Generally this is done with the FSB attached to the barrel. I wouldn't call it risky.
Of course, if you're really worried, you can send it to me for disposal....free of charge.
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A properly installed taper pinned FSB will not come loose under hard use. I would recommend removing the FSB prior to cutting. Removing the FSB is 10 minutes of work.
The problem with removing the tapered pins of the FSB is that sometimes the holes enlarge a bit and you have to drive the pins in a bit deeper to get them tight again. It's much simpler to just tape off the area around the FSB and shave it down while it's still installed on the barrel
I've shaved the FSB on the barrel before. I used a hack saw to cut the top off (takes less than 5 mins.), then used dremel sanding wheels, a file and sandpaper for the final shaping. Just mask your barrel like Dead Man said and pay attention. I seriously doubt a dremel can get this part as hot as a couple mag dumps.
One more thought on the pins: If you pulled them and gave them a good cold blue (clean with acetone, heat the part nice and hot and apply), you'll add a few microns of thickness to the pins, as well as greatly increase their frictional coefficient. Much easier than reaming. Probably just as unnecessary, but it's something, and you expressed anxiety over it.
I'll also point out that the handguard is easily removable once installed. It's not something you'll do as a part of regular maintenance, but certainly to fix a problem.
I have done several of these, but after the last one I did I started to rethink this. Instead of doing this, I removed the FSB and then dimpled the barrel for use with a low profile gas block, Loctited the screws and put it back together. Having out 10's of thousands of rounds down range with this set up previously in other uppers, I am not sure that the juice is always worth the squeeze.
I used ceramic high-heat paint since it will get hot.