Tom - I didn't know they were so light. I would start with a 405gr. Factory load and go from there, if I were you.

A PAST recoil pad on your shoulder under a jacket or shirt sure helped me with my .375 when my sholder was really hurting with it's unjury.

The 405RP bullets are about the cheapest. See Rod Halverson's post about penetration on water, steer heads and wood. If you don't have it, I'll mail it to you. It is quite enlightening when compared to the 300gr. Swift partitions from a .375H&H.

Once seeing this, it is not surprising thousands of buffalo were killed by the original 405gr. lead bullet and black powder round.

a 400gr. to 420gr. cast bullet at up to 1,600fps should be OK in the light gun, with a PAST Magnum Recoil Shield.

How you hold the gun and canting the barrels can really effect impact.