Biggest problem of rifling is that the "lands" wear down quickly. For a pistol it's probably good thing that it wears down before it has time to break in other

Biggest problem of rifling is that the "lands" wear down quickly. For a pistol it's probably good thing that it wears down before it has time to break in other ways. For tank gun, you can use some other, more accurate way to keep track of shots fired. Probably second biggest down sides of rifling is that it weakens the barrel. How much depends a lot on how the rifling is done. But anyhow this has to be compensated with thicker/stronger barrel. But that gets problematic at extreme pressures. You wan't to use pressure vessel steel, because such steels 1. have good fatigue strength and 2. they tend to send around little less shrapnel when they fail. Problem is that such steels are typically low alloy steels with somewhat low yield strengths. So you would like to make thicker barrel, but the problem here is that you quickly hit diminishing returns. There is usually very little point making barrel thicker if the outer diameter is twice the inner diameter. (Except if you need more rigidity to improve accuracy. But that's whole another subject.) Anyhow, the brits still use rifled tank gun.