What are the advantages of using an aftermarket metal barrel instead of the one supplied with the armor kit

For starters, no seam running the length of the barrel and muzzle brake (if the barrel is a two piece assembly) which needs to be removed. Also, a solid, machine turned metal barrel will be perfectly round and straight, something which can't be said for multi-piece, kit-supplied styrene offerings. It looks waaaay better, too. If the particular kit you're building contains a turned metal barrel as an option, definitely use it.

No seams!

On a lot of kits, covering different manufacturers, the barrel comes in two halves. It's a real PITA to get rid of the seams.

If the barrel is one piece, then you have to contend with mold seams. That's not so bad, but scraping them off sometimes leads to flat surfaces. Then hope the barrel hasn't warped.

Usually, with turned metal barrels, the length and shape is more accurate.

Steve

It just looks better .

Yeah, just plain better. Just glue it and it's done. One plus for me, especially on larger guns, is the rifling. Just adds a little something more.