wanderer, on 27 September 2015 - 08:54 AM, said:
And we should apply a common mechanic to minimum range rather than different functions for different weapon systems.
Different weapon types have minimum ranges for different reasons, however.
LRMs' minimum range is attributed to the warheads not arming prior to launch, a common trait in real-world missile systems.
BattleTech canon uses this reasoning, and the TT game even has additional rules specifically designed to work with this - namely, the "hot-loading" rules found in
Tactical Operations (and, before that,
Maximum Tech).
"The minimum range of LRMs and ATMs in the standard rules reflects the time it takes for the internal guidance systems to lock on to targets and for the explosive payloads to arm. Hot-loading enables a player to arm his LRM or ATM warheads before firing the missiles." -
Tactical Operations, pg. 102
For standard PPCs (which, for obvious reasons, would not share the LRMs' need to arm warheads), the minimum range is attributed to the risk of feedback along the path of the ion beam/bolt & the "field inhibitor" component intended to prevent damage from said feedback. Again, BattleTech canon uses this reasoning & there are TT rules found in
Tactical Operations (and, before that,
Maximum Tech) that specifically address the situation.
"The field inhibitors restrict the dangerous charged-particle feedback produced when a PPC is fired, but also prevent the weapon from firing accurately at targets closer than minimum range. Disengaging a PPC’s field inhibitor removes the minimum range modifier, at the risk of subjecting the firing unit to particle feedback." -
Tactical Operations, pg. 103
Arguably, the minimum range on the lighter autocannons could be justified via a need for the shells' warheads to arm (example: the 120mm M830 High Explosive Anti-Tank Multi-Purpose Tracer shells used by the M1 Abrams MBT arm approximately 60-100 feet from the muzzle of the gun after firing, and are armed "through the ballistic environment of firing, set back, and deceleration";
source), with the larger autocannons having such low muzzle velocities (which also explains their short effective ranges

) that they are able to arm before traveling any significant distance.
(Personally, I think that all of the ACs should have had minimum ranges to reflect the shells' need to arm their warheads, in one-hex/30-meter increments - that is, AC/2s with a 4-hex/120-meter minimum range, AC/5s with a 3-hex/90-meter minimum range, AC/10s with a 2-hex/60-meter minimum range, and AC/20s with a 1-hex/30-meter minimum range. But, alas.

)
In contrast to all of those, Gauss Rifle Slugs are solid ovoids or spheres composed of a nickel-ferrous alloy & about the size of a watermelon (see
here,
here,
here, and
here).
Gauss Rifle slugs have no warheads to arm, no booster rockets to ignite, no sabot petals to separate, and they rely purely on kinetic energy (a product of the slug's mass & velocity), and will be at their greatest velocity (read: damage potential) at muzzle-exit.
Thus, the Gauss Rifle's minimum range would have to be explained as a property of the weapon itself, and a significant firing delay (in the form of the charging mechanic) accomplishes this (in addition to its primary purpose of imparting a desynchronization effect versus other weapons, particularly the PPC family).
The broad effect ("being less effective at closer ranges than at greater ranges") is common to each minimum range implementation, but the underlying mechanisms & other particulars are (
and should be) different in each case.