I'm afraid it cannot be that simple. At least under the current rules, as clearly allow a swarming conventional infantry unit to remain on a 'Mech even if it drops below 16 troopers. See below for a detailed explanation.
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EXPLANATION:
The table on p. 221 refers to a so called "swarm attack". According to p. 220 (third paragraph under "Swarm Attacks") "The swarm attack to-hit roll determines only if the infantry unit manages to gain secure footholds on the 'Mech. The infantry unit does not inflict damage on the 'Mech during either the Weapon or Physical attack phases this turn."
Later on p. 223 under "Swarm Attack Damage" it says "If the swarming infantry unit stays on the 'Mech, it may make normal arm-mounted weapon attacks during the Weapon Attack Phase of the turn after it successfully swarmed the 'Mech. [...] All attacks automatically hit."
So conventional infantry needs to roll only to climb a 'Mech, and only then it the modifier from the table on p. 221 applies. No to-hit roll is needed to stay on a 'Mech in subsequent turns and to apply damage.
The last paragraph under "Swarm Attack Damage" before "Critical Hits:" (p.223) lists conditions under which swarming infantry unit leaves/is forced to leave a swarmed unit: "Infantry units can continue to make weapon attacks on the 'Mech in subsequent Weapon Attacks Phases until the 'Mech is destroyed or manages to shake off the attacking unit, or the swarming unit chooses to end its swarming attack". Nothing about swarming conventional infantry units under 16 troopers being automatically forced of a 'Mech either here or (as far as I can tell) anywhere else.
The way I understand the rules as they are written a conventional infantry unit has to have at least 16 troopers only to climb a 'Mech, not to stay on it. Once an infantry unit successfully swarms a 'Mech, it leaves the 'Mech only under one of the following conditions:
- the swarming infantry unit is destroyed (obviously),
- the swarming unit is shaken off (see "Fighting Off Swarm Attacks" pp. 221-222 - nothing there about a conventional infantry unit below 16 troopers shaken off automatically),
- the swarming infantry chooses to end the attack (according to "Swarm Attacks" right before "Aimed Shots:" p. 220 "A swarming unit may end a swarming attack during any subsequent Weapon Attack Phase. [...]".
Furthermore the last paragraph under "Mechanized Battle Armor:" on p. 222 explicitly states that a BA unit may attempt to mount a 'Mech swarmed by less then 16 conventional troopers. The rules state "Even if the swarming enemy infantry have fewer troopers then will provide a Modifier from Swarm Attacks Modifier Table (for example a conventional infantry unit with fewer than 16 active troopers), the friendly battle armor attempting to mount must still make a standard Swarming Attack, but no modifiers from that table are applied."
This one sentence from the rules confirms my interpretation of the rules. A BA unit may attempt to mount a 'Mech swarmed by fewer then 16 conventional troopers, so it must be possible for a 'Mech to be swarmed by less then 16 conventional troopers before a friendly BA mounts the 'Mech. I'm unclear if the BA mounts a swarmed 'Mech at the end of Ground Movement Phase (as under Mounting on p. 223) or during Weapon Attack Phase (as a special case of swarm attack in which BA "swarms" a friendly unit), but either way it happens before any damage is applied to the conventional infantry (unless the damage happens during Movement Phase, but the only situations when it may happen are described in the left column on p. 222, and all of them end with the conventional infantry either shaken off or destroyed before friendly BA may attempt to mount a 'Mech).
So the rules state, that a BA unit may attempt to mount a friendly 'Mech swarmed by less then 16 conventional troopers. At the same time the conventional infantry can not take damage during that turn and remain on the 'Mech before BA unit makes it's attempt. This means, that under current rules it is indeed possible for less then 16 conventional troopers to begin a turn swarming a 'Mech. This in turn means, that the conventional infantry unit could not be forced of a 'Mech at the end of the previous turn just because it had less then 16 troopers.
