You need to be able to do at least 1 SRM damage in AS (10 damage in BT) to get the special ability. A single SRM-6 is only .8 (average of 4 missiles from cluster hit for 2 damage each is 8 BT damage or .8 AS). Thus, Warhammers, Orion and Quickdraw don't qualify.
Two SRM-4s would be .6 each for 1.2. however, that has to be adjusted for heat, and that's where most of the Crusaders have a problem. They have more than a 20% heat inefficiency, so they usually drop below 1 and therefore don't qualify. The 3K reduces the lrm racks (lowering overall heat) and adds heat sinks, and thus its SRMs are not as affected by heat and can qualify for SRM.
Units with artemis don't qualify for SRM, because the primary purpose of the SRM special ability is to use alternate ammunition. If you're using artemis ammunition, you can use alternate non-artemis ammunition.
