Space Marine Realism?

[TomsDad]

It's the Long Fangs that suffer. A Devastator squad has five extra guys to hump ammo. I've actually modeled them up for my Battle Company. Every single one of my heavy weapons Marines has a counterpart who is carrying extra ammunition like this guy:

Heavy Bolter and Assistant Gunner with extra ammo

They look pretty cool!

I guess I've seen to many devastator squads split of as combat squads (been playing on and of since RT), I always picture it as the five over marines dumping all the extra ammo at the guys with the heavy guns feet while they rush of to have fun. A bit like being left at the check in counter of a hotel while your girlfriend goes of to jump in the pool!

[TrooperTino]

Take what you will from the mention of low recoil. It could be understood that a bolter has low recoil by comparison to a "normal" .75 calibre firearm, not by comparison to a 5.56mm rifle, for example.

I stumbled over a nice description of bolters on a fansite once, it was not official, but very sound. Bolters for unaugmented non-spacemarine humans didn't use the kicker charge because of the high recoil. A "normal" bolter would use just the rockets, so caseless, with nearly no recoil but low impact over short distances because the rocket needs a few metres to come to lethal speeds.
A spacemarine bolter uses both kicker charge and a rocket that ignites when it is already travelling at lethal speed, so the space marines bolter is better at close quarters which fits the role of the marines. I have adapted to this, it explains most contradictions, leaves room for every artwork, spent shells or not, and makes a difference between human carried bolters and the uber spacemarine bolters with recoil that rips a normal man's arm of.

[RBLFunk]

Its not really a misconception as far as this discussion goes though, because it's a real life failure of technology and how it could be compensated for in sci-fi tech. I.e your saying no a bolter can't be described as a gyrojet because it doesn't fail like a gyrojet, we're saying it's more like a gyrojet that works.

I get what you mean but hypothetical gyrojets that are very different to real life ones, aren't useful in a comparison to real life weapons, because they don't exist. A hypothetical gyrojet could be as identical to a bolter as information on bolters allows you to conceive. As could a hypothetical modification to any existing firearm.
There are other real life weapons that are already closer to a bolter and would require less hypothetical modification to replicate the fantasy weapon. Manufacture rocket assisted rounds for the PAW-20 or XM25 or XM109, or rocket assisted Frag-12 shotgun shells, and you're pretty much there.

[Polaria]

I think the closest "real life" equivalent of what bolter technology is supposed to be is 2A28 Grom. For those who don't have their degree at Soviet weapons the 2A28 is a 73 mm low pressure smoothbore semi-automatic gun, with cyclic rate of fire between 8 and 10 rounds per minute, which fires fixed fin-stabilized rocket-assisted rounds available as HEAT and HE fragmentation. A small powder charge is used to boost the projectile from the gun barrel at 400 metres per second (1,300 ft/s). Once the projectile has travelled 10–20 m (33–66 ft), the rocket motor starts and accelerates it to 700 m/s (2,300 ft/s).

[ChaplainCharlie]

Except that Bolters would still have kick even with caseless ammo...
They aren't bazookas! They have no real venting potential for the erupting gasses by design, just like the SM missile launcher, so even with rocket propelled ammo, they would have kick. Having fired a few "recoilless" weapons, I can say they do still kick. It's a relative term though. A 105mm rocket launcher is recoilless in comparison to a 105mm tank gun, but it still kicks like a small mule.

The most interesting thing about bolters is that the dreaded "kick" isn't neccessarily that big. .75 calibre was commonplace during the American Revolution and many muskets fired that size of round. Yet, negligable kick. Sure, bolts are a lot heavier, but still. And I doubt bolters even have a high muzzle velocity. If they are rocket propelled, they biggest issue with them would be accurate shooting when the slow rocket kicks, but takes its sweet time to exit the stub of a barrel the gun has. Hence, Marine use only.

[Polaria]

2A28 Grom is not a bazooka. Its a semi-automatic self-loading cannon with rocket assisted ammunition. It uses the recoil of the booster charge for loading and chambering next round and does pack quite a kick as the receiver weights dozens of pounds alone.

[ChaplainCharlie]

I was speculating on the notion of "non-marine" bolters not using the booster charge and just firing the rocket. Should have quoted that, sorry.
But in counter, the AS50 can fire 5 rounds in about two seconds and has very mild recoil for a gun its size. Similarly, we also have the Desert Eagle, which fires .50 cal ammo. I think that comparison could work. A desert eagle vs a bolt pistol. The bolt pistol has a heavier projectile, but only needs to get the bolt out of the very snub barrel, so the booster charge can remain small. A Desert Eagle has a smaller bullet, but has to get all its energy from one propellant, so the two ought to kick about the same amount.

[The Warmaster]

Since people have given explanations for the other stuff...

or is it more like starcraft do they have some kind of system like the bat pole where they go in one side and when they come out its all snugly on them?

Automated backpack removal bays have been mentioned here-and-there, and would be necessary if they were putting on their armour alone, for obvious reasons. Corax uses one in Deliverance Lost.