I'm not sure if you are looking for a multiple-role AC, or an AC that can (effectively) fire any ammo of any type, and any diameter. If you are looking for the former, this may work for you.
On the main forums someone was looking for some fluff on how to "improve" rifle cannons. I suggested porting (usually, a port is used to let pressure out, but it is possible to reverse that) the barrel and using a secondary charge to maintain peak pressure during the entire time the projectile was traveling down the barrel, resulting in a higher muzzle velocity. Whether this translates out to extra range or damage is the subject to interpretation. While this is fine for a slow firing weapon, I'm unsure that it would work on an automatic or fully automatic weapon, BUT, this is BTU
. (Grabs handwavium wand, one MagiktekusBattleteckus later, and POOF! It works just fine!)
On a slow reload weapon, IE rifle cannon, the idea would be like placing a tank(or capped pipe) along side the barrel. Ports run from the tank to the barrel, the projectile is loaded into the gun while at the same time, a charge is placed in the tank. When fired, the projectile is accelerated down the barrel by both the primary charge and the secondary charge. (NOTE: IRL it doesn't quite work this simply, and someone (Cray) could blow a hole through this idea big enough to fly an overlord through. BTU tech again)
With that leap applied, I can conceive of a system that (subjectively) might give you most of what you are looking for.
The Multiple Role Variable Velocity Gun System. (MRVVGS or Mister Veegis.)
There are two barrels, one primary, one secondary. The primary barrel fires the projectile and is ported, the 2ndary has a computer controlled "restriction" mechanism on the end of the barrel and is also ported. The ports run from the 2ndary to the primary, and, due to timing requirements, would need to be computer controlled. The "charge" for the 2ndary is a round (or rounds, if necessary) of the same type (ac5,ac10) as is fired by the primary with the projectile removed. The Projectiles are removed (and then discarded) by a "shearing" (really more of a pinch or twist and pinch) mechanism after the ammo is loaded into the bin, but before the weapon is fired.
In high-range mode the 2ndary barrel is fully restricted to improve velocity as much as possible. However, the time for the propellants to fully clear the barrel(s) takes longer in this mode requiring a reduction in the rate of fire (and damage) in order to receive range improvements. During mid-range operations the 2ndary barrel is partially restricted so that the ports in the barrel(s) are kept clear. The range and damage are typical of an AC of its' type (5,10). In short-range mode the shearing mechanism is disabled, and the restrictor is opened far enough to allow a projectile to pass essentially doubling the rate of fire of the gun(s).
The MRVVGS is only produced in "light" (based on AC5) and "Heavy" (based on AC10) versions due to high ammo usage at the heavy end, and the damage being too low at the light end.
Rules:
While the MRVVGS uses either AC5 or AC10 ammo, the system uses 2 rounds for each shot. Result: Light MRVVGS ammo is 10 per ton, Heavy is 5.
Heat is doubled.
Range and damage is based on the type of AC used, with "short-range" being one class heavier, and "high-range" being one class lighter. Short-range damage is allocated as 2 clusters of "mid-range" damage, however either both clusters hit, or both miss as determined by the "to hit" roll.
Stats:
Clan Light MRVVGS (AC5)
Tons: 7 Crits:4 Heat:2 (I may increase this to 3) Ammo: 10 per ton
Range: Damage by Mode
High-range: 2/9/18/27 2
Mid-range: 7/14/21 5
Short-range: 6/12/18 10 (Two 5 point clusters)
Clan Heavy MRVVGS (AC 10)
Tons:12 Crits: 9 Heat:6 Ammo: 5 per ton
Range Damage by Mode
High-Range: 7/14/21 5
Mid-range: 6/12/18 10
Short-range: 4/8/12 20 (Two 10 point clusters)
Notes:
1. This weapon IS compatible with the Targeting Computer (as this is the only way to reliably "stack" the hits when using the short-range mode, Clan warriors will sometimes "call" their shots as a demonstration of skill)
2. This weapon "could" be an LBX type weapon, however, due to the porting in the barrels, this weapon would require that LBX Cluster ammo have the "buckshot" be contained in a container or "canister" that does not disintegrate (or be shed) until after the shot leaves the barrel. This is to prevent damage to the ports.
3. If this weapon were to use a standard barrel, I see no reason to prevent the use of specialty munitions, so long as the ammo count (per ton) is halved, and the round doesn't fragment while in the barrel. I would highly recommend using the ranges of the intro tech AC's if specialty ammo is allowed (used)
I realize that this may not be what you are looking for MA, but sometimes a different take on the same problem can generate the solution you are seeking.