The "shotgun" implementation is actually the correct, canonical implementation.
LB-X ACs were always described as "BattleMech-scale shotguns", with the cluster munitions being consistently represented as
shotshell rounds (which fragment immediately upon leaving the weapon's barrel) rather than proximity-detonation "
Shrapnel shell" rounds (so named after its inventor, an English artillery officer named
Henry Shrapnel), the latter of which most people tend to (incorrectly) refer to as "flak rounds" (also,
"flak" is actually a role rather than any particular type of shell; it is an acronym that comes from the German term "
flieger
abwehr
kanone",
meaning "Flyer Defense Cannon" or "Airplane/Aeroplane Defense Cannon") and/or confuse with actual
canister shot rounds (which, as it happens
and somewhat ironically, actually operate in a manner closer to shotshells than Shrapnel shells).
LB-X ACs are also specifically described as smoothbore weapons (
TechManual, pg. 207; "These materials, coupled with a
smooth-bore, multi-munition feed mechanism, make the LB more expensive than standard autocannons."), a descriptor that is used in such a way as to indicate contrast with other AC types... and one which is significant specifically because
rifled barrels, due to the effect rifling has on spread, are generally unsuitable for firing shotshells, an issue that would not exist with Shrapnel shells.
Additionally, the CBT Master Rules (Revised Edition, #10984) specifically states (on page 132), "The LB-X autocannon can fire cluster munitions, which act like an anti-BattleMech shotgun in combat. When fired, the ammunition fragments into several smaller sub-munitions."
The description indicates that the cluster munitions fragment "when fired", as opposed to "when the shell is within X meters of the target", further supporting the "cluster rounds as shotshells" LB-X model.
Moreover, many (if not all) of the novels in which they appear specifically refer to LB-X ACs and their specialty shells as "'Mech-sized shotguns", "glorified shotguns", and so on.
In conclusion: TT/canon LB-X cluster ammo was always intended to be a shotshell and is consistently described in a manner indicating such (as opposed to being a Shrapnel shell), and MWO's implementation of LB-X cluster ammo as shotshells is consistent with BT canon and therefore correct.