Hillbill-
I also have the Browning 53. Bore for mine slugs .3110 - .3113". I've just started load development but found this rifle to be very finicky with regard to powder selection so far. Tried each of the following powders...
AA 9
W 296
SR 4759
AA 1680
Blue Dot
2400
IMR 700X
SR 7625
Conclusion - Patterns with all but 2400 & a bit further behind but still showing promise - AA 1680.
Details
Bullet: Mountain Molds 30115FC cast WW + 2% Sn air cooled, BHN =14.5. This is a GC bullet, weight = 106 grs. without GC or lube, sized .312 & lubed with a soft lube.
Primer: WSR
Powder: 2400, 9.9 grs. - 1664fps SD 20
Accuracy: Original open sights off sandbag rest, 50 yds. 1 1/8"
I will experiment with harder bullets, size diameter, and perhaps milder primers to see if more can be had but I think I'm approaching the limit of my ability for open sights accuracy. (Still hoping to get a load to shoot under an inch consistantly.) Other bullets may be an option but buying moulds to "experiment" can get expensive.
The next closest powder - AA 1680 running in the same velocity zone was running around 1 1/4" - everything else was 2" to >>2".
I wouldn't give up on the CB's - but it may take a bit of experimenting to find what the rifle likes.
Best regards-
Sky C.