Well the first thing is your accuracy might be very different. Also that is a maximum load in your rifle. Powerbelts normally shoot better with less then magnum loads although this is not always the case.

I would instead, perhapstry some Pyrodex Pellets instead of the Triple Se7en if you're that concerned about a fast second shot. Otherwise, make the first shot count, take your time, swab the rifle, load the next load like normal, and by then if you placed the first one right, the animal should be down or well on the way.

Another idea is find something that can be loaded as a second shot, with reasonable accuracy for a follow up instead of the powerbelt.

It is an interesting delima.