- Industrial Diamond Shot Shells (i.e. Buckshot made of diamond shards instead of lead pellets). Industrial Diamonds are relatively affordable, although they certainly cost more than Lead.
- Standard buckshot fired from the in-game Coilgun would have more range and cause a lot more damage considering the speeds involved.
- I agree with an earlier post that improvements to Laser power sources would be a feasible option. After all, you're studying alien Plasma and Antimatter technologies, among other things. Same weapon, different ammo cartridge, different color beam...
- An electromagnetic Assault Rifle is theoretically doable with a good enough power source, but this falls into the realm of a new weapon. I can see this happening before a five-barrel coilgun in the real world. Actually it would probably go Sniper, then Semi-Auto, and then 3-Round Burst, but anyways... The way I imagine this would be solid metal bullets/slugs/whatever with a basic automatic feeding mechanism as used in modern weapons. The difference would be that the barrel would be a rail or coil accelerator and the ammo would probably feed in as far back as possible, potentially having the magazine attach to or near the shoulder stock. The weapon could release several projectiles each time the accelerator "fired". Full auto wouldn't be possible unless you had the power to keep the accelerator on for several seconds, but firing a 3-shot burst wouldn't take any longer than firing one. The rounds would probably be smaller than those used in a sniper weapon.
- Chemical liquid or gas grenades would certainly work. Biological possibly as well. Biological darts as in X-Com Apocalypse probably wouldn't be a good option since the darts would have to penetrate the alien armor to be effective, which kind of nullifies the whole idea. A toxic gas would probably be rendered harmless since aliens use a breathing aparatus. Something that could quickly be absorbed through the skin and lead to violent convulsions or temporary paralysis (once it kicks in) would probably work best. We have plenty of such nasty things in the real world today so Phalanx would certainly have access to it. Although, a Concussion grenade might work even better. Armor won't offer much protection with those.
Just some thoughts after reading this thread.