Sam

Welcome here & welcome to muzzleloading.

A 24" barrel muzzleloading bullet goes downrange just about as far as another 29" barrel muzzy. The difference is insignificant... the 29 driven bullet may be 200 fps faster... which in yards.. is really maybe 10 yards further maximum. You'd be surprised as to how many deer are dropped at 200 yards using even shorter 22" barrels. The rifle you have is sufficient for all muzzleloading use.

The twist you have in that Rem is the kindest, most-friendly twist there is. The 1-28" likes just about anything but roundballs. It'll shoot light & heavy sabot/bullets... light & heavy conicals not quite as well as sabots... but pretty darn good. That's because your Rem bore has shalllow rifling... as do all 1-28" twists -- better suited to fire sabots than conicals.

So you really don't need a new rifle. You need to find the right powder & the right powder grains to shoot your Rem accurately. Your 700 ML has a couple issues, but after you find a load you're happy with, come back here to discuss how to remove your bolt easier for cleaning & how to lessen the blowback that Rem 700 MLs are famous for,

You can go buy a new gun... but I seriously doubt you need a new gun or a gun with a different twist.