I did some testing by playing around with the ccm_k0 values of the AIM-120C and testing it against a closing ace-level Flogger at high altitude yesterday. According to notes on some of the missiles, a value of 0 should represent perfect countermeasure rejection capability while a value of 1 represents "medium" countermeasure rejection. Indeed, as Tippis noted above, the way missile seekers are coded in the Lua has the more advanced missiles with CCM values progressively closer to 0, and most missiles in the game fall between 0 and 1.
When I tested values between 0 and 1, which is where most missiles in DCS fall, I could not discern any practical difference in countermeasure rejection. At a value of 0, which should supposedly make the missile immune to chaff, as well as values like 0.01 and 0.000000000001, the missiles still went dumb against chaff in well over 95% of my tests. Things were the same when I tried a value of 1 and anything in between.
Following this, I tried arbitrarily larger values. Setting the values to 10, 50 and 100 still had the missiles miss for chaff almost every time, but the missiles seemed to gain the ability to reacquire the target after falling for chaff. At 1000 and 10000000000, the missiles would flip-flop between tracking chaff but ultimately reacquire and either get very close or hit the target almost every time (though a factor affecting affecting this is that sometimes the AI would stop notching when it thinks the missile went for chaff).
I haven't tested the effects on IR and SARH missiles, but I strongly suspect that countermeasure rejection might not currently function correctly in DCS as a whole, at least with respect to the values found in the Lua. The values which should give perfect countermeasure resistance clearly don't while the values that should give lower resistance seem to give the missile a progressively higher probability of kill, though the behaviour at those values is really wonky, with the missiles constantly shifting between tracking the chaff and the target.
I'm attaching several tracks of shots at ccm_k0 values at 0 (which should be considered typical of all the shots I made at values between 0 and 1), 10, 50, 100 and 1000.
AIM-120C CCM Test 1000 N1.trk
AIM-120C CCM Test 1000 N2.trk
AIM-120C CCM Test 1000 N3.trk
AIM-120C CCM Test 1000 N4.trk
AIM-120C CCM Test 0.trk
AIM-120C CCM Test 10 N1.trk
AIM-120C CCM Test 10 N2.trk
AIM-120C CCM Test 10 N3.trk
AIM-120C CCM Test 50 N1.trk
AIM-120C CCM Test 50 N2.trk
AIM-120C CCM Test 50 N3.trk
AIM-120C CCM Test 100 N1.trk
AIM-120C CCM Test 100 N2.trk