I'd try the Ruger.And try the Lyman slug I mentioned. Stay with light loads as barrel isn't as heavy as NEF. But regulating is problem in both OU and SxS. But a 12ga Ultra Rifled slug gun don't cost much and it can handle real hairy factory loads or hairy reloads.
RIP on AR forums been testing starter powders loads, with shotgun primers,with Blue Dot starter and HBMG main load. And I just tested some variety of ones myself. Doing some testing with starter powder, 15gr Blue Dot, with slower powders that fill the cases with minimum wads. Tested in 3.5" RMC case in NEF with shotgun primer, with 36" added to barrel.Remember I have extra foot of bbl.
These 4 top loads expanded case, where I check it, just above thick base section to .811" from .807" resized.
1000gr jacketed, 230gr HBMG, 1800 fps, 7200 ft lbs, with 1/8" cards and 1/4" felt wad.
715gr jkt, 270gr HBMG, 2200, 7600 ft lbs, 1/8" card.
600gr Dixie, 250 gr Retumbo, 2400, 7600 ft lbs, 1/8" card.
385gr jkt AL core, 250 gr RL25,2900, 7200 ft lbs. This is faster than lighter loads in NEF, using 4227 powder.
Now the starter powder loads, which a few guys like, increase powder speeds so that HBMG acts like RL25. In first example, with 1000 gr in RMC case you couldn't use 230gr of RL25 as that would expand RMC brass too much and stick the case. A few guys have found that slow powder, with starter powder is easiest for 1000gr and heavier loads.
On another note, I've heard around the grapevine that if enough of us ask for NEF to make available a 12ga Ultra with a 28" rifled bull barrel that they would do it.Start calling folks, maybe do some good. Just tell them these new sabot and fullbore slug loads need more barrel to get the velocity out of them. 1-866-776-9292. Ed
