Strangely enough, nowhere where I've ever read anything about the WWI British range of "Mark" tanks have I seen anything about gun range. I'm assuming the range would be a lot less than the original naval version though, because I DO know the barrel length was shortened twice, once for original fitment in the Mark 1s, and a second time before fitting to the Mark IVs and all following Marks. So you can reckon that range would certainly be less....and THEN there's the question of effective range - would it have the same range of elevation as in its Naval installation? Also, there's the question of sighting, something I don't recall reading - mostly I've read about use rather than design. Certainly there'd be no blind-firing or directed fire, but more fire-on-acquisition...so whatever they used for sights may simply not have let them accurately fire at targets out to that range!
"Well, my days of not taking you seriously are certainly coming to a middle." - Malcolm Reynolds