Respectfully, I disagree with the proposed rules. Also, you might think it's comparing apples and oranges, but the core introductory weapons are designed to work together. When you are talking about game balance you have to consider what the other weapons do.
What you propose is flipping the ranges and changing the damage mechanic to make them more realistic. In my opinion, none of this works.
1 - Smaller Autocannons don't have much of a use anymore when compared to energy weapons. Arguably, even more so than the originals.
2 - Larger Autocannons DO resemble LRM launchers. You might not agree, but the weapons now don't have much of a difference from an LRM system. An LRM-20 is a much more efficient Autocannon 20. (I would also argue that this is what other weapon systems that currently exist suffer from, like HAG's and Rotary AC's).
I appreciate any effort to try and improve the game but I really don't see how these new rules help. I'm even trying to improve the game and have backed off on a lot because of criticism. Nothing wrong with being wrong.
Strongly agree with point 1 and I 2nd point 2 with the following caveat: I'd argue the "Preacher AC/20" would be a more flexible LRM-20. The LRM-20 has a big minimum range but the Preacher AC/20 has none, and its range brackets are slightly better in general. A little bit more heat, crits and tons is a small price to pay for marginally higher firepower combined with
far more flexible range brackets.
My approach to trying to "balance" the ACs and such is usually more minimalistic. Removing AC/2 and AC/5 minimum ranges, reducing AC/2 heat to 0 as will that really hurt anything? Increasing AC/2 ammo supply to 50 rounds, in lines with the rest and their 100 damage potential per ton. Fluffwise/Campaign wise, lowering C-Bill Costs and possibly tech rating of the cannons themselves (the "cheap" AC/5 costs more than the high-tech Large Laser). Marginal improvements that's not going to "wow" anybody but nonetheless makes them a little less gimped. I think the biggest 'change' I ever did was let ACs double-tap like UACs with a +1 to-hit modifier (and made UACs work a bit different as well). Those are the simple changes anyways, I dabbled a bit with alternative AC ammo like ATGMs and HV ammunition too but they're more rules-heavy and out of range of the discussion.
(I try to avoid revising weight/crits since that can potentially invalidate canon units or give them "dead weight". Makes the balancing job harder though.)
But back to the OP. "Powder charge" is an overly simplistic justification, and not correct either. I'm
really tired and lazy so I'll try to demonstrate without too much words or going math heavy.
If you're in a vacuum environment with a gravitational field like the moon (not completely irrelevant to BT as conflicts do occur in such environs from time to time), your absolute maximum range is only dictated by muzzle velocity and the force of gravity and the round will essentially follow a true parabolic arc. If we set gravity constant, the higher muzzle velocity weapon will have the greater range. In this case, the higher muzzle velocity of the 5.56mm NATO round at ~850m/s will have a nearly identical maximum range in a vacuum to the real Long Tom, the M1 155mm Howitzer, also at 850m/s muzzle velocity, despite the Howitzer's obviously far larger powder charge. We can also consider this maximum range the "maximum theoretical" maximum range as that's the range the weapon would have in that gravity field before considering aerodynamic drag.
Aerodynamic Drag is basically the reason why the huge 155mm Howitzer has a higher actual maximum range than the 5.56mm NATO round on earth despite similar muzzle velocities. Basically, the NATO round will slow down in flight due to the air pushing back on it, like an object thrown underwater slowing down rapidly. Assuming similar drag coefficients for the NATO bullet & the Howitzer shell, the bullet weighs 4 grams while the typical howitzer shell weighs 45,000 grams. The ratio of the weight of the bullet and the shell is 11,250 bullets per shell. The ratio of areas for a 5.56mm bullet to the 155mm howitzer shell is about 777, so 777 5.56 cross-sectional areas are equal to 1 155mm cross-sectional area. Divide the ratios and you conclude that the small arms bullet is 14.4x more effected at a given velocity by drag than the artillery shell, thus lower range in an atmosphere.
Even in an atmosphere with these rules, higher calibre, heavier shell & more powder doesn't translate to higher range, as a good-enough shell with a higher velocity can make up the difference. Ex: Compare the M116 75mm pack howitzer to the Japanese type 38 15cm howitzer. The Japanese howitzer has 2x the caliber of the American howitzer, the Japanese shell weighs 36kg while the American shell weighs 8-9ish kg, but the American howitzer has a typical muzzle velocity of 390 m/s versus the Japanese howitzer's 290 m/s. You can do a simple calculation to find muzzle energy: .5*m*v^2, and you'll see the Japanese's is higher and so should have more powder. Yet, the American howitzer has a typical maximum firing range of 8.8km while the Japanese one has a maximum range of 5.9km. So simply having higher muzzle energies and caliber is no guarantee to higher maximum firing range.
Either way, maximum firing range isn't the same thing as effective firing ranges, and real-life engagement ranges are generally far below that of their maximum theoretical values as it's actually
pretty difficult to actually
hit something 10 kilometers away, howitzer or not. Something that's allegedly with an effective firing range of, say, 5 kilometers with a 500 m/s muzzle velocity is going to take more than 10 seconds just for the shot to hit the ground, so take those numbers with a mound of salt.
In real life, guns tend to pay dearly in the weight department for improved performance. Take the famous Bofors 40mm guns. You have your L/60, an old WWII workhorse, and your super fancy high-tech L/70, which some ships use in some variants for anti-missile defense with 100 m/s increased muzzle velocity, faster turret traversing rates for tracking targets, and nearly a 3x increase in rate of fire. This comes at the cost of increasing the weapon's weight from 2 tons to 5 tons. Or the Bushmaster 25mm cannon as seen on the Bradley compared to the Bushmaster II 30mm cannon, a weapon that allegedly sees a 50% increase in firepower for a similar increase in weight (110lb -> 160lb)
The basic answer as to why small ACs have longer ranges than large ACs is: They don't pay the weight for it. The AC/20
doubles the AC/10s firepower for only 2 tons extra mass, 14 tons vs 12 tons. You just can't get BOTH massive increases in firepower and extend, or even keep your current range brackets at the same tech level for only such a small increase in weight. We're not going from a L7 105mm cannon to a Rheinmetall 120mm gun, we're going from a L7 to some weightier prototype L7 with a snub-nose but weighty automatic reloader, or some other junk. If we wanted to keep an extremely high range weapon with a hard-hitting punch, we'd either have a 30-40 ton AC/20 or some crazy crap, or we'd have a Gauss Rifle.
