There are specific rules about arm braces involving length of pull, etc. Might want to become familiar with them, I'm sure they're on BATFE website somewhere. DO NOT write them and ask. Really, most of these charges are used as "add ons" for folks robbing banks and stuff. Like the man said, no one cares about you at the range. If you're really concerned, get an honest to gosh stock, efile the form and pay 200 bucks. SBR forms filed that way have been coming back approved in about 2 weeks.