AR 15 Noob question -- low profile gas blocks and FSB

[germs]

What exactly are low profile gas blocks for? I'm building my first AR, and one of the things I would like for it is to have the front sight base as far forward as possible, to get the maximum sight radius. Could this help me do that? BTW, I do know that the FSB on an AR15 is where it is so that there is plenty of barrel past the gas block.

I have done a ton of searching tonight, learning about dissipators and such -- but would something like the following be possible?

Could I get a 16 inch barrel with a low profile gas block, put on a longer than usual handguard that goes almost to the end of the barrel, and then put the FSB on right at the end of the handguard, just an inch or two from the end of the barrel? Obviously, the FSB won't be involved in the gas system, it will only be there to provide a front sight.

What I am looking to do is something like this:
http://www.pewpewtactical.com/wp-content/uploads/2015/03/Bushmaster-MOE-Dissipator.jpg
(I know this link is a picture of a dissipator, it just seems low profile gas blocks made them obsolete, right?)

I would like to use the normal FSB for all this, by the way, I am not interested in using a rail of any kind. I think what I typed out is possible, but I just can't be sure without asking!

 

[Inebriated]

Low profile gas blocks are used to fit under a rail.

What you want anyway is a "mock-dissipator" with a mid-length gas system and a dead FSB up at the rifle position. The mid-length will be less finicky than a rifle-length gas system on a 16" barrel, simple as that. Spikes offers that for sure, and I think PSA or someone else makes one too. BCM prototyped the same thing, and landed on a mid-length system... but they never produced it, and for that I wept.

But if you have a barrel you want one installed on, you'll have to turn the barrel down to .625" and have a .625" FSB installed, since a gov't profile barrel isn't quite .750" ahead of the gas block. ADCO for one will do that work, and I'm sure local 'smiths would as well.

 

[germs]

Thanks for the excellent info! I'm starting from scratch, so any barrel at all will work for me.

I wasn't able to find anything by Spikes -- their pages for uppers and barrels don't seem to have anything:
http://www.spikestactical.com/enhanced-uppers-16-c-147_176_235.html

However, it looks like PSA has some, in theory -- though most are out of stock.
http://palmettostatearmory.com/catalogsearch/result/?q=dissipator#

I'm looking for as much of a pencil barrel as I can get, though it looks like the options aren't too many here, so that might not be an option

Also, unfortunately for me, as a lefty, I'll need the brass to eject to the left, but I guess I can just replace the upper and keep the barrel and handguards that PSA (or another company) offers to get what I need.

EDIT: I am surprised more options for this don't exist actually, since getting a longer sight radius seems like something everyone would want, right? I know the handguards being longer adds a little weight, so maybe that is why? Just wanted to be sure I wasn't missing anything.

 

[greyling22]

you could also put a low pro gas block on and add a 15" rail, then stick on some flip up iron sights.

 

[Inebriated]

I could have sworn Spikes was still making their dissipator style uppers. It may've been discontinued.

But if a lighter profile is what you want, it actually might be the better decision to go the custom route. Get a standard mid-length gov't profile barrel, and have someone turn the front down to .625". ADCO is who I've looked at using, and they'll re-profile and re-park for $80. Their FSB install cost, however, is... proud... at about $100 plus the cost of the FSB (~$30 from BCM for their .625"). No idea if that's the going rate, though. Just know that a lot of companies won't work with surface-hardened barrels, so a nitrided or similar treatment makes it a no-go.

But having $200 wrapped up in barrel work before you even pay for the barrel itself is something you have to consider. I'm at a point where, if I build another AR, I'm just going to eat the cost and do it exactly as I want... but it's because I just want it, and not having it annoys me more than not having that $200 would. In all reality, the practical benefits of the mock-Dissipator just aren't all that great. Mid-lengths offer a lot of pro's, not the least of which being additional sight radius and handguard space over the carbine, but still keeping the balance point further back on the rifle.

I love the concept, I've had it in the back of my mind for years, but I bet market research would show a larger preference for a free float and rail-mounted front sight, and that's why we don't see many off-the-shelf options. At least, I'm guessing that's the case...

 

[Cheesemaker]

Sounds like you are looking for a rifle-length gas system. This will put the gas port as far down the barrel as possible.
You wanting to keep the normal FSB means the front sight and gas pick up are together.

 

[grampajack]

you could also put a low pro gas block on and add a 15" rail, then stick on some flip up iron sights.

This is exactly what I was going to suggest. If you plan using only irons, then you can get fixed iron sights as opposed to flip ups. They're fully featured, namely the rear sight, with big elevation and windage adjustments.

In any case, I wouldn't recommend building a dissipator. The gunsmith fees alone would run into the hundreds. You would have to pay him to turn down the barrel and install the FSB, then just pray that he pinned it on straight. If it's a dissipator you want, just buy one off the shelf.

 

[edwardware]

I looked at the Spikes and PSA mock dissy, and bought the PSA. I like it.

 

[Sam1911]

As others have said, a low-profile gas block is to fit under a fore-end tube or rail system. It doesn't have a front sight riser on it. Here's a thread about how I made my own, if you're interested: http://www.thehighroad.org/showthread.php?t=758773

As someone else suggested, you really don't have to have a regular AR front sight block installed on your barrel at all. I used a 15" handguard on my build and mount a front sight on it, which puts it almost at the very end of the 16" barrel. Like so:

If you peek through the holes in the rail/tube you can see just how much farther forward the new front sight is from the original mid-length front sight block/gas block location.

 

[grampajack]

As others have said, a low-profile gas block is to fit under a fore-end tube or rail system. It doesn't have a front sight riser on it. Here's a thread about how I made my own, if you're interested: http://www.thehighroad.org/showthread.php?t=758773

As someone else suggested, you really don't have to have a regular AR front sight block installed on your barrel at all. I used a 15" handguard on my build and mount a front sight on it, which puts it almost at the very end of the 16" barrel. Like so:

If you peek through the holes in the rail/tube you can see just how much farther forward the new front sight is from the original mid-length front sight block/gas block location.

What optic is that?

 

[Sam1911]

It's a Russian PK-AS red dot, which I really like a lot ... except that no matter how much I fool with it (over the span of about 12 years!), I just can't get it to work consistently and hold a zero. So it's retired now.

Great concept, though. Cool dot-in-a-doughnut reticle and brilliant clear huge field of view.