WARNING: this is one of those pointless "I wonder" posts. It might not really have any value, to your games, or to you, in general.

I wonder...why don't the Guard have a tank with a turret-mounted lascannon? I think at one time in what GW uses for fluff/canon, the Guard had most of what the Space Marines get as tanks (Predators, Land Raiders), and the Space Marines either worked more with the Guard, or did like Space Marines, deep strike/drop pod-deployed in, did one big thing, and fled, before their movie marine shirts failed them, and they got 100 of anything-ed to death, but other than "the Space Marines have the Predator, so the IG don't have such a tank", I don't see the logic. They have hull-mounted lascannons, so I won't really whine here, but the Predator mounts sponson lascannons, too, and I don't know if the turret-version would really be so different. Almost none of the Guard tanks have a turret that hits at Lascannon strength, and the one that does, the Demolisher, is the low-range Leman. They make up for it with AoE/Blast, I suppose, and Guard would like numerous kills more than one "big kill" (their usual BS wouldn't usually pull it off, anyway; might be the Vanquisher's biggest fail-point), but I'm still a bit surprised that with all the "plasma turrets", "melta turrets", and shell-hurling things, there is no real "lascannon turret"; just the multilaser, which lacks strength, in order to increase RoF. Even the Vanquisher is a bit less than the lascannon, in tabletop, while the Basilisk, though not a Leman Russ, just lobs a shell from the state next door, and prays to the Golden Throne it hits something relevant.

So, was it just to give the Space Marines a decent tank? Is it the Guards' frequently low Ballistic Skill? Any thoughts would be fun. Still wish some cheesy Guard, and also Rogue Traders, could field Land Raiders, oh, and Predators, too, of course.

Assuming we are talking purely about the fluff, not the standard game, you're looking for the Leman Russ Annihilator

Have a link : http://wh40k.lexicanum.com/wiki/Leman_Russ_Annihilator

Pretty sure it's a Forgeworld / IA thingy

So, was it just to give the Space Marines a decent tank? Is it the Guards' frequently low Ballistic Skill? Any thoughts would be fun. Still wish some cheesy Guard, and also Rogue Traders, could field Land Raiders, oh, and Predators, too, of course.

I think it may be more an issue with the rules and how they may have evolved over time?

Looking at the old Epic 40k stats, a Leman Russ' battlecannon certainly outperforms a Predator's twin-linked lascannon by dealing equivalent damage (both listed as AT 4+), but having +2/3 range (45 cm -> 75 cm). It gets even more obvious if you look at the Leman Russ Vanquisher which has AT 2+ and is described as a dedicated tank hunter. For comparison, the next best weapon after the Vanquisher's main gun in that game will have you look at Titans.

[edit] Okay, even after looking at current TT rules, the battlecannon doesn't look so bad to me! Yes, it has 1 less Strength, but this is negligible if you compare it to the higher Penetration (giving it +1 on the vehicle damage table) and the Ordnance rule allowing it to reroll penetration.

Coupled with not even having to roll to hit thanks to the Large Blast, it does look at least as useful against enemy armour as a lascannon. The latter is basically a sniper's version of an AT gun, the greater focus of its damage allowing it to pack more punch into a (comparatively) smaller package, but it still is not without its drawbacks.