11 gr. unique and 405 cast
This "Unique" load is netting 970 fps in my Ruger#3's 22 inch barrel ...however shot in Springtime.... we don't get the Cold weather the rest of the country does in winter to test for chrony discrepancies between regular temps. and severe winter temperatures . I would guess you will lose about 50 fps max. in your 45/70 due to minus winter temperatures at this low velocity. But it should still work for you if it was accurate to start with in your rifle.
My rifle gives good accuracy with a Lee 405 gr. plain base at 15.0 grains Unique at 1190 fps ...and if your load needs a few more fps. in winter to remain accurate just go up by a half grain from 11.0 gr. until it comes back.
As you may know, some people change to magnum primers in the extreme cold for use in bottle-neck ctgs. This is not a safe option for a 45/70 or any rifle really useing any pistol/shotgun powders like Unique unless the initial load workup was started out with mag. primers.
