I may have missed it, but are you sleeving the barrels, lining them, or making a new set? I did this project a few years back, lined a 12 bore Cogswell with Hastings 20 bore barrels. Tack driver at 75 yrds. I used factory Hornady ammo. Now the RBL Proffessional is offering basically the same type gun. I'll bet your costs are at least that when your done. You have to love projects for projects sake....
JZ