Pappa, many years ago I built up a couple of these using 45-70 in the rifle side. Just turned the insert barrels to fit, closely, at the muzzle and the chamber ends- in the middle reduced the dia to save weight. Brazed a piece on the extractor for the rifle side.

Accuracy was very acceptable. The tube could be rotated before cutting for the extractor to get the rifle to roughly shoot to the smoothbore POA, particularly if the muzzle is turned off by a few thousandths. A simple set screw in the very back of the chamber, where the brass section of the shot side is, will hold all in place. remember the insert barrel will recoil foreword in relation to the shot barrel surrounding it, so it won't move if turned with a rim.

As far as 7x57r, you likely will need to bush the firing pin- not likely necessary in 45-70.

Fix the trigger problem regardless!

Doug