Here is picture of 4 long cases of
RIP's on the AR forum.
Top being a Rocky Mtn Cartridge
3.5" turned brass 20ga. In 20ga
getting these and lengthening chambers
allows use of the 3.5" 20ga plastic also.
Some factory guns are 3.5"

Next a RMC 3.5" turned brass 12ga.
Chambered for these you can use
3.5" 12ga plastic also.Some guns are
3.5" from the factory.

3rd down our 3.85" 12ga FH we made from BMG brass.
Bottom a RMC 3.85" turned brass 12ga FH case.

Next picture shows one of the old ways they locked slugs
into paper cases. Many old slugs had a wide deep groove
which made it possible to crimp that way.You can
see one of those slugs in the picture.

Testing 490gr 12ga full bore Lee style slugs in 3.5"
plastic at 2200. Works in the long barrel rifled NEF and
the Khan 12ga 3.5" auto smooth bore.

In same guns the 570gr US-S brass slug with
locked on base at 1900 in 3.5" plastic worked also.
It is great to see a smooth bore slug like the US-S
that is as accurate as most fullbore slugs from
rifled barrels.Maybe they will be imported soon
from European Cartridge.Ed