an ar15 barrel nut wrench is commonly used to remove them. a standard ar-15 armorer's tool will have it. its just pins that go into the holes for the gas tube at two points.
https://www.midwayusa.com/product/77384 ... r-15-steelthats what i have, but there are many many variety's of armorer's wrenches out there that will work.
you'll also need a vise block of some sort to hold the upper while you're torquing that barrel nut off, as well as installing the new one.
if you dont have them already, might as well jump on a kit and just be done with it. if you're gonna play ar-15 builder, you're gonna need most of this stuff eventually
https://www.midwayusa.com/product/49133 ... ntials-kitat the very least you'll need the following
armor's wrench multi tool
torque wrench
vise block
the gas tube alignment tool is also a huge bonus for ensuring your barrel nut is properly aligned when you're torquing the new one on.
dont forget to put a little anti-seize on the threads when you put the new nut on too. dissimilar metals and all that.
also be warned - its VERY easy to blow out a gas tube hole in those factory style barrel nuts - thats just milled aluminum. as long as you keep the tool square, and fully inserted against the barrel nut you'll be ok. i found this out the hard way. as a precaution try to use the holes that are not near the 12-o'clock position when you do put the barrel nut wrench on there.
you *may* also be able to remove it with a strap wrench, but thats iffy at best. bushmaster is known for using locktite in places we usually dont want to deal with it - including these barrel nuts. a strap wrench will be handy though for removing the handguard from the barrel nut as they do use loc-tite to secure that connection and time the handguard.
hth