Not only are the lower powder charges usually more accurate but if you use 120 gr of BH 209 (roughly equivalent to 138 gr of black powder/pyrodex) a powerbelt will probably come apart on impact and you will get less penetration than if you use 90-100 gr of powder. Pure lead bullets (all of powerbelts are pure lead regardless of what the skin is) don't do well if pushed too hard.
120 gr will also kick harder in two places: your shoulder and your pocketbook. Do NOT buy into the magnum hype. 90-100 gr of powder is enough to kill any elk walking. My elk load this year is 95 gr of BH 209 and the 350 gr FPB (killed an elk with this load for the last two years).
I would use either the 338 platinum or the 405 aerotip if I were using a powerbelt. The 350 gr Hornady FPB is a lead alloy bullet that is also a good choice.
For toughness of bullet, the Thor can not be beat.
txhunter58
venor, ergo sum (I hunt, therefore I am)