There are a 3-4 bores. A good setup will run a couple pistons through each bore if you run enough and good oils, not this 50 or 100 -1 crap for indoor.
Its not cheap to build a big bore Correctly.
I wouldn,t expect it to be cheap.Ya want to play ya gotta pay.Simple as that.

What size carb are guys generally running on the 310,s for MX?