I was reminded that the ATF has no authority in Australia, so this post is to anyone inside the USA who is thinking of doing this! (read my follow up post) My advice to US citizens is, tread lightly here!

I think you are looking for a visit from the ATF, if you go about lopping off barrels of a double rifle shorter than 16", or building a pistol on a shotgun action, or building a pistol on any rifle or shotgun action. These are considered sawed offs by ATF, and will net you some time in the federal lock-up! Even if you make a double rifle on a shotgun action it can have barrels no shorther than 18", and must have an over all length of 26"! One that started out as a double rifle from the factory, cannot have barrels less than 16"!

Dangerous area, fellows, to even talk about, in public, and a definent problem to actually do!

Find yourself a good clean 577 Howah pistol, pay the price! It will be cheaper in the long run, I assure you!