I know of a set of early full length 20 tubes in a 30" 12ga. Elsie and it is the proverbial 'pig on a shovel'. Not a pleasant gun to try and shoot, too much weight forward. Today, lighter tubes are available. Were it I, I would call Briley and give them the dimensions you are playing with and see if they can approximate the tubes' finished weight for you then I would insert that amount of weight in the bbls, perhaps a wrap of plumbers lead on small dia. wooden dowls or something similar and then practice mounting and swinging the gun to see if it still suits your intended useage & vision. If it does then go for it. I'd just try that first to avoid a disappontment since it will involve some expense once you have committed. Briley does good work.
