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According to sarna.net's BattleTechWiki*, the Autocannon/20 (or AC/20 for short) inflicts 20 damage on a direct hit; the Long Tom Artillery Piece inflicts a maximum of 30, diminishing with distance from blast focus; yet both weapons have 5 ammo per ton at an ammo cost of C10000 per ton. In other words: Both weapons fire 200kg shells costing C2000. How does the Long Tom get so much more bang per buck?

* I have no experience with BattleTech beyond the 2018 eponymous video game, so I could be looking at this all wrong.

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    Note that the shell as a whole may be 200kg, but that doesn't tell you the relative proportions of propellant to projectile. The Long Tom has a longer barrel, so all else being equal, it will have a higher muzzle velocity, hence better range and more damage. (All being equal includes using an artillery piece in direct fire mode.) – DavidW Jun 10 '19 at 2:48
  • @DavidW The propellant-projectile ratio was why I brought up the cost of the rounds. Maybe propellant and projectile have about the same cost per weight, in which case that's a viable explanation. Maybe they don't and it isn't. – michaelb958--GoFundMonica Jun 10 '19 at 4:25
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    Not sure why this has been closed - "In-Universe Explanations — Even Based On Real Science — Are On-Topic" as per scifi.meta.stackexchange.com/questions/7364/… – user22478 Jun 10 '19 at 23:45
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These are very different weapons systems. The Autocannon is a short range, direct fire weapon designed to be mounted on a manouverable Battlemech, whereas the Long Tom is a long range, indirect fire artillery weapon mounted on a large, slow, tracked vehicle.

The Autocannon/20 rating actually covers a range of different weapons with varying caliber and rate of fire, however the "20" rating means their damage output is equivalent. In the sarna.net description for example the "Crusher Super Heavy Cannon" is a 150mm caliber weapon that fires ten shells per "round" of ammo (i.e. each individual shell is actually 20kg, not 200kg).

The Long Tom on the other hand fires single very large shells (200kg per shell) that are large enough to have an area of effect when detonated (hence the varying damage depending on distance from the centre of the blast). The potential damage from being at the centre of the detonation of one of these very large shells is therefore much greater (30) than being struck by a number of smaller Autocannon shells (20).

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    Note that there is a smaller version of the Long Tom, the Long Tom Artillery Cannon, designed for 'Mech use. Even so, though, it's 20 tons and 15 crits vs. the AC/20's 14 tons and 10 crits, or roughly 50% larger. – Cadence Jun 10 '19 at 4:13

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