What's the fix? If the 8.8cm does less than half damage, how much damage do the smaller cannons do? 5 damage doesn't seem like a lot for a 120mm cannon but the range is great. I don't really expect Black Powder Weapons to do much damage, unless they're really big weapons. Considering how armor works though, I would think they'd do something. Not a lot but something. More than guys with clubs but less than those with WWI weapons.
The fix is: Reduce damage by 20% and range by 25% for each level of BAR above 6. The reductions are applied to the new values for each level, so BAR 7 = 80% damage and 75% range of values vs BAR 6; vs BAR 8 = 80% damage and 75% range vs BAR 7 values; vs BAR 9 = 80% damage and 75% range vs BAR 8; and vs BAR 10 = 80% damage and 75% range vs BAR 9. Black Powder guns begin these reductions for each BAR above 3, and so they suffer through 7 layers of similar reductions by the time they hit BAR 10. Use normal rounding for all values, to minimum values of 0. (0-damage weapons at BAR 10 are possible, but not very likely.)
Damage from physical attacks (such as ramming), and nuclear weapons are not reduced by BAR. Artillery and Minimum ranges, when given, are not affected by BAR. (It is therefore possible to see weapons reduced in effective range to a point where their Minimum range modifiers overlap across multiple range brackets.)
Some fun examples:
64-pound Black Powder Cannon (vs BAR 3): 16 damage; Ranges (Min/Sht/Med/Lng/Ext): 5/22/43/65/173
64-pound Black Powder Cannon (vs BAR 10): 3 damage; Ranges (Min/Sht/Med/Lng/Ext): 5/3/6/9/23
--
16-inch Naval Gun (vs. BAR 5-6): 40A damage; Ranges (Min/Sht/Med/Lng/Ext): 5/25/50/75/200
16-inch Naval Gun (vs. BAR 10): 16A damage; Ranges (Min/Sht/Med/Lng/Ext): 5/8/16/24/63
--
125mm Smoothbore Tank Cannon (vs. BAR 5-6): 12 damage; Ranges (Min/Sht/Med/Lng/Ext): 11/56/113/168/280
125mm Smoothbore Tank Cannon (vs. BAR 10): 5 damage; Ranges (Min/Sht/Med/Lng/Ext): 11/18/36/53/89
--
30mm Fighter Cannon (vs. BAR 5-6): 3 damage; Ranges (Min/Sht/Med/Lng/Ext): 0/23/45/68/180
30mm Fighter Cannon (vs. BAR 10): 1 damage; Ranges (Min/Sht/Med/Lng/Ext): 0/7/14/22/57
--
30mm Gatling Fighter Cannon (vs. BAR 5-6): 11 damage; Ranges (Min/Sht/Med/Lng/Ext): 0/35/71/105/175
30mm Gatling Fighter Cannon (vs. BAR 10): 5 damage; Ranges (Min/Sht/Med/Lng/Ext): 0/11/22/33/55
--
.50 cal (U.S. [1945]) Machine Gun (vs. BAR 5-6): 2 damage; Ranges (Min/Sht/Med/Lng/Ext): 0/15/29/44/145
.50 cal (U.S. [1945]) Machine Gun (vs. BAR 10): 1 damage; Ranges (Min/Sht/Med/Lng/Ext): 0/5/9/14/46
--
7.62mm (European) Machine Gun (vs. BAR 5-6): 1 damage; Ranges (Min/Sht/Med/Lng/Ext): 0/14/28/40/135
7.62mm (European) Machine Gun (vs. BAR 10): 0 damage; Ranges (Min/Sht/Med/Lng/Ext): 0/4/9/13/43
Cassettes are limited to 2 tons, right?
If you're building them as robotic battle armor, yeah, but you can build the non-BA cassettes bigger.
I hope I did this right. It's been a while since I made a small support vehicle.
<snip>
I forget, are they supposed to have Remote Operating Systems and Sensors too? I thought, no, but I don't remember. If yes, then it's back to the scribble pad.
A quick review of your figures tells me you got it pretty much spot-on. Below 10-tons, robotics doesn't add any extra weight to a vehicle's control systems, and the Syberian version of AI is actually smaller in that they can fit their version inside a standard 3-ton 'Mech cockpit, so I think your control systems are fine. I was briefly worried you might have exceeded the vehicle's equipment slot allowance, but you were even ok there. Nice job!
- Herb