Trying to fix my canted front sight, pins are out but DAMN, the FSB is really on there! I soaked it in penetrating oil for 24 hours then whacked it with a rubber mallet. Nothing. Im really trying to avoid buying a barrel vise. I searched the threads but couldn't find one specifically about this problem. So please, lets hear it.

Put some heat on it, then whack.   What are you clamping the upper with?

I considered heat but Im trying to damage to the finish as little as possible. Ive been trying all sorts of different angles in my vise. Im just using a good old fashioned 5" bench vise. Im clamping on the barrel where the hand guards would be but Im a bit nervous to damage the barrel. I was thinking maybe there is a good barrel vise alternative?

Get a thread protector or a sacrificial muzzle break and a brass drift. Pit the muzzle down on your bench and use the drift on the flat of the fsb. Whack with hammer. You could use a press. Just get two pieces of flat stock that's all you really need.

I suspect you meant to PIN the muzzle down on the bench. If so, what do I use to pin it down. The jaws on my bench vise don't seem to have much grip. I've seen a product that essentially makes a barrel vise out of the regular bench vise. any opinions on these? Sells cheap and looks it too. $20.00 at brownells. One other question, how much pressure would have to be exerted on a barrel before it eggs?

Clamp receiver to bench, support barrel in front of fsb under muzzle device, and behind fsb. Whack with hammer. This is all about how secure you can support it. The fsb will move if you do this right. If it is extra tigh, it might move too much.

To decrease tension, stick the whole assembly in the freezer for 30 minutes.

I am in the same boat. Pins were removed. I tried to heat the FSB with a torch and spray it down with liquid wrench, with the bench vise just under the FSB I tried to use a hammer to strike a block of steel on top of the muzzle device but it wouldn't budge (Same method as using a press). I did have an 1/8" gap between the muzzle device and FSB to allow the FSB to move.
I feel like heating the FSB also heats the barrel so they are both expanding.

I also tried to squeeze the tip of the muzzle device and the back of the FSB with the vise. Also wouldn't move.

I am out of ideas. I definitely don't want to damage my brand new Romanian CHF barrel. But I need to get this FSB centered.

To pull the bock on an RPK, I used a gear puller (from nappa auto, about 20$) and a crescent wrench that fit snug around the barrel and wouldn't slip past the FSB, then used a brass nut to protect the muzzle.

I put the crescent wrench just below the FSB, put the brass nut over the muzzle and hooked the gear puller hooks under the wrench, tightened the puller, then used a cheater bar for leverage.

it will probably be really difficult at first, and the brass will deform, eventually it will "pop" and then be fairly easy after that. I think whatever finish used tends to kind of glue or fuze the parts together.

after it pops it will be much easier to move, if you need to press it back down, use a muzzle nut and a wrench to screw it back down. You may still need a mallet to straighten it out.

Same method as using a press

Finally got my FSB off the barrel after some heavy duty persuasion. I purchased a gear puller from my local auto parts store and after 5 minutes of use, I bent the puller. Went back and got another, this time a thick forged steel version. I tried using an adjustable wrench like some suggested snugging it up behind the FSB, but since I couldn't get a balanced grab on the wrench due to their shape, I had to use something else. I found a #15 wrench would work but the grab points were too close to the barrel and the gear puller would slip off after applying a nice amount of pressure. I cut a large steel washer into a U shape to put behind the wrench, this provided a nice even parallel gripping surface.
Cranking on the gear puller with a 3/8" ratchet and an 18" leverage bar on the end I cranked it until it wouldn't turn any more. Still wouldn't budge. With all that force pulling on the FSB I used a torch to heat it up, sprayed liquid wrench in the pin holes. Tried to crank one last time with everything I had and it finally popped. Then it kept popping 6 more times as it was being pulled forward. No wonder I couldn't get it off with a big hammer, that damn thing was really tight.
No damage to the barrel either as I used a small block of aluminum on the muzzle and that worked pretty good. Hope I don't need to do this again.