look at the timeframe your campaign is going on in. A rotary autocannon is grossly overpowered for 3025, and somewhat limp for 3150, where all the other experimental techs show up.
so start with logical variation.
For example, the canon Heavy Machine-gun for the Inner Sphere weighs as much as a medium laser with a small laser chaser, ranges 2, and only provides a boost against light (unarmored) infantry for the armor damage of a single small laser-at the risk of a bin of explosive ammo and a fairly huge burden in terms of carriage space.
compare to the Clan version, which, with half a ton of ammo, weighs one ton.
The problem is a logical one; what kind of idiot would build the Inner Sphere version, which is qualitatively inferior to virtually anything you can put into that ton-and-a-half?
Esp. since you can get 30% more range and an extra D6 of anti-infantry damage by carrying standard inner sphere machine guns) while having less of a torso bomb if your ammo bin gets critted (since you'll use it up faster, and per-shot damage is lower)?
answer: Nobody. really, outside of forced canon exceptions, anyone arming to actually WIN isn't going to take this weapon, it's a dog.
So...let's do an alternative, by ocnsidering the idea that not all researchers just blindly copy the performance of something because "Duhr, it's Klanner!!! duh-huH!!!"
The Alternative Inner Sphere Heavy Machine Gun:
.75 tons (meant to be used in pairs, obviously, or to absorb that loose quarter ton you sometimes get with fractional accounting).
Damage: 2 (2D6)-just like a standard machine gun.
Ammo Explosion: 3 per round in the magazine (like Clanner HMG's)
Range:
Short: 2
Medium: 4
Long: 6
The extra weight is reinforced breech and the extra range is more and hotter propellant for the same projectile, further, you're not spending the equivalent of a Medium Pulse laser plus heat-sink to mount two of them to do the same job. The damage curve is a little lower at the top end, but it can inflict that damage further away than a standard machine gun (though not really that much further), making it good as a support weapon for covering infantry embark/debark from transporting units, or greasing that riot at ranges they can't reply with PISTOLS.
This, in an AU, can replace the "inner sphere heavy machine gun" which is outperformed by pistols and shotguns, with something that actually justifies its existence (and eliminates the "Light" machine gun that weighs in the same as a standard for one point tickles).
thus, a factionally different development curve, which doesn't rely on "Well duh, Clan Gear is Better".
further, this can slide into earlier, or lower-tech eras without massive disruption or being grossly overpowered, or replicating the performance of something fundamentally already better. which makes it easier to balance.