I am considering an Armoured Campaign and the tank gun I'm realy missing is the Punisher.
Based on the Codex I would say something in the line of:
Range 100m, -/-/10, 1d10+5 I, Pen 1, Clip 400, Rld 2 Full, Inaccurate, Storm
Useless versus vehicles but absolutely devastating against light infantry.
Being fully aware that fluff and rules rarely enjoy a common link, I thought I might mention the following:
The Vulcan Mega-Bolter (as mounted on the Stromlord Super-Heavy Tank) is noted as having an onboard ammo supply (not Clip) of 10,000 rounds, which allows the weapon to fire for upwards of 20-25 seconds (roughly 4-5 Rounds). Side-by-side, the Punisher Cannon appears to be the obvious go-to option in terms of engagement length.
Rather than giving the weapon the Inaccurate Quality, why not let its short range act as the limiter? Inaccurate would severely hamper the weapon (and its vehicle) in confined spaces and/or against infantry targets; there being penalties to hit due to the vehicle being in motion, targets may be running (additional penalties), and so on, and I feel making the weapon Inaccurate is unduly harsh. You already have its range 50m less than an Astartes Assault Cannon, and 20m less than the "civilian" (Dark Heresy) pattern.
Personally, I'd also be inclined to increase the Damage output, say to 2d10+5. In terms of the weapon's relative strength when compared to an Assault Cannon, it is marginally lower. I know Damage scales differently between game lines, but Dark Heresy and Deathwatch both show the Assault Cannon as 3d10+6, Pen 6, Tearing. Tearing may be a FFG 40K RPG equivalent of TT's Rending. If the Punisher Cannon is not a Rending weapon, the Storm Quality is perhaps an accurate assessment of the weapon's astonishing RoF (Heavy 15, is it?).
I'd also be inclined to increase the Pen to 3, maybe even 4 (it defeats Flak in TT, yes?) so bring it more in-line with this fluff (again, knowing the fluff-rules link is weak). If you don't feel comfortable increasing the Pen value (which increases its potential against vehicles), you might perhaps give the weapon the Proven (2) Quality? You should also consider that only fool enemies will fail to make use of Cover when confronted with or facing off against a tank; Flak (AP 4), TB 3, moderate Cover AP 6 (even with Pen 1) means a Severan Dominate infantrywoman soaks 12 Damage. It is a game of dice, after all, and dice are fickle; what you first think is "devastating aginst light infantry" soon becomes near-ineffectual. Are you putting all potential hits against one target? Then that target dies. Are you putting all potential hits against multiple targets? Then you have many wounded targets left with a desire for revenge against your vehicle, some of whom may be toting melta weapons…
In Deathwatch, the Assault Cannon is shown as having a Clip of 200. Assumedly, this is the ammo box capacity of a weapon carried by Tactical Dreadnought Armour. Perhaps increase the weapon's Clip? 400 seems low. I'd double, if not triple its Clip; 800-1200; and increase the Reload to 3 Full. This Reload time may seem overlong, but the vehicle is likely to have at least a hull-mounted weapon, perhaps a pintle Heavy Stubber, maybe even sponsons, so there are plenty of opportunities to fire upon the enemy while the main gunner reloads her weapon. If things are well and truly Klybo, some of the crew might dismount and defend the vehicle until its main weapon is reloaded, providing potential additional depth of play/role-play to encounter scenes.