Autocannon Vary by the Manufacturer... A description that works for one design will be totally indalidated for another...
For the Most Part, Autocannon are Automaticly loaded and Fired Cannon.
Vehicle Guns are more likely to be Straight Barreled than Rotary Barreld (they have the geometry for it)
Standard Autocannon are Straight Barreled or Rotary Barrel arranged Autocannon that Fire between 1 to 100 Shells of calibres that range between 20 mm and 203 mm, 30 mm to 203 mm being published in the Canon body of work.
Ultra Autocannon are more likley to be Rotary Autocannon that have a set spin/fire rate with the capability to double the spin/fire rate at the cost of risking damage to the weapon... TTBMK No canon artwork has depicted an Ultra Autocannon as a Straight Barreled Design (the closest is an Armoured Cowling that covers all but the port that rounds are shot out of - this is depicted in a Caesar or Catapracht artwork in IIRC BattleTech Master Rules... before introduction of RAC in Canon... Unlikely that would have been depicted in Canon product while only published in meerly Official Mechforce UK Products
LB-X Autocannon... Variously seen as Large Bore - eXtended or Lubalin Ballistics - eXperimental no canon source has really given indication of what LB-X Stands For. However, the Cluster type Shells are true cluster Shells like the real world piece fo equipment (fire, leave barrel, then deploy submunitions... rather than fireing something like grape shot, or true proximity fused ordnance)
Light Autocannon are likely to be straight Barreled, but this is only a guess considering that they are the predecessors of the Rotary Autocannon... Note, While the RAC has a Spin / Fire Rate that varies from 1 - 6 times... the Light Autocannon only has a spin / fire rate of 1x (barring dueling rules or Rapid Fire rules for BattleTech)
Rotary Autocannon are always rotary autocannon, and, sinc they are descended from the Light Autocannon, they are more likely to be fairly close to 30 mm in calibre
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Christopher Robin Perkins
It is my opinion that all statements should be questioned, digested, disected, tasted, and then either spit out or adopted... RHIP is not a god given shield