I just purchased the M&P 15 it's the model with the detachable carry handle. I was thinking of replacing the front iron sight with a gas block that has a rail on top. I've seen seen several on different sites but noticed they have different diameters. I don't know which one is the right one for my rifle. (I don't have the rifle yet, it should be here Friday)

If anyone can help me or point me in the right direction I would greatly appreciate it!

What are you going to put on the rail? Ive seen somewhere that there is a rail that will attach to the front sight. I dont think I would bugger up the FSB which is F marked just to put a rail on it. Google FSB with rail and you may have a much better option.

I have a M&P15 OR (optic ready) which has the railed gas block on it (comes without any sights).

The barrel diameter, just forward of the gas block, is .760". I doubt S&W is using a different barrel profile between our two models.

Here is a photo of it.

__________________
Stu

Your barrel is a standard profile barrel, so it's 0.750 at the gas block.

If you do put a rail gas block on your rifle, do NOT put a plastic sight on it (like the Magpul BUS). It *will* melt from the heat of the barrel.

Also, if you put a rail gas block on your rifle, try to get one that is the same height as the rail on the hand guard. That will give you a much broader choice of sights to put on it - maybe a dozen sights (make sure the sight you buy is a "rail height" sight). If you get a "gas block height" rail gas block, you might have a choice of two or maybe three sights to put on it.

Thanks to you all for the information. I was going to switch out the FSB and put on a magpul back up sight. That is until jsimmons warning. Looks like there's no need to continue this lol.

On another note. I've read on several other boards that the longer the gas tube the "softer" the recoil. I also read that a longer gas tube will lengthen the life of the rifle. Has anyone looked into this? I'm curious if it will actually make that much difference in the recoil and longevity of the rifle.

To have a longer gas tube, the gas block needs to be further forward, so the gas hole in the barrel would need to be further forward, so you'd have to plug and drill the barrel.

BTW - I also have the S&W M&P 15 OR. I put the Magpul MBUS on it before I saw the waring about them "melting". I have only 600 rounds so far, without issue. That includes once dumping 4 30 round mags as quick as I could (end of first time out with it). It got hot enough to smoke all the oil off the barrel (it was impressive ), the front sight was very grey, and being sure it was toast, I thought, "guess I deserved that, sorry little MBUS". But it cleaned up fine, and keeps working prefect. I like the Magpul MBUS a lot, we have them on our M&P15-22 also. But to not have to worry about the heat issue, and to get just a little solid bottom rail, I'm changing the standard gas block out to the Yankee Hill Hooded folding sight/gas block with bottom rail this weekend:
Flip Front Sight Towers With Bottom Rails
I want the lower rail to put a bi-pod on from time to time, and a light/laser at other times. I have the Magpul hand guard (carbine length=short) and like it a lot, and want to keep all of it's space for my hands, not hanging stuff off of.

That's one of the things that makes these AR so much fun, there are so many options and different ways to configure them. That's also what makes it confusing at times.

If your not set on the Magpul MBUS, there are many other folding front sights that fit the std rail height. Many less for the "gas block rail height". They each have their advantages.

Thanks CBR240 for the insight. I found a great deal on the magpul MBUS (got both for $55 shipped). I figure I'll keep the FSB the rifle came with and use the magpul rear. For some reason I'm just not crazy about the rear sight on the carry handle. At some point I'm going to get an optic but that's going to have to wait a while.

I'm thinking of a reflex red dot or an eotech type because my rifle is going to be a 200 yard and under. Here in South Florida there just isn't wide open space for much more than that. Either way a quality sight will cost me a bundle. One I just don't have right now.

Would you mind telling me where you found the deal. I've been looking and that's a great price.

Gunbroker. It took me about a week to find it but I did. What I've learned about that site is that sometimes people don't spell things correctly or perhaps place things in the wrong category. I actually found it under flip up sights instead of MBUS or MBUIS...and I was mistaken. I actually paid 65 not 55. Still a pretty good deal for both.

I got my cheaper than dirt order of 5.56 ammo today. My buddy dropped off a used UTG free float that he said he didn't like (if it's free it's for me!). I hope it'll work as I want to mount a forward grip with a light/laser. I'm going to take off early from work tomorrow to pick up my new rifle! I feel like a kid at Christmas!!!

It's not the length of the gas tube that makes for softer recoil - it's the distance from the muzzle to the barrel gas port (the shorter the distance, the shorter the dwell time, which means less pressure build-up in the barrel). The shorter the distance, the softer the recoil. Typical "soft" barrels are 14.5-inch mid-length, 16-inch mid-length, 18-inch rifle-length, etc...


You can also reduce felt recoil by changing to a M16 BCG, a heavier buffer, and a heavier action spring. I'm in the process of building a new rifle with all of those mods (mid-length barrel, and the stuff I just listed).

Thanks jsimmons. I picked it up today and I'm stoked! Can't wait to get to the range and start shooting. Going to work with the irons until I save up the money to buy an optic.

Got another question though. I want to change the stock grip but don't know what kind of tool is required. I thought it would just be a screw but it isn't. Looks like some kind of a starred key but never seen anything like that. Unless that's a grip washer. Any suggestions?