I am in the middle of upgrading my M&P 15 sport II and have already replaced the stock and pistol grip. It is my first AR but I have a pretty good understanding of the internal workings after field stripping it and watching videos on youtube.

My rifle came with the standard "carbine length" Magpul hand guard, as well as the A2 front post/gas block. I want to slide the front post to the "rifle" position and replace the carbine gas tube with a rifle length tube, rather than removing the A2 and replacing with a low profile gas block.

Has anyone had a negative outcome from attempting this? I have done the other upgrades so far without too much issue, but this seems to be a little more advanced.

I'm not sure I completely understand what you want to do, but let me say this. If your barrel came with a carbine gas system then that means the gas port in the barrel is drilled in the carbine position and you are stuck using a carbine length gas tube.

Correct, you can't just move the front sight post/gas block forward and install a longer gas tube, you would also have to replace the barrel.

OK that answers that for me, thanks. I've seen videos of people replacing gas tubes but wasn't aware that barrels were setup for a specific length tube. I guess I could use an internal gas block then and then simply adjust the A2 sight for a rifle length hand guard? Provided the guard fits over the gas block....or am I just grasping at straws.

You will have to replace the barrel ,
There is a hole in the barrel under the front site post that taps off the gas and directs it down the gas tube into the bolt.

Imo much more cost effective to just buy an inexpensive complete upper from PSA etc.
They have complete units in the $300 range,
If you get one without a bcg and charging handle they are even less.

You don't need to replace the gas tube unless you're changing the length of the barrel.

I have a M&P sport II it never held very tight groups even at 50 yards. I put a Wilson Combat 20" barrel on it now it shoots sub MOA at 100.

The front sight post is the gas block, if you are going to change it to a low profile and change hand guards you will need to drive out the two pins that hold it on the barrel. They should come out left to right. The pins are tapered. small end goes in right to left. On my S&W M&P the pins were opposing meaning one was left to right and the other was right to left from the factory. (someone's first day on their new job at S&W)

Yeah I've seen that in the videos I've watched, apparently the pins are setup opposite to how most other manufacturers do it.

Getting the A2 front post off might be a bear. That's doesn't mean you shouldn't do it, but just know they can be tough to get off from the factory.

You can also get another carbine length gas tube for a few bucks with a roll pin and not have to get the pin out of the front post too.

Replacing a gas block is not a huge deal, but you do have to do it right or you'll get undergassing and cycling problems. The port on the block has to line up with the port on the barrel.

Don't be intimidated. It sounds daunting at first, but even if you screw up, that's how you learn. The good news is that doing this operation wrong is not dangerous. The weapon just won't work right.


Don't get discouraged if that happens. There's plenty of help and you'll get it right. The AR is a fairly simple machine overall.

Look at a "Dissapator" set up. Carbine length gas system hidden under full length handguards and front sight moved forward.

Thanks for all the info guys! It's been super helpful. A friend of mine found an easier solution for my problem, so I won't be needing to move the sight post right now.

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There are those who love to hate on UTG gear, but not all of it is chinesium junk. The Pro series hand-guards are American made and while not as good as some costing a whole lot more, are pretty good, especially for the money.



I have a couple myself, but they are full length with low pro gas blocks. I've never used this model.

There is also

Reading your post, it appeared your intent was to only lengthen the sight radius. At least that was my interpretation. Glad to see the problem solved. Plus you got a quick lesson on a vital part of the function and design of the gas system.

Absolutely, my main intent was to be able to put a rifle length handguard on the rifle. I wasn't so much worried about the exact placement of the front site post but I was under the impression I would have to move it in order to put a rifle hand guard on my AR. Super appreciative of all the input on this thread!

Can we laugh at this part now?

Go right ahead!

You aren't the first to trip up on that one, and you won't be the last!

I'm a big believer that experience is the best teacher!

Figured I'd drop in and show the final result. Thanks again to everyone for the pointers!

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Well done. Looks good. You might as well drill out the rivet and remove the front sling swivel. It'll look even better.

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Will do, good idea!

You could remove the front gas block/sight and install a low profile gas block and put any length handguard over it if that’s what you’re trying to get. Then add a front pop up sight, but you’ve accomplished it a different way.

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