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Regarding the in-game function of standard 2, 5, 10, and 20 ACs: compared to the alternatives, ACs are a poor alternative to missiles and lasers, even taking heat into account. Consider the Large Laser versus the AC/10. The AC/10 gives you a better heat to damage ratio, but at what cost? 12 tons plus at least one more ton of ammunition. If you invested a total of 12 tons into a Large Laser, you spend 5 for the laser, and 7 for heat sinks. That amounts to 8 damage and 1 heat at the same range, with fewer critical slots taken up. On top of that, the Large Laser costs half the C-Bills of an AC/10. Apply this same critique to all ACs and you will quickly find that you are much better off going with the alternative weapons. PLEASE NOTE: this may or may not apply to more up-to-date ACs. — The preceding unsigned comment was provided by Kendrick (talkcontribs) on 29 August 2008.

-Well, the Autocannon is quite an effective piece of equipment, and if you are not playing with Double Heat Sinks, the Autocannon can be a high damage, low heat alternative to the very hot Large Laser. In particular, I prefer to arm my Marauder's "head" weapon as an Autocannon rather than a Large Laser if I have the choice.

Also, the Autocannon has several useful variants, such as the Ultra (potential double damage if you roll an eight or higher on 2D6,) or LBX, which has the very useful -1 to hit bonus and can be used for solid slug or pellet shot (which is practically instant-death to anything that flies.) Either way, I like Autocannons for their high power and low heat generation. That, and with them, you don't need to burn up crits slots or tonnage with extra Heat Sinks. -jregley

-If you're looking to mount a weapon on a vehicle powered by an ICE, a large laser, along with its required 8 heat sinks and half-ton of power amplifiers, actually weighs a half-ton more than an AC/10 and one ton of ammunition. Ironically, choosing the large laser with its 8 damage could require you to reduce your tank's armor coverage by 8 points.

Beyond that special case, it then becomes a question of how much you value the additional two points of damage. If you're looking purely for a way to punch through armor (e.g. shooting at vehicles, installations, DropShips, etc.), then you're correct about the laser being a better choice. But higher-damage weapons have additional affects beyond merely flaying away armor. An AC/10 hit to a 'Mech's head is guaranteed to generate a critical hit roll, while protecting against critical hits from AC/10 hits on fighters will require 20 extra points armor (1.25 t) on the desired location. And if a Behemoth hits a 'Mech or a fighter with both of its AC/10's, it triggers piloting skill rolls that two large laser hits would not.

Finally, you claim that all autocannons are inferior in general, but the AC/10 is about the only class of autocannon where there are close alternatives. The AC/5 does only half the damage of a PPC, but has negligible heat, and while the AC/2 is considerably bulkier and more expensive than an LRM-5, it shoots farther and will literally pay for itself with its ammunition prices. It's all apples vs. oranges and, like everything else when it comes to design, requires determining the right tool for the right job. — The preceding unsigned comment was provided by (talkcontribs) 2 December 2009.

I find the autocannons to be very useful, and a good alternative to LRM's. There is definately no substitute for a Large laser or a pair of mediums, but add the additional punch of a good autocannon, especially the ac10, and you've got a good combination for a viriety of uses.
add to this the fact that like lasers, autocannons require zero lock time, and it just gets better and better. call me oldschool, but I always take an autocannon along, in addition to lasers or ppc's. Even with a gauss which serves the same purpose for mechs, I'll usually take an ac2 or lbx5 for anti-air anyway.
don't get me wrong, LRM's have their place, but are certainly not a 'necessity'. A lrm5 with 1 ton of ammo is great for dispatching turrets and vehicles for example. but against another mech, even the lrm20 is wasteful both in terms of heat, and ammunition cost. plus factor in AMS, ECM, and the required lock time, and you're better off with an AC or a gauss rifle.— The preceding unsigned comment was provided by (talkcontribs) .