I just got my AR-15 and installing the Blackhawk Rifle Length Quad Rail.
What do you do with the gas tube? I took off the metal front sight so now how do I attach the gas tube?

You are going to need a low profile gas block if installing a rifle length quad rail on a carbine gas system.

Looking at what you have

installing a low profile gas block will that dampen the wobble of the quad rail?

No. If you are using a free float handguard, it should secure to the barrel nut. Some manufacturers use the original barrel nut some use their own barrel nut. Difficult to say without knowing what you are working with.

OK, I think the issue is you have a two piece drop in for a rifle length gas system and trying to use it on a carbine length gas system.

I have the M&P 15 Sport 2 with blackhawk Rifle Length Quad Rail

So far I have removed the stock plastic hand grip, removed the 2 rivets which hold the metal front sight and removed the gas tube pin, and pulled back the gas tube. I can not get the front sight off the barrel because I have to remove the stock flash suppressor but the front sight is moved full forward

AR-15 Rifle Quad Rail Forend (Two-Piece) - BLACKHAWK!

Do I have to modify the gas system to fit a rifle length quad rail? or can I install a low profile gas block?

a low profile gas block will fit under your new rail. You will need to remove the stock flash hider to remover the old gas block and install the new low profile gas block. This is the route that i would take.

It's not that hard, but you do need something to hold the upper in a vise.

others have just dremeled off the sight to where it will fit under the rail. I wouldn't do it.

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SDMF

Also, the gas port on your barrel dictates whether it is a carbon length or rifle length gas system, not the length of the rail.

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SDMF

I hate to say but you have the wrong handguard for what you are trying to do. Drop ins (two piece hand guards) are made to be user friendly/zero modification. If you had the correct length all you would have had to do is remove the plastic handguard and "drop-in" the two piece quad rail. Delta ring secures the breach end and the plate behind the front site secures the muzzle end.

A little advice? You really needed to understand how the rifle is put together and functions before you start ripping it apart. However, we are here to help.

If that Blackhawk rail you linked is the one you have, it will not work with the rifle you have. That rail is a two piece (drop in) that requires a front sight block to mount to. 2 piece rails use 2 attachment points, one at the barrel nut and one at the front post/gas block. Not to mention, it's a rifle length and thus needs a rifle length gas tube and system (rifle length is longer). The Sport has a carbine length gas tube and system, so that means the rail you have is too long to fit your Sport.

Now my suggestion (as others have stated). Since you already have the front sight post unpinned, remove the flash hider and take it off entirely. Then return the 2 piece rail you have and purchase a free floating one piece rail and a low profile gas block. The reason for the free floating rail is it is only one piece (as opposed to 2) and mounts at one location only, the barrel nut end of the rifle. It doesn't need the front sight/gas block to mount. The free float rail IMO is a better way to go anyway (for various reasons) so it's a win/win for you. That then allows you to install the low profile gas block using your existing Sport gas tube and then install the free float rail over the low profile gas block. Since it doesn't use the gas block to mount, the rail just slips over it and mounts at the barrel nut. That's what I think you should do.

Your only other option is to re-install the front sight post and tube and buy a carbine length 2 piece drop in rail and return your rifle length 2 piece rail.

Sorry for the bad news, but as I said originally if you had done a little research you probably would have known the rail you bought wouldn't work. Good luck, let us know what you decide.

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Still carrying my S&W 642

When I replied, I did not realize that the blackhawk was a 2 piece drop in. I assumed it was a free float rail. I have since looked it up and can't believe that Blackhawk is asking that kind of price for a drop in two piece.

I also suggest returning the 2 piece and buying a free float. When I built my AR, I used a 13 1/2" keymod Megafins 2 freefloat rail from Strike Industries. The price for that and their low profile gas block was less than the price that Blackhawk has the drop in rail listed at.

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SDMF

It's obvious that you have no idea what you are doing. While we are here to help, you'd be better off going to your local gun shop and talking with someone who is knowledgeable.

If you just want a quad rail, reassemble everything and by a carbine length drop in rail. If you want a longer handrail, you have a couple options. You can retain the A2 front sight/gas block and buy a Troy delta or Charlie rail. These rails use the stock barrel nut. Or, you can go further down the road by switching to a low profile gas block and free float rail. You'll likely need to change your barrel nut for the free float rail though. Best of luck to you.

Solution A
Exchange the rifle length 2pc handguard for a carbine length 2pc. This will allow you to leave the front sight in place and the handguard will be secure.

Solution B
Get a free float handguard of whatever longer length you want and a low profile gas block. This will require removal of the front sight and flash hider that you are already struggling with. It will also require the removal and replace of the barrel nut. Specialty tools required AR receiver vice block and AR armorers wrench. You'll also need a torque wrench but that isn't AR specific. And it would be a good idea to get a FAT wrench to properly torque the gas block screws for whatever gas block you get.

Solution C
Leave the rifle alone until you learn how an AR works and what you actually want and need. Then decide if it's better to buy the rifle or replace the parts on your current rifle.

Good luck