Don't take this too seriously, but 46" seems to have been a favorite length for Virginia rifles. So much that in a description of one (if I am recalling it correctly), Shumway says something to the effect of "The barrel length is 46", which is appropriate for a Virginia rifle". For at least one of the rifles that you are building, don't forget practical matters like "will it fit in the car I have" or "can I see to load it without tripping other people with the buttstock", etc. I think 42" standard is a little on short side for most originals through ~1830's or so, but not outrageously so.