8 hours ago, Lynata said:

Hmm, are you sure about the 12G ?

Like I said, it's pretty much an automatic shotgun, you just have to add a rocket motor to the bolts. The conventional firing mechanism also deals with the downside that a pure gyrojet would have in terms of lacking penetration at short range.

Absolutely sure, but this will not be the first case when western and eastern measurement of calibers works differently.

And while two-stage mechanism will deal with short range problem, it will only worsen the situation with round instability on high speed/long range .

9 hours ago, Warior4356 said:

The Vanaheim is from the Inquisitors handbook in the forgeworld section.

The arbites gun is from the Book of Judgment, its name is the Vox Legi

Ahh, that's why it seemed familiar. It's been some time! Though I've only checked the Forgeworld section in passing, anyways, and was more interested in the Hive and Warzone stuff.

4 hours ago, Jargal said:

Absolutely sure, but this will not be the first case when western and eastern measurement of calibers works differently.

Yeah, wikipedia uses a different table as well. It seems that with shotgun gauge it comes down to the mass of the ball, too, so maybe whoever wrote the aforelinked site used a different formula. Either way, 18.5mm is cal .73, so ... close enough?

4 hours ago, Jargal said:

And while two-stage mechanism will deal with short range problem, it will only worsen the situation with round instability on high speed/long range .

True. The AA-12's high explosive rounds are fin-stabilized slugs, but I don't think I've ever heard of bolts being fin-stabilized. It could be possible that bolts are spin-stabilized, with the rocket motor being something like a quadruplet of tiny thrusters angled outwards to induce rotation. This might even have the effect of slightly increasing penetration as the projectile would act like a drill.

That being said, we should probably remember that boltguns are intended as short-to-medium range weapons, anyways. The only instance where they're used for sniping was the Stalker-modification mentioned in the original Deathwatch rules, where you'd slap a sniper optic and a suppressor onto the gun, and use it with a special projectile noted for its slow (= subsonic) velocity and a different, gas-powered accelerator.

2 hours ago, Lynata said:

It could be possible that bolts are spin-stabilized, with the rocket motor being something like a quadruplet of tiny thrusters angled outwards to induce rotation.

And this is why I've mentioned Gyrojet - truster stabilisation just don't work, at least in such "small" calibers. But my main problem with bolt weapon is that its description contradict itself. The only point of two-stage scheme on personal weapon is to reduce recoil while saving high speed at target/kinetic energy on impact - but bolter is notorious for recoil.

3 hours ago, Jargal said:

And this is why I've mentioned Gyrojet - truster stabilisation just don't work, at least in such "small" calibers.

Wasn't the issue of gyrojets that they didn't have enough ooomph on close range as they need a couple meters to accelerate and increase their potential kinetic energy? I seem to remember the later models were quite accurate .. at least for ranges where you'd use a bolter in 40k.

3 hours ago, Jargal said:

but bolter is notorious for recoil.

That probably depends on the source. The only times I see recoil mentioned at all (usually in some Black Library novel) it's usually just to hype the weapon's coolness or the masculinity of its owner rather than any technical explanation.

From the specs, I'd assume that a bolter kicks about the same as a 12G -- which can be surprisingly little, once you account for the existence of recoil compensators like mercury weights or spring mechanisms (example: the AA-12 could probably easily be handled by a 10 year old kid, on full-auto). The rocket motor is added to increase the kinetic energy of the projectile, and possibly the penetrating effect of the diamantine tip.

One could ask why they didn't just go for a larger calibre instead, to which I'd say:

• In-Universe Reason: Larger calibres mean that ammunition requires more space to store, which can be a problem both on the field of battle (where it directly affects a warrior's carrying capacity, or how many rounds you may fire off a single magazine before you need to reload) and interstellar logistics (just increasing projectile size by 1/4 effectively results in an Imperial transport being able to carry 25% fewer ammunition to the armies it supplies, which can cripple a war effort considering that it may take weeks or even months for a ship to reach its destination).
  Furthermore, larger sizes may make bolt weapons more difficult to wield by Non-Astartes, which would necessitate the creation of additional, smaller calibres, in turn reducing the flexibility of Mechanicus weapons manufactoriums as well as removing ammunition sharing capabilities between the Space Marines and other Imperial armies.
  
• Out-of-Universe Reason: Because rockets are cool. Duh.

Gyrojet was designed for silent shooting with high speed at target. Low starting speed and excessive scatter after acceleration buried it.

Comparison of recoil by millimeters of caliber is pointless - 20x83 mm sniper rifle weighs 26 kg (14.5x114 - 29 kg) and even with very complicated compensation system it can bury the shooter into the soft soil .

2 hours ago, Jargal said:

Gyrojet was designed for silent shooting with high speed at target. Low starting speed and excessive scatter after acceleration buried it.

Hmm, wikipedia claims a later review revealed that the inaccuracy was caused by a production flaw. Maybe some day the idea will be picked up again and we will know for sure -- until then, anyone who thinks it might be an issue could also assume that the projectile may be capable of self-correcting its trajectory. We know that Executioner rounds are homing , and a a missile that simply tries to maintain a stable course should be even easier to do.

2 hours ago, Jargal said:

Comparison of recoil by millimeters of caliber is pointless - 20x83 mm sniper rifle weighs 26 kg (14.5x114 - 29 kg) and even with very complicated compensation system it can bury the shooter into the soft soil .

Bolts aren't 83mm long, though .

When codex fluff presents bolts as shotgun-slug sized rounds and describes a shotgun-like velocity with its first stage of launching the projectile, then I don't see why we have to assume that the recoil would be any different from a shotgun as well. Actually, the recoil would probably be less, given how the weapon's greater mass would soften the blow, as moving it would require more kinetic energy. And that is before any potential recoil compensation is considered, of course.

Could the bolt, in theory, weigh 200 kilos and as such necessitate a massively larger ignition that results in humongous recoil? Sure .. but this is by no means a quality in any way supported by the fluff, unlike projectile size and extrapolated velocity. When the codices mention bolters, the limiting factor is usually the weapon's own weight, as these guns are supposedly rather heavy.

That being said, this is 40k, and by the very people who write the books there is no canon here -- so ultimately it's up to you as an individual gamer/reader, depending on what you think would be cooler and/or what would make more sense. Even FFG's RPGs and GW's codices cannot agree on these things.

So I did some reading, it seems to me everything a shotgun can do, a flamethrower can do better. For close range death that is.

Flamers are almost ineffective against high-Agility characters who have enough space for maneuver , but against hordes of mooks, especially in confined spaces, they are great.

Yeah. I'd say flamethrowers are more .. specialized, but they're great at what they are supposed to do: light people up, assuming you can hit them. Note that they can also be subject to environmental effects such as sprinkler systems or the vacuum of space. By contrast, shotguns are more reliable, more flexible thanks to the special ammunition you can load, especially if your GM lets you adopt some stuff from the other RPG lines as well.

Or at least that's my impression. Ultimately, I would say they strike a nice balance in that they complement each other nicely. It's why the Enforcer/Arbites squad from Necromunda has one flamethrower among its models, in addition to shotguns, bolters and a heavy stubber.