I would say your fortunate to have rifled barrel with
3.5" chamber. You can do so much more in the way
of wads to cushion keeping peak pressures down
and making cases last longer and resize easier.It
means you can load brass cases with protruding
slugs about 3.1 overall, and use the magazine.
So with Magtech 2.5"brass don't cut it off, and with
600gr slugs use 95gr of 4759, 100gr 4227, 140 gr RE17.
And you can use same loads in 3.5" plastic, with roll crimp.
Of course fill space with seals and wads.Ed