You may mock AC/2s but wait until you get a platoon firing in rapid fire while using Armor Piercing rounds. Or precision rounds at extreme range.
Waste of ammo.
The extra crit using AC/2s happens 1/36 of the time due to the hefty penalty for being an AC/2. Normal TAC checks on a 'Mech happen 1/36 of the time and the result of this TAC check averages to .6111 crits per check. Non-AP ammo ends up having about 11/648 crits per hit (~1.7 crits per 100 hits), and AP AC2 ammo when including its "free" check will have about 29/648 crits per hit (~4.5 crits per 100 hits).
4.5 is of course quite a bit better than 1.7, but AP ammo also hits significantly less often than standard ammo due to its +1 to-hit modifier, and even less often than precision ammo. For instance, if the base to-hit number is 10, AP ammo hits only 1/12th of the time (modified THN of 11), standard ammo hits 1/6th of the time (base THN of 10), and precision ammo hits about 42% of the time (modified THN of 8, -2 to-hit bonus). At a base THN of 10, the average number of crits per shot when adjusting for hit frequency becomes:
Precision-.708 crits per 100 shots
AP-.375 crits per 100 shots
Standard-.283 crits per 100 shots
Note that AP ammo is only somewhat better (~30%) than standard ammo at causing crits despite its bonus, and standard ammo is hitting twice as often so more damage is actually being done. Also note that Precision blows AP out of the water at THN 10 at its own job of causing TACs.
Change the THN to 9 and the results become:
Precision-.991 crits per 100 shots
AP-.75 crits per 100 shots
Standard- .472
AP finally blows past Standard shells, but Precision gets 3.5x more hits in, which is important for wearing down opponents and getting internal crits.
Change the THN to 8 and the results become:
Precision-1.23 crits per 100 shots
AP-1.25 crits per 100 shots
Standard-.708 crits per 100 shots
Finally, at THN 8, AP shells starts to get more crits in than even Precision rounds... by less than a 2% difference. At such a small difference they're functionally equivalent, and considering Precision's still hitting a lot more frequently, you should have an extremely good reason to be using AC2 AP rounds over Precision in this case, such as "I am out of Precision rounds".
Changing the THN to 7 results in the following:
Precision-1.42 crits per 100 shots
AP- 1.875 crits per 100 shots
Standard-0.99 crits per 100 shots
AP now starts to get a decent lead on even Precision ammo, causing 32% more TACs on average. Even now, the AP ammo is hitting less than half the time while Precision is hitting 5 times out of 6, so you have to
really like those TACs to go with AP ammo in this case, but at least a reasonable case can be made for it. Of course, if you're regularly getting THNs of 7 on your opponent's perfectly-healthy 'Mechs, AP ammo is the least of their problems...
TL;DR version: AP ammo is maybe OK at to-hit numbers of 7 and below, but honestly just take Precision ammo.
