Correct. The 8-inch was AKA the 203mm. The same vehicle mount and recoil system was used for the long barrelled 175mm gun, which the cannon cockers sometimes called *the sniper's rifle. And there was some shared technology between the Army and Naval 8-inch projectiles, too.
Haven't seen one anywhere, and I'd have loved to have seen one.
The highest we have currently is a 155mm.
But the biggest in the world IIRC is in South Africa.
Don't remember the size of the round offhand though.
They played with a 245mm gun-mortar, IIRC- not sure if it was rifled or smoothbore. Wisely, they pushed the development of the SRC G5 long-range 155mm artillery instead. Of course the *Superguns* designed by SRC founder Gerald Bull for the Iraqis were 350 and 1,000-MM smoothbore tubes, though happily not mobile. I wouldn't bet on the Soviets not having something of similar size, though; during WWII they had at least one field piece [305mm BR-18] that used the ammunition of their 305mm TM-2-12 and TM-3-12 railroad guns...which were NOT their largest mobile artillery; the 500mm TG-1 probably holds that record.
And no, there aren't any 8 inch howies around anymore.
Well the Israelis still use 'em, to be sure. As do a few of our NATO allies. But we've gone to the MLRS rocket launcher instead. And the Israelis are developing their lightweight 20-ton RASCAL 155 SP gun with 45-cal barrel on a modified Merkava tank chassis, which should outrange the 8-inch...unless the SRC developments for the 155 are applied to the 203mm guns and projos too. 
