3.32" should be plenty of space for medium fine ball powder like Big Game, at least for any 286 gr other than TSX.
I'd also look at 2000MR - it's finer than Big Game, I believe. I had no trouble getting 62.5 gr of it under Hammer 258 gr all copper, which is a much longer bullet than any of the 286 gr out there, with a COAL of 3.30 initially.
I believe RL15.5 is about the same level of fine-ness as RL16
I can't help you with TSX 286 gr loads, but any other 286 gr you should easily achieve 2400 fps with 3.32" and Big Game or 2000MR, even with that 21" barrel. I know you want 2500, but 2400 is still plenty.
I keep editing this.
Do a 10-shot ladder, start with either 58.0 or 59.0 of Big Game/2000MR, go up in half grain increments until you see pressure signs, then back off a grain. I'm at 2475 fps with 286 gr A Frame and 62.5 gr of 2000MR, COAL of 3.325.
I've started LD for my son's Sauer 9.3x62, all the lead core bullets I've tried, I could probably seat them out to about 3.34 or even 3.35 and still have at least .02 jump