Finally we come to the problem which started this whole thread:
- under current rules it is possible for friendly BA unit to attempt to mount a 'Mech, while the 'Mech is swarmed by 1-15 conventional troopers (as explained above),
- the rules state, that in this case "[...] the friendly battle armor attempting to mount must still make a standard Swarming Attack, but no modifiers from that table are applied." ("Mechanized Battle Armor:" p.222, second paragraph),
- in a paragraph just above the rules state, that under normal conditions (when there are more then 15 swarming conventional troopers, or a 'Mech is swarmed by hostile BA unit) a friendly BA unit attempting to mount "[...] must make a standard swarm attack, with all appropriate modifiers. This includes those from the Swarm Attack Modifiers Table for the number of active enemy troopers swarming the friendly carrying unit versus the number of active friendly battle armor troopers attempting to mount; this number is applied as a negative number to the target number. For example, if 21 enemy conventional infantry were swarming a 'Mech and a 4 trooper friendly battle armor attempted to mount the 'Mech [...] in addition to all standard modifiers, a -3 modifier (the +3 from the table, turned into a negative number) would be applied to the target number."
So according to the first paragraph under "Mechanizad Battle Armor:" on p. 222 it looks like a friendly BA gets a bonus to roll to mount a 'Mech swarmed by enemy BA unit or at least 16 conventional troopers. The bonus ranges from 0 to -7 (minus what is written in the Swarm Attack Modifiers Table) not counting bonuses for battle armor equipment, 'Mech being prone or immobile etc., but according to the second paragraph in that section if a 'Mech is swarmed by 15 or less conventional troopers, the friendly BA does not get the bonus. This means that it is always more difficult for friendly BA units to mount 'Mechs swarmed by 15 or less conventional troopers (they get no bonus), then 'Mechs swarmed by 16-20 or more conventional troopers (at least -1 bonus depending on friendly BA unit size).
This is not logical because for 16+ swarming conventional troopers the larger the swarming unit, the more difficult for friendly BA unit it is to mount the 'Mech (this part is logical), but when there are less then 16 conventional troopers left the roll to mount suddenly becomes more difficult (no bonus from the Swarm Attack Modifiers Table) (this part is not logical).
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POSSIBLE FIXES:
I can see two possible ways out of this entire situation:
1) Change the rules to fit your ruling in the post above (conventional infantry units below 16 troopers are forced of a 'Mech at the end of a turn) - THIS REQUIRES AN ERRATA, BECAUSE CURRENT RULES SPECIFICALLY STATE HOW TO HANDLE A SITUATION, WHEN A FRIENDLY BA ATTEMPTS TO MOUNT A 'MECH SWARMED BY CONVENTIONAL INFANTRY WITH 15 OR LESS TROOPERS, which is impossible under you ruling;
2) Allow a swarming conventional infantry unit to remain on a 'Mech after it has been reduced below 15 troopers (as seems to be intended in the rules), and change the second paragraph of of "Mechanized Battle Armor:" on p. 222 to something logical. Personally for simplicity I would go with one of the following:
- give friendly BA unit a bonus of either minus what is in 16-17 troopers line of Swarm Attack Modifiers Table (so mounting when there are 1-15 swarming troopers is just as difficult, as when there are 16-17 swarming troopers),
- give friendly BA unit a bonus of either minus what is in 16-17 troopers line of Swarm Attack Modifiers Table, minus extra 1 point (so mounting when there are 1-15 swarming troopers is a little easier, as when there are 16-17 swarming troopers),
- make mounting automatic, when there are less then 16 enemy troopers left (simplest solution).
IN THIS CASE YOU WOULD ALSO NEED AN ERRATA.
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SIDE ISSUE:
Aside from this main problem I believe there is another smaller issue that i noticed while writing all of the above. When do friendly BA make decision to mount a 'Mech swarmed by enemy infantry declared? When is the roll to do so made? I would lean towards "at the end of a movement phase" as opposed "during weapons attack phase", as it avoids a potential timing problem, but the rules under "Mechanized Battle Armor:" on p. 222 don't say one way or the other. The timing problem is that if both enemy and friendly infantry try to climb a 'Mech at the same turn. If the friendly BA does so in the weapons attack phase, you could theoretically get a situation when at the end of a movement phase a BA unit refuses to mount a 'Mech (using rules on p. 223 for infantry carriers), then during weapons attack phase enemy infantry declares it will attempt to swarm the 'Mech, and then the friendly BA unit has to declare to attempt mount the 'Mech (using rules on p. 222 for fighting off swarm attacks) before it is known if the swarm attack by enemy infantry succeeded or not.