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 Wulfmar wrote:
The guns appear to have a forward assist and a breech bolt. I expect they will eject casing of the bullet.

You can argue that caseless ammunition is a current thing and that it would be expected in 40K, but then they are technologically backwards and so having bullet casings could also be a thing.

Caseless rounds are not more advanced than cased rounds. They're a step backwards in bullet technology, only thing they save is on material cost to make the actual bullet. But the drop in reliability and shelf life simply isn't worth it. Spent casings can be reused too.

"Advanced" is based on performance. If you get a reliable caseless round, it's more advanced because of its other benefits.

The benefits really aren't that impressive given the shortcomings.

 Wulfmar wrote:
Looking at the Heckler and Koch etc, I can't see a breech.

It has an ejection port on the bottom of the weapon.

If you look at the dreadnought base, there are ejected shells on the ground. Now, I know that's not a basic bolter, but same tech, right?

Assault cannons are technically from the Autocannon family of weapons. Not related to the bolter, but it also uses cases too.

In the fluff a bolter is a .75cal weapon, meaning it fires a projectile .75 inches in diameter; as a matter of perspective a 12ga slug is .72 inches in diameter. There exists automatic 12ga shotguns that are fully manageable. For semi-auto shotguns there are 20 round magazines manufactured. In that way bolter is entirely in the realm of possibility.

There was only really one bit of fluff from way back when that described it as a caseless munition, while everything else has portrayed it as a cased cartridge. There are two advantages to caseless ammo: reduced weight of ammo allowing you to carry more and the time it takes to cycle the weapon is reduced as there would be no ejection cycle that translates into a higher rate of fire.

A weapon like a bolter would benefit from the reduced weight, but that would be unnecessary for a power armored space marine. A weapon like a bolter would not benefit from an increased rate of fire. High rates of fire from infantry weapons are predominantly for suppressing fire, but given the Space Marines modus operandi they don't really need to rely on rate of fire for suppressing fire.