Rob and RIP on AR forums are doing doubles.
I haven't the resources to do one.

The Vias brake bored out in above picture will help
RIP with recoil as he wants to shoot all heavy slugs
in the NEF Ultra. Guys like the Vias brakes.They
can be bored out and fitted to anything.
I just like ported internal brakes and more gun weight.

Here are some results( had 1 nice day) with 8 gauge and
10ga light slug loads of my testing. I call them my real
big bore varmint loads. A 770 gr slug in 8ga fired in
Enfield at 2400 all 3 shots nearly touching at 20 yds.
A 515 gr Lyman, NEF 10ga at 2400, same tight group
at 20yds. Lyman is originally for 12ga, but is fit in
10ga thickwall BPI shotcup cut off and slug glued in so
it is like a impact discarding sabot. Both smoothbores.

Smaller varmint load. NEF rifled 12ga with RG's 385gr
aluminum cored jacketed HP slug at 2700, 2 overlapping
and one half inch away using RMC 3.5" case. Ed