My 300 blackout pistol is sighted in at 50 yards using S&B FMJ 200 Grain ammo. I'm interested to take it into a tree stand tomorrow using Barnes VOR-TX TAC-TX 120 Grain ammo to take a hog at 30 yards. Is there a free ballistic calculator that will compare the POI between two types of ammo and output what the compensation should be?

You have to verify zero, on paper with supers. Then verify zero on paper with subs. A ballistic calculator can only do so much. It has to be shot for real to know.

Plus there is a thing called "first round pop", and hopefully I am explaining it correctly. A barrel, and more importantly a suppressor, full of baffles and void space are full of air. Air is 21% oxygen, oxygen is a necessary component of fire. The first round fired can land somewhere completely different from each subsequent shot, often higher. Because of more oxygen available, means more muzzle velocity. First round pop happens with my suppressor, and shot number 1 does shift from the rest, but it is slight. I'm only talking supers here. If I load a sub, on in an unshot barrel, the first round pop point of impact shift is more pronounced.

30 yards? Just earhole em with the subs.

You will certainly have a POI shift between those 2 types of ammo. Not only will your trajectory be different, but you will have to shoot them both to know what POI shift you get. No one can answer this for you until you shot and tested it.

What I can tell you is, if you are sighted in with the subs, the 120 grain super sonic ammo will impact high. How much higher I can't tell you until you shot it. But it will be enough to miss a pig.