steel cased polymer... 50/50 for failures. I was wondering if maybe I got the alignment off. Thats a tiny port hole in the barrel, so I imagine being a tiny bit to one side or the other could have a big effect on gas flow. Basically my question is how do I know if I did it right? Is there some kind of tool to use to align the holes? Or is it all with eyeballing, which i think i did a pretty good job at.

[shodog805]

what does the primer look like on the ftf? is it dimpled? dont really know how to align the front sight/ gas block but i would seem that if the port was partially obstructed it would cause your rifle to short stroke not ftf. just mho.

[NSR500]

Can you give us more information on your build? Is this a Carbine, Mid, or Rifle system?
Are you using a Rifle buffer system or Carbine buffer system?
If you're using a Carbine buffer system, what spring and buffer combo are you using?

The reason I ask is because playing with buffer weight and spring can affect cycling too. It is not always the gas system.

[Grumpyoldretiredcop]

Aligning your FSB and gas tube

Remove your upper receiver and take the handguards off (assuming you don't have a free floating handguard - if you do, there's no need to remove it).

Pad your vise and lightly clamp your upper receiver in it. You won't be able to use most armorer's vise blocks and you don't want to bend your upper. If you have barrel blocks, use them instead.

Lay a torpedo level (about 6" long, inexpensive to buy at your local hardware store) across the rails on your upper (assuming that it has rails - if it's an A1 or A2, you can lay the level across the top of the carry handle), move your upper until the rail is level from side to side. Back to front isn't as important but it's easier if the receiver isn't too far out of level from front to back.

Now lay the level across your front sight base/gas block (hereafter referred to as "FSB") to see if it is level from side to side. In the case of the OP's YHM flip-up base, leave the sight down and lay the level across it.

If the FSB isn't level, loosen the setscrews or clamping screws and gently move it until it is. If you are using standard handguards, make sure the FSB is firmly seated against the handguard cap also. If the FSB is level and firmly against the handguard cap, the FSB is not likely to be the problem.

Tighten one of the screws and check the FSB again for level. If it has moved, loosen the screw and relevel it. If it is correct, tighten the remaining screw(s).

Reinstall the handguards and check for level one more time.

Apply carbon black, marking fluid, or soot from a candle flame to the gas tube inside the upper receiver.

Install the bolt carrier group, reinstall the upper onto the lower, then cycle the action a few times.

Open the action (you need not remove the front pivot pin), remove the bolt carrier group, then carefully look at the gas tube. If the marking you applied to the tube is rubbed off on one side or the other, the gas tube is rubbing on the inside of the gas key. Using a small screwdriver or other instrument, GENTLY pry the end of the gas tube away from the side that is rubbing. You don't want to bend it so far that it rubs on the other side. Recheck and repeat as required.

Clean the end of the gas tube and the inside of the gas key, reinstall the bolt carrier group. Your gas system is correctly aligned.

[ar15barrels]

Well, I just bought a YHM flip up front sight/gas block a while ago. I already have it installed and it shoots XM193 fine. Not a single hiccup. With wolf steel cased polymer... 50/50 for failures. I was wondering if maybe I got the alignment off. Thats a tiny port hole in the barrel, so I imagine being a tiny bit to one side or the other could have a big effect on gas flow. Basically my question is how do I know if I did it right? Is there some kind of tool to use to align the holes? Or is it all with eyeballing, which i think i did a pretty good job at.

Do you have a handguard cap between the gas block and the barrel's shoulder?
If you do and the front sight is standing straight up, then the port is aligned.
If you don't have a handguard cap, leave a 0.025" gap between the barrel's shoulder and the rear face of the sight.
Make sure the sight is standing straight up.

[Grumpyoldretiredcop]

Randall has a better eye than I do - and gets more practice, too. When in doubt, listen to the expert - that would be RR, not me.

[ar15barrels]

The hole in the gas block is generally 0.125" or larger.
The hole in the barrel is between 0.062" and 0.096"
Due to the size difference in holes, the sight can be up to about 7 degrees left or right and not impede the flow of gas at all as long as it's correctly centered front to back.
The hole in the barrel will be 0.295" forward of the barrel's shoulder.
The hole in the gas block will be 0.275" forward from the rear face.
This allows for the 0.020" thick handguard cap.
Now, the difference in hole sizes will generally allow you to run the gas block all the way back against the barrel shoulder without accounting for the handguard cap.
I have rarely seen this to be a problem.
Even still, it's wise to account for the handguard cap anyways as that will ensure the gas port is perfectly centered within the gas block.

Also, go fire a few rounds and then remove the gas block.
The gas will make a ring on the barrel around the gas port, showing you exactly how well aligned the gas block was...

[nrakid88]

Wow, i never thought about the thickness of the hand guard cap. I am running free floated so it just never went through my mind. I will peel it apart and check for that ring and see if its in proper alignment, its sitting right against that shoulder, if its not properly aligned then I am guess it will probably just need to be moved forward.