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Just a minor correction on smoothbore vs rifled cannons on tanks. All cannons started off as smoothbore for ease of loading, which caused a loss of energy due to windage (loose seal of round to the barrel caused burning powder to blow pass tge round). The switch over to rifling occured with precision machined breachlocks allowing safe rear loading, while the rifling improved muzzle velocity with added accuracy via spin conveyed by a tighter seal of the round to the barrel. The switch back to smoothbore occurred with the adoption of discarding sabot, fin stabilized rounds which gain even greater muzzle velocity via tapered smoothbore barrels that conpact the sabot seal as it exits, building up more pressure from the slow burning cordite.

The best ratio of barrel length to round diameter is 50 to 1, of .50 caliber. This is also known is gauging, hince 12 Gauge (12 to 1 length to width ratio). At some point gauging and caliber got mixed up with round diameter measured in inches, which lead to .50 cal rounds being 1/2 an inch, despite the 14" rounds lobbed from an Iowa-class Battleship being .50 cal due to the length of the rifled cannon barrels.

The terms "bullet", "round", "ball" and "shot" mean the same thing, which refers to round sling stones which were later replaced with round lead shot, which is French is a bullet. Lead shot was use in the new Draguuns, or Guuns, which is where the words "gun" and "Dragoon" are derived. The familiar bullet shape comes to use from the Minie Rifle, the first common use conical round which was designed to expand on its powder side to grip the rifled barrel forming a tight seal. Etc.

SJ

 Ouze wrote:

The real question is, after seeing that - at some point could all marines fly/jump, and not just assault marines? After all, that is a "jet pack".

I do not know much about the older lore.

I think that was really referring to the zero-G movement thrusters which the marine backpack is equipped with. Not much use on a planet's surface, but in zero gravity it would allow for the marine to maneuver.

 KingmanHighborn wrote:
Well considering the real gyrojet weapons have a very low velocity BEFORE they leave the barrel it's likely there is a case to FIRE it up to high velocity like a multi stage rocket. So the casing is ejected and the bolt is traveling at a high speed already for close range, then the rocket kicks in for range/stabilization and impact force. Not to mention penetrating flesh and exploding once it's dug in.

Yes, I agree with your points, but bolts seem to have several different wounding effects based on the fluff they come from. There are fluff images and Dark Heresy things that indicate that there are numerous different types of ammunition that all fall under the blanket description of "bolts". It seems like bolts generally only explode if they hit a soft target, but when they strike armor or enemy cover they just create a huge impact.

It just occurred to me that there are a multitude of different relative gravitities in the galaxy depending on the mass of the terrestrial body they are on. I'm sure Space Marines compensate for these differences somehow. I'm just thinking aloud

 Ouze wrote:

 AndrewGPaul wrote:

The real question is, after seeing that - at some point could all marines fly/jump, and not just assault marines? After all, that is a "jet pack".

I do not know much about the older lore.

My understanding was that they were for use in zero g, and for stabilization especially in high gravity environments.