Are Boltguns and their rocket powered ammo less effective at point blank range?

No.

One might think about changing that due to the rocket needing to accelerate, but given the frequent nature of close range and melee combat in the 40k universe, the signature weapon of the system certainly will have some way to compensate for that problem - possibly, the bolt is fired from the gun at high speed like a regular SP projectile before it ignites. That would go to explain the recoil problems the Space Marine weapons have for normal people.

Cifer said:

That's how it's explained in Imperial Armour 2.

Ah good, the rocket is more of a sustainer of velocity than a booster. What i thought. Good, all is right in the world.

I can't remember which novel it was, but I'm sure I remember an instance where a bolt round passed through it's target before detonating, as the target was too close

user4574 said:

It has happened in several of the space marine novels. Seems to be a common problem, doesn't seem to affect the lethality, just the explosion.

I would imagine that this would mostly be an issue with unarmored and very lightly armored targets.

The bolt rounds might think that the meat that it has just passed through was armor and that the less dense air behind the target is the flesh that it wants to destroy so it explodes after it exits the body, effectively becoming an air burst grenade.

If you wanted to model it in game, a very rough idea anyway...

If the Pen of the round is 3 or more higher than the targets armor and greater than the targets toughness then on the roll of a 9 on the damage die the round goes clean through the target doing its normal damage, no tearing, and detonates as a grenade in the space behind the target. The air burst effect does damage as normal, no tearing, 0 Pen, using the Pen value for the blast radius.

That could actually be quite entertaining in a room full of unarmored cultists.

ItsUncertainWho said:

The blast of a bolt shell is relatively small, and I can't imagine it being all that effective in what is otherwise a 'failed' shot - there's special kinds of ammunition for 'airburst' shots, afterall.

IMO, the "blowthrough and failed detonation" effect is best represented by just playing the game as normal... if you roll a 1 for your bolter's damage, the shell might well have just passed straight through the victim before detonating uselessly behind him.

In one of the Horus Heresy novels, they mention that there are different types of bolts. They debate wether to use the standard shells, low speed or mass driven (i.e. they detect the mass of the target & explode based on that).

I always pictured the explosion as being in the m80 range mysef. Dangerous if too close, but safe at any real distance.

I would imagine that the explosion is just enough that it would do injury even if the shell failed to penetrate armor. (I don't mean Terminator armor, I mean "ordinary" 40k-era personal ballistic armor.) So the "blast radius" would be maybe a couple feet, if that much.