Generally speaking I'm not a fan of hybrid weapons like this, since the general result is that you wind up, as @AndreiROM pointed out "Turning two good weapons into one crappy one."
There is an exception scenario though, so that's what I'm going to design to. That exception is when you have something big and fast charging you, and you want to be able to both shoot it AND stab it without letting it get close enough to you to do damage. African hunters solved for the shooting part with the Elephant Gun back in the 19th century, and European hunters solved for the stabbing part with the boar spear even further back.
I'm going to just give up on the idea of slashing, really. A weapon that's going to be useful against anything in the weight class of a Rhinoceros or a Tyrannosaur is inevitably going to be too heavy for any kind of kung-fu bo staff fighting. You want something you can point at the threat and start pulling a trigger and keep pointed at the threat right up until it impales itself trying to get to you.
Given the requirements you've specified, what I would use it were my skin on the line is a KSVK. Really, any of the .50cal Anti-Materiel rifles would work, but if you want a rifle that you can beat something with and still shoot afterwards, you always want to go Russian.
Likewise, Russian construction is going to be more forgiving of the need to use hand-loads, and even potentially the slightly smaller .50 BMG rounds that western AMRs fire.
Obviously the scope needs to go, that's not going to survive melee combat, but that's ok, because we're not using it as a sniper rifle, we just need the muzzle energy for dealing with those Dinosaur-scale threats you mentioned, since ~13,000 foot/lbs or so is what you want if you've got several tons of whatever with thick hide and thick bone coming down on you and you want to put holes in it that will actually matter.
One advantage here is that putting a polearm blade on the end of the barrel is actually going to make the whole thing more well balanced for melee combat, since by default the rifle starts out pretty heavy at the back. The whole business is going to wind up weighing about 14-15kg, so using it is going to be a workout.
As far as the blade is concerned, I think you want to go with something like a boar spear. If big stuff is going to be charging you, you want a blade that's going to pierce bone and muscle deeply enough to get to something vital, and you absolutely need a good broad crossguard to keep whatever it is off you while it's bleeding to death.
So, there you go. You've basically got a REALLY big rifle with a REALLY big bayonet. You see a big nasty thing, you point, shoot, and keep shooting till it's almost on you, drop the stock from your shoulder into the ground and lean into it and let said big nasty slam right into that long blade and hope you can keep it off you till it notices you've killed it and stops trying to eat you.