The only advice i could give you is to not sleave the old barrels, that is if you not planning to build a 16 bore. It will be to barrel heavy and wont have any balanco left. I now as i did it and wasnt pleased with the result. I would cut the old ones and use it as a monoblock for a ,577 or something like that, but not sure if you are able to pusch it to 2000fps with safety.