gas tube length on pistol build SBR question

[hardcase]

I'm finally getting my first SBR build with pistol length gas underway as I now have all the necessary parts.

I did a finger tight fit to make sure stuff was meeting as at should before permanently installing the parts.

I notice the gas tube extends into the upper about an inch+- a bit further than my carbine AR fifteens. I put the bolt in place and everything seems to fit but I am unsure about the extra length of the gas tube.

Is this a common finding with pistol length gas tubes?

[Sithlord]

i have two barrels with pistol gas... The shoulder on one of the barrels is a bit further back. When I installed the gas block, I put it against the shoulder, and ended up with a big pita to get the bolt out. It was maybe 1/4" further back, but....

Moral: make sure the gas block is properly aligned. Properly installed, the gas tube shouldn't come out much, if any, further than any other length gas tube.

[hardcase]

I went back and rechecked now that I have the barrel installed. My gas tube measures 6 7/8 inches as it rolls around on my build table. When I held it in place without the gas block, just eye-balling it looks like the gas tube extends into the upper about 1". This seems to be appropriate. A carbine length gas tube is about 9 3/4 inches as those listed on Brownells.

[r.tenorio671]

...you will find variation in lengths depending on manufacturer, i.e., Midway lists the VooDoo pistol gas tube at 4.846", Noveske at 6.75" and the DPMS at 6-5/8".

If during your mock up assembly the BCG would close properly and the bolt would close & lock up properly without the tube butting up against the rear of the BCG's gas key, you should be okay. If during the mock up test you see the gas block being shoved forward when the BCG is closed under finger pressure, then you may need to get a shorter one. The distance your gas block moves will give you an idea of how much shorter you need to go.

HTH

[hardcase]

While I've got you guys, how is the best way to line up the port on the barrel with the port on the gas block?

[Guest762]

While I've got you guys, how is the best way to line up the port on the barrel with the port on the gas block?

Is it not dimpled on the back?

[r.tenorio671]

While I've got you guys, how is the best way to line up the port on the barrel with the port on the gas block?

...you can use your calipers to measure the distance from the rear edge of the gas blocks port to the rear edge of the gas blocks body and compare that to the distance from the rear edge of the barrels gas port to the shoulder behind it. Some barrels have the gas block seating journal cut in such a manner to accommodate the thickness of the seating plates typically used in the standard M4/M16 handguards. As for centering it, you can use a straight edge to pencil in an extended line on the barrel as well as the fore & aft 90 degree edges of the gas block, that bisects the center of the port holes, then mating the lines up when installing.

If you don't feel comfortable doing all of that, Brownells offers some alignment pins on thier site:

http://www.brownells.com/gunsmith-tools ... 68371.aspx

[dellet]

To set the depth, slide it on upside down, mark it, rotate into place. The hole for the gas port is drilled from the bottom and becomes a set screw.

Some clamp on types this will not work

[hardcase]

I love this rifle. I had a few dumb moves but got it, finally. I put the gun into one of my gun rests and kind of sneaked up on it with my right hand (I'm a lefty) for a couple of test shots. The first two shots cycled. Next one a no go. I field stripped and cleaned it. Tried again and no-go again. Frustration big time.

I looked at the gas block through the key-mod handguard and it was forward about 1/8 inch. I had forgotten to tighten the Allen screws on the gas block to tighten it down and it moved forward I fired it three times in our back yard into a dirt bank and it seems to work perfectly now.

I would say this 9" barrel seems quite a bit louder than a 16" barrel. My next step is optics and put one of my cans on it.