Went to the range today with a couple of different bullets in .455 ammo. One was a commercial 250 gr SWC and the other a commercial 250 RNFP. Powder charge for both was the same - 4.5 grs Trail Boss. Brass was Dominion.
At 12 yds offhand, my MKVI put 5 rds of the SWC into a group that measures 1-1/2"H x 1"V, centre to centre, just a tad right from point of aim! Not believing this, I fired a second group, this one clustering 4 rds into a 1" group with a called flyer 1" outside the group. The RNFP load gave slightly wider groups, but impacted at the same place.
The Colt NS put 4 SWC's into the same size group as the MKVI, with again, a called flyer I" away. This performance was repeated with the RNFP load, one called flyer ruining an other wise great group.
At 25 yds, the MKVI put 5 rds of SWC into a group 2-1/2"H x 2"V, the group printing 8"H and 4"L of the POA.
The New Service put 5rds of the RNFP's into 2-1/4"H x 2"V, printing 2"H and 4"L of the POA.
Again, these groups were fired offhand. I prefer the discipline of offhand shooting to using a rest, rifle or handgun. In competition, I'm not going to be able to use a rest and they would impact differently in any case.
Needless to say that Trail Boss is proving a powder worthy of consideration when loading for the .455.
But - a warning!!! These are not wimp loads. If you are at all concerned with the integrity of your .455, you may not want to shoot them in your gun.