Steel is way harder than copper. We use hardened wheels at work for cutting steel or stainless, the regular wheels will chip and break. Also, unless your barrel is cylinder choked (and maybe still if it is) it will taper from the breech to the end. This will cause the pipe cutters to track and they'll leave an unsightly spiral down the barrel without cutting through anything. Best bet is probably a good bandsaw with a vice to ensure a square cut. You could do it with a hacksaw and clean it up with a file, but I'd make friends with someone who has the right equipment. Good luck.