Blast weapons are quite good for harming hordes, but many of the larger vehicle weapons are moddelled as a low rate of fire and a high blast. This seems problematic as the turbolaser on a Thunderhawk is actually less lethal to some amsters and vehicles than an attack with special ammunition from a Storm Bolter or other high RoF weapon. The solution my group has been using modifies how blast interacts with larger targets, allowing tanks to finally be better at killing eachother with main guns than a horde with stubbers.

For every 3 points of blast a weapon with blast trait has, it can score an additional hit on a target of sufficient size. The size modifiers are used here. So a hulking target can be hit one additional time and enormous target 2 additional times, and so forth.

So a blast 6 battle cannon could hit a Carnifex three times for its damage rather than merely once, actually inflicting a resonable quantity of wounds, and similarly certain missiles and grenades are much more useful against large targets.

This obviously increases damage output of certain weapons against certain foes, but most weapons with blast are quite uncompetitive compared to high RoF weapons, this change brings them more on par, and is reasonably easy to implement across several different weapon types. In particular, this makes the plasma cannon considerably closer to viable as an anti-vehicle heavy weapon as it should be.

My suggestion (and what I've done) is, instead of having the blast rating affect their damage, make them a little bit more potent. The issue as I see it is, using your system, hitting a land raider with an Earthshaker round would be far more effective than hitting it with one of the Leman Russ special AP tankbuster rounds, or a shot from a laser destroyer. But all of that fragmentation doesn't help at all against armor. Same thing with a Tyranid monstrous creature, you want to hit one with the biggest round you can, as opposed to a bunch of frag grenades.

As far as the stormbolter's rapid fire being better against some targets, it comes down to the target's armor. A completely unarmored target will almost always be devastated by rapid fire weapons, whereas a heavily armored target will be far less threatened.

When you come down to the numbers though, frags are still sueless against vehicles, rest assured. Doesn't matter if I roll 2d10+2 ten times and my las cannon once against a carnifex, the carnifex will get hurt by the las cannon, not so much the frags. With vehicle level armor, or the combined armor and toughness of bbegs like a fex, the rules still work fine in our playtesting. On the other hand, if someone requisitions vengeance rounds, or even hellfire rounds for that tyranid, they may well destroy their target with far more success than a RAW las cannon. Consider:

Against a Tyranid Warrior a lascannon will do about 46 damage, a stormbolter will do about 110 with the right ammunition. Even against a tyrant guard the SB average damage is approximately the same as that of a lascannon. That is kinda problematic. Extremely heavy tanks will ignore SB fire, it is true, but I have used few land raiders against my players, tyranid creatures and tau barracuda I have used rather more of. But tau vehicles are in the same boat. Since they are not as heavily armored my tactical marine is better off shooting them with his stormbolter than resorting to a heavy weapon. I would at least like the big blast single shot weapons like the plasma cannon to be worth considering.

You are right of course, anti armor rounds on a tank and the like should work really well, and I have already implemented special rules and damage increases for many heavy weapons including krak missiles, las cannons, and others, for exactly this reason.

Which set of storm bolters are you using? And refresh my memory, what's the Toughness/Armor of a hive tyrant/guard? (We havn't run into 'nids yet, mostly Tau in my campaign).

SB stats are taken straight from the errata, Tyrant Guard from page 54 of MotX and Warrior from page 370 of the Core.

Ok, I'll give you an update on my thoughts when I get home and get back to my books.

But based on TT recollection, I think a warrior would actually be faster and easier to kill with a heavy bolter than a krak missile launcher. The tyrant guard should be another matter entirely. Which of the "special ammunitions" are you using?

(Aforementioned Tac here) The rounds being used against the tyrant guard are Vengeance rounds. The 9 pen still gets through basically all of their 10 armor, and felling is a super handy trait since they have UT. SB damage is 1d10+15 (1d10+9 + 2 for master crafted + 2 for Tac Solo Mode ability + 2 for Mighty Shot). I have the Master of Arms distinction to add another level of felling, but I didn't at the time, so we'll ignore that for now. I skip 6 of the 18 TB and there's 1 armor left unpenned, for a total DR of 13, making my wounds/hit 1d10+2 Tearing. 8 hits end up with an average of just over 80 wounds after averaging in RFs.

