When I replace the muzzle break, do I have to remove any parts of the grenade launcher?
I'm not clear here on what you are asking. If there
is a muzzle brake on it, the grenade launcher would have been removed to install it.
Here's a shot of the original equipment (OE) GL:
If yours looks like this:

It could be simply a drilled OE GL, or it could be a USA made replacement. You'll have to look for the markings for mfg and "Made in USA", etc..
If it's OE, drilled or not, plenty of PB Blaster soaking, drive out the pin, and screw it off.
www.surplusrifle.com used to have a good tutorial, but they seem to be down
