I have a M&P Sport 5.56/223 16 inch. I just purchased a 20 inch barrel and I would like to know if the Front Sight Base can be removed and put on my new barrel or do I need to purchase a new FSB. Is the .750 gas block a part of the FSB or do I need to purchase one? (New to building or changing parts Thanks guys.

I'm not an expert on this, hopefully others will chime in here.....but you can't just swap different barrel lengths like that without swapping other parts. At that point it is almost cheaper to just start over and buy a rifle length AR if you want a longer barrel.

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Still carrying my S&W 642

The only difference is the barrel length 16' to 20' and the gas tube length. both use a .750 gas block and have extended m4 feed ramps. Just asking if the FSB can be removed from the present barrel and put on the new barrel. Would like to know this before I go to the gunsmith to have the new barrel installed. The FSB looks to be a permanent fix.

The M&P Sport uses a standard A2 style front sight. And, yes, the gas block is integrated into it. It is held in place by two tapered pins. The pins come out left to right.

The problem comes when trying to install it on the new barrel. Look at this pic:

See that hole indicated by the arrow? That is from the old sight I removed. If the new barrel doesn't have two holes like this (the other is under the new low profile gas block), you can't install the sight on it. Of course you can drill these holes yourself or have a gun smith do it, but you don't need to.

A new gas block can be had for around $25. A new integrated sight/gas block will be about $75-$200 depending on what you get.

Because the Sport is a carbine, it is likely the forearm will need to be replaced as well. Does the new barrel have a mid or rifle length gas system?

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You may have to change the spring and buffer to get the rifle to run reliably. The spring and buffer in the Sport are set up to run by pressure created with a carbine length gas system. The rifle length system creates less pressure and in theory, may cause malfunctions if you use the same spring and buffer weight.

You could probably re-use the FSB, but why? If you are not doing the work yourself, it might be cheaper to just use new parts than pay for labor to remove and re-install them...

And I agree with the other poster... I would just buy a new upper. They are so cheap right now. You will have to buy new handguards as well, so buy a new upper and either sell the Sport one or keep it.

My concern would be if the pin holes in the barrel taper the same direction as the FSB since S&W at factory installs them the opposite of other makers. Unless they changed procedures in recent years.

The new barrel has not been pinned. I have purchased a new FSB, rifle length gas tube, buffer/spring and standard A2 stock and hand-guards. Think I am ready to go. Do I need a new flash suppressor as well. Did not want to sell the Sport because it belonged to my deceased cousin (sentimental value) and was willed to me. Thanks guys for all the info. Looking forward to CMP competition season in the spring.

Did the stock come complete with the proper receiver extension (buffer tube)? If not, you will need one of those.

You can re-use the flash hider.

Yes, you can re-use the flash hider. Some say to use a new crush washer and some say it's no big deal to use the old one. I don't really know if it makes a difference.

I re-used the old crush washer on mine and haven't had an issue. Even so, I'd be curious to hear if there is a valid reason to get a new one.

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[QUOTE=cyphertext;138796218]Did the stock come complete with the proper receiver extension (buffer tube)? If not, you will need one of those. (yes it did)

You can re-use the flash hider.

Thanks guys, you all were most helpful.

It can make a difference if the crush washer is flattened too much and the muzzle device needs to be oriented a specific way. Some flash hiders and muzzle brakes have a solid bottom to minimize dust flying from the muzzle blast. The only way to know for sure would be to try using the old washer and see how the muzzle device times. Or you could do what I did an just get a jam nut and not worry about it. Personal preference. You can twist the muzzle device to about where you want it then and tighten both the nut and the device to the proper timing without worrying whether you have the right sized washer and/or spacers to get it right.