I don't feel a tyrant guard should be impossible to damage with a bolter, but I definitely feel like it should have solidly entered the range where a bolter is sub-optimal. RAW, a Plasma Cannon will do 2d10+12 (and lets throw in 4 for quality and mighty shot, making it +16). It will fully pen the armor and then lose 18 from toughness, making it do 2d10-6, for an average of around 6 wounds. I think we can agree that's absurd, which is why we've buffed plasma from several different angles.

Firstly, we buffed plasma across the board. We had already done this before the errata since a bolter did more than a plasma gun (per shot) even without special ammunition, but even with the errata bolter nerf and the plasma buff, it wasn't anywhere near enough. We basically added a d10 and felling to all plasma (relative to pre-buff), and then ended adding a couple points of damage to guns and pistols and Devastating to Cannons based on my modeling spreadsheet showing plasma to still be a subpar choice is most all situations. Final stats for the Plasma Cannon are 3d10+11 Pen 12 Blast(3), Devastating (1), Felling(1), Volatile. The final change was to have maximal up pen by 4 instead of 2 and increase Cannon Blast to 6 on maximal fire. In the end, you have the follow against a tyrant guard: Armor fully penned, 12 TB applies. Total damage is 3d10-1 with 2 hits. This gives around 35-40 wounds, making it about 1/2the SB damage. Firing on maximal, you then up to 4d10-1 with 3 hits, giving you around 70 wounds, pretty similar to the SB. The advantage is then of course that blast might hit something else, so the Plasma Cannon has succeeded in having a niche.

Another attack greatly affected by the change is smite. We've mostly okay with this, because librarians have pretty poor damage options vs. masters. Getting a few more hits with smite goes a long way. It's actually started to become an issue with pushed smites, though even without the change the blast template just starts becoming silly at middling ranks. We ended up changing the blast to be rank meters instead of PR, but that's really more of a stop gap, since that makes smite kind of suck at early levels. Something like PR/2 rounded up might fit better.

I see two things that need addressing/could be addressed as far as the rules go.

Again, don't have my book, but aren't vengeance rounds supposed to explode every now and again in your chamber? And are how much requisition are you paying for them? If I recall it's around 10%, which means that, statistically, after doing a little more than half the damage to kill the guard your gun explodes in your hands. I see that as being a decent balance.

They are supposed to be really nasty rounds, but if you assume a full Stormbolter will require two purchases of ammo, you're suddenly putting down a whole lot of requisition for some burst out of that weapon. Right now you're getting a LOT of per-hit bonuses. This is something I've never cared for in the system; an exceptionally crafted Lasgun gets a +2 damage bonus, about 25% increase to average damage. A master crafted lascannon gets a +2 bonus. about a 4% increase in damage.

As much as anything I think a large portion of the problem is the per-shot bonuses greatly favor spam weapons, and the heavy hitters become the unloved redheaded stepchildren of the game. that becomes extremely obvious in an example like this. My solution (which I'll likely implement now that it's had such a clear representation) is to change the damage bonus rules to being +2 per damage die. Mighty shot and a mastercrafted Lascannon should have a meaningful increase. Bolters are average of 12 damage with a +2 bonus for each, so assuming a +1 to damage for every 6 average damage, both master-crafted and big shot would give a Lascannon a bonus of 40, for a +7 damage bonus from each. Having both makes it a +14.
Using your plasma cannon (which is very close to what I use in my home brew), it'd give a damage bonus of +4. So a total of +8. You might also go with a slightly more top-heavy scaled system, although that'll require some more thought time and playtesting.

The other one sounds like an issue with Vengeance rounds being a little too potent, so you can either reduce their potency, increase their requisition cost, or reduce the number of them issued.

I definitely see your point though, the current system seems to have some strange issues. Right now as I see the vengeance rounds are the entire reason it's so devastating; a regular stormbolter would be doing 1d10-8, or somewhere thereabouts.

Conceptually, I just don't like the idea of blast weapons hitting a large target more often, because it benefits certain kinds of weapons over others. It means that single punch AP weapons are less effective against tanks than highly explosive ones. Laser Destroyers (Forge-World made IG tank hunters) are less effective against tanks than the standard Leman Russ Battle Cannon. I don't see that fixing everything.