I am currently in the process of doing the same short barrel bushpig set up. I started from scratch with buying a rifle to suit and after my digging around and mr google research I came to the conclusion a 308 is best suited to the job so I am using a tikka as starting point.
If it helps try googling 270 barrel length v velocity and see what comes up and do some research as to whether you think it is worth doing and just how much performance you will lose.
But it seems to me the 308 appears to be the best choice for what you are thinking of doing.
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