I've been thinking/working on a hybrid "Phoenix Command/TW2K" modern mini's skirmish ruleset and each of these systems seem to use the action point system to track things (by the way, you should be able move AND shoot as an action, but you can't move as far or as agile (no running across a beam while shooting) nor do you shoot very accurately). I've thought of having an action point track for each character on the table with action markers. You count up how ever many squares a declared action will take. You plot out your actions and then execute the half second (full second or whatever) turn based on your plotted actions. The problem though is opportunity. I may have intended to run behind cover, aim and shoot but during my sprint to the obstacle, I observe another target...I may try to utilize the opportunity to change my intended action during the turn. That needs to be factored in. BUT, the biggest problem for me has been sequencing.
I'd like to be able to play with a few squads worth of characters per side for a combat sequence. Tracking the actions and who goes first (usually based on initiative values) just won't work when you have that many characters, so I want to come up with another way. Phoenix Command breaks down a half second turn into .2 of second combat turns (or sumsuch) and that's just way too much overhead. You can say that all actions in a half second turn are simultaneous I suppose but like what Boot Hill did with the "quick draw" should have a chance to be applied...say I get my shot off just a split second before you do...you are hit and die and thus your shot never happens.
That's where I'm stuck. Not sure if this is beneficial to you at all but thought that I'd throw it out there for you.