I have a 14" bull barrel. Mine likes the 180gr XTPs but also does about as well with the Hornady Single Shot Pistol bullets. I've had very good performance from them with AA1680.
I assume you must be talking about these:
Hornady Manufacturing Company :: Bullets :: Handgun :: Choose by Bullet Type :: InterLock®

What it really likes is the Rem 180 JHPs with the petal type jacket. Using them, it rarely delivers groups over an inch and a half at 100yds if shot from a good rest, often less than that.

Oldflatfoot, it would be interesting to see how the Hornady SSP bullets work in your double rifle. That would certainly make an appropriate looking cartridge to use in a double rifle. I imagine the velocities it could reach are a good bit higher than the T/C pistol barrels, particularly with a slow powder. A .358 rifle bullet might even be appropriate since a hollowpoint bullet designed for pistols could be driven so fast it exceeds it's performance envelope. A gaschecked hard cast bullet may be OK. No doubt about it though, a 357 Max double rifle made from a sleeved Damascas barrel shotgun certainly sounds like a very interesting gun and concept!

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