Has anyone gotten a form 1 approved for a modular can?  For instance a .308 rifle can with a short 5" primary section and a second removable 3" section bringing it to 8"?

If so, how did you describe the length on your form?

I've done several. Input the longest length and leave it at that. I've yet to inform them it's a modular design as there's no law saying I must.

If you can swing the cost, I recommend another short section. You can stuff a bunch more baffles in it. This allows you serious flexibility with making multiple style silencers, from one stamp.

OP, you most likely already know this but pay attention to that word right there, that design gives you the option of multiple styles of one silencer that you can configure.  Don't make the mistake of thinking that this will allow you to have more than one silencers on one stamp
I have seen conversations on modular designs where someone will ask "how would they know" and the truth is they probably wouldn't.  But IF they did . . hope you don't have a dog