180Pilot- Those shotcups are slit to start with.
Maybe Rocky Mtn wants to make sure that your chamber isn't short
as with brass it could raise pressures.If you have
a set of inside mikes you could also measure where the
forcing cone starts.I found an old set of 1.5 inch thread
big Hollywood 12ga dies.Mine has no trouble holding slugs
against recoil.It's smoothbore with cylinder bore.
The 600 gr is a hardened hollowbase Dixie slug.I plan on one
with a longer heavy rifled barrel, hate short barrels.
In the TC shotcup the 600NE slug perhaps could be paper
patched tighter.If we can find a supply of 50 cal sabots, like used in Federal Barnes saboted 12ga loads, we could
use the front half of surplus 50 cal borerider bullets,
shown in picture next to barnes saboted copper slug.
Third in picture is Asquare 600NE in sabot.Ed