here are the parameters you have to work with by the Feds, your states figures may vary.
Shotgun barrel minimum of 18"
Rifled barrel minimum of 16"
Overall length minimum of 26"
A Handi "pistol gripped" firearm legal by federal law if you meet or exceed those measurements. A (my) Handi Rifle "pistol grip" says on the side to use only with a barrel length of 22" or longer. I just measured my legal "pistol gripped" 20 guage H&R 20 guage shotgun. It has a factory 22" barrel and with the factory pistol grip the overall length is 26.5".
My belief is if you used a 22" .357 Mag barrel it would meet all the criteria including the required overall length but with an 18" barrel it would be at least 3.5" short by federal law...Good luck and I wish you luck, I think with the proper barrel it would be legal and should work....<><....