Lets compare the oldest and newest official descriptions word for word (something Sarna isn't allowed to do).
Quote
LB 10-X AUTOCANNON
This weapon is closely relc.ted to the common
AC/10. The design of the LB 10-X uses several types
of lightweight, rapid-heat-dissipation alloys. Though
this makes the weapon costlier, its lighter weight and
need for fewer heat sinks easily makes up for the
expense. Another important feature of the LB 10-X is
its Mercury-IV fire control equipment. This electronic
system gives the cannon a better hit probability at all
ranges, as well as extending its maximum effective
range by 20 percent.
In addition to firing standard Dual-Purpose Armor-
Defeating Rounds, the weapon may also fire a
special Cluster Round that acts much like an anti'
Mech shotgun. After being fired, a set fuse triggers
the round to release several smaller submunitions
This improves the chance of striking a critical location
on the target, but also reduces the overall damage
done and spreads it out over the entire target area
rather than concentrating it in one location.
TRO 2750, page 8 (NOT the revised version)
(This description sounds more like a flak cannon with a smart round releasing small cluster bombs, since each of these 'submunitions' [usually short speak for explosive payload] has the same power as an LRM).
(Either these shells are really big to carry such deadly explosives, or there's a lot of smaller ones that make the overall blast.)
And this is what I found when looking up cluster munition.
And the 2018 description in the Battletech Battlemech Manual, page 98.
Quote
LB-X AUTOCANNON (LB-X)
“Nothin’ says lovin’ like a face full of ’Mech-scale buckshot.”
Special Munitions: Yes (cluster munitions only)
Technology Base: Clan or Inner Sphere
Type: DB, S
Such descriptive skill, much research. Also the over simplified misconception taken to an extreme.
[but to be fair its a "concise" "hardcore" rulebook not a fluff book, so the simpler it makes things the better it does its intended job].
Now to compare with autocannons.
Quite a difference. Also note that while the type of shells is not specified here, ACs do fire HEAP rounds. LBX does not, its standard shells by comparison even on Sarna...
(At the 10 rating)
"IS = 12,000 (slug), 20,000 (cluster)" LB-10 X
"Ammo Cost (per ton) 6,000" AC/10
And looking it up in Megamek, standard LB-10 X (non-cluster) ammo is always twice as expensive as AC/10 ammo in three tested time periods (2750, 3025, and 3063, though the actual prices varied a bit as while the first two were near the center of the IS, the last one was on the periphery for my Corsair testing and the price of regular AC/10 was nearly 90,000 and LB-10X was just about 180,000, this also had supply rules and manufacturing in play). Why would it be more expensive for LB-10 X if it fires the same ammo? It gets the same results, sure, but it clearly doesn't fire the same ammo.
I have a better comparison, but first...
Note how AC ranges are.
These are the expected effective ranges for an average pilot/gunner.
AC/2: 24 hexes (720 meters)
AC/5: 18 hexes (540 meters)
AC/10: 15 hexes (450 meters)
AC/20 9 hexes (270 meters)
Now this isn't how "bigger guns with bigger bullets work", the bigger the bullet, the farther it goes.
But since many of these ACs share the same size bullets, lets look at the most common shared between 5, 10 and 20, the 120mm HEAP shell. If it fires the same bullet, how come one does 5, one does 10, and one does 20 if the shell is the same? It isn't, but an AC/10 similar to a UAC/5 fires twice as many shells as the AC/5 in the same amount of time, thus reducing its weight. An AC/20 fires four times as many shells as an AC/5 in the same amount of time. And finally an RAC/5 fires 6 times as many shells as an AC/5 but while its range for making reasonably accurate hits is considerably further than the AC/20, the accuracy is all over the place [as your hit locations spread across up to 6 different regions on a given target]. (Notice a pattern here?)
But lets compare Rifles, the actual one shot FLD cannons of BT.
Light Rifle: (120mm) 12 hexes (360 meters) [3 to 0 damage]
Medium Rifle: (150mm) 15 hexes (450 meters) [6 to 3 damage]
Heavy Rifle: (190mm) 18 hexes (540 meters) [9 to 6 damage].
Note the bigger the gun and the bullet, the farther it goes. (Also fun, AC rounds have a max of 2k meters, a Heavy Rifle exceeds it considerably for max range.)
Consider that the MR has 9 shells per ton so each is 166.67 lbs if you assume the ammo bin, feed-from-bin mechanism, etc. all weigh 500 lbs in total -- compare that to an AC/10, assume the same weight for the bin and take the fluff for the Centurion's 10 cassettes per ton at 10 shells per second [as 10 damage] and you have each shell weighing 15 lbs [a real life 120mm sabot weighs 20 lbs], though if you consider ACs to fire for several seconds at a time they can be even lighter...
So giant hefty shells.
What if the LBX was a cross of these two?
Rapid fire, similar to autocannons. Large shells, similar to Rifles. (Notice PGI's LBX barrels are much bigger than AC barrels).
This way at most it fires a couple to a few cluster shells to net the intended result, counting as one "round" (or cassette/magazine, as it does for ACs). This clearly means it can't fire standard AC shells as these are much heftier, but are more than the simply oversized and overweight tank-like shells of the Rifles. As such, it fires shells like slugs from a shotgun, which individually hit much harder than any standard AC (but much fewer in the same amount of time).
The alternative, in order to fire standard AC rounds, is that the LB-10 X pumps nearly a dozen or more shells each with dozens upon dozens tiny explosives that altogether for an LB-10 X make ten damage and having to slug the enemy that many times just to get the full 10. Pretty impractical, and I imagine all the mini explosions would basically conceal the enemy. It also doesn't explain why they are always twice as much as standard AC ammunition for the same net result of damage.
Anyway going beyond that, no implementation of LB-# X AC in any Mechwarrior or Battletech game to date has ever been like the canonical one of the original lore because they all take the "shotgun" oversimplification too literally.
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