HC police: there are a few around, but are you doing this build for them or you?
Personally, I like to stay within the 'realm of the possible' on my builds. That means the hardware and lock, stock style, carving, etc, should fit within a certain time period; and within that constraint, reflect a certain mix of cultures, etc.
The caliber you're talking doesn't bother ME. The few Beck's I've seen have been fairly slim, so the only danger would be that you'd end up with a fat Beck if you use a big barrel.