Best shell for 150mm x 2 meter Autoloader?

[Mad_House]

Hey guys, I'm looking for a good shell for my guns. Basically; six-barrel guns (on one firing piece) with high-rate of fire (highest you can get with autoloaders). What I want to do is equip them with a solid round for most ships; currently, I'm using the HEAT head with HE down to a solid base & bleeder. It seems to work, but, it takes a while to kill something. Shields and engine hold up pretty well, so I can go at it for about 15 minutes, but I'd like to have more penetration. What other rounds can I use that will be effective?

Also, should I mix-and-match rounds as well, or is that doable? I've got like 16 autoloaders for one gun, 12 for another, I assume I can do a few good ratios. Any advice is welcome.

[FourGreenFields]

15cm is relatively small for anything based on HE, EMP, or flak (and flak is only good for CIWS anyway). Might still work, but probably not the best choice.
Frag, possibly with hollow-point. Can pass one shield (with inertial fuse, obviously), but not layered ones. AP/Sabot could also work, but is stopped completely by shields.

You could use a mix of Warhead - Warhead-tracer rounds (warhead being HE, flak, EMP, frag, smoke). Replacing the rear-most warhead with a tracer module will keep the speed the same (at least up to max module length of the tracer). Gives twice the accuracy (like tracer-only), and you lose less firepower than when using tracer-only belts.
Doesn't work quite as well for HEAT and HESH (for those you generally want the fuse all the way back, for max hollow-charge/squash-head effect; other shells need in front of the warheads for slightly higher speed), but tuning powder-density should still allow for same speed for both shells - or replacing the tracer module with a warhead that's not contributing to the HEAT/HESH.

Also, what makes you think you can't reach higher RoF with autoloaders?

[Doombringer]

If using base bleeder with 2m shells, you can reduce the shell size to 148.1mm to get 14 component shells (or 142.8mm without base bleeder).

I love hollowpoint frags (I tend to use 90degree frags) mixed with some disruptors to deal with shields. (The disruptors need a little more gunpowder to reach the same speed as other shells.)
You can use smoke shells if laser defenses are causing you problems, but with that few autoloaders I would not bother with them.

Also at ~150m sabot spam cannons work too: sabot head, half/half sabot/solid AP body, lots of gunpowder. I think a 14 component shell (with bse bleeder) would be best with ~8 gunpowder, but I can't remember off the top of my head. It should be easy to play around a figure it out though.