OK, can't seem to find any add-on slot trouble (unless I'm missing something) so I'll keep looking for what the trouble might be with this.
I know this is just a test version, not even a beta, but I figure I might as well list what it does.
1st, no model or icon changes at all. All I've done is text file changes so far. I've tried messing with icons and models before and not had much luck (admittedly I only tried putting in a model of mine once, a long time ago).
1- 2 new damage types: Laser Beam, and Armor piercing.
Laser Beam is the non armor piercing version of normal laser (which is considered pulse laser, though I don't think I can change the name of the basic laser type damage). Laser and laser beam is now a little longer ranged than other dam types, laser beam is a little more accurate, does a little more damage overall in lots of little packets. Basically does a handfull of points of damage but rapid fire. (I'd like to get a beam visual effect that lasts a hair longer so they merge together and similar for a 'bzz' style sound effect that merges into the next shot, thus giving it a 'beam' feel rather than lots of individual shots. But that's for another time if I ever figure that out. I may end up going for the plasma beam FX eventually, though I'd prefer something more like a needle thin laser graphically)
I altered the damage resistance of enemies a bit vs lasers (both laser and laser beam) and both Matriarchs and robots are not as vulnerable to them as before. They still will hurt them, and the inherent high accuracy of lasers means they still are good vs these, but with good marksmen it might be better to go for AP bullets or something. As I said, lasers are the AP versions of beam lasers in this, so reticulan lasers and human pulse lasers are better vs heavier foes (if lasers affect them) and beam lasers are better against weaker, unarmored enemies.
Armor piercing projectile damage is a little more extreme than I've seen before, basically normal projectile damage is good vs unarmored targets (beastmen, shaman etc) and armor piercing is good damage vs heavily armored targets (rollers, matriarchs etc... though accuracy vs matriarchs is still low... they dodge bullets

) Armor piercing damage tends to blow through unarmored targets and do very low damage. You can't just switch to AP bullets late game to deal with everyone! Martians too are particularly immune to AP rounds, more so than normal projectile damage.
Also, I replaced the existing High damage bullets with AP bullets (could this be a problem with the mod? Hmm, maybe I can't alter the existing ammo type? I might try disallowing the high damage bullets, but leaving them as normal, then adding AP bullets separately)
I altered the long range bullets (for all guns that use them) to work over a longer range, be slightly more accurate, but do much less damage than normal. I think this should open up a bit more strategic options, though I might have to reduce the damage more to balance it out later (right now it's at about 60% normal)
2- Changed quite a few existing weapons:
Scientific laser now does laser beam damage and I've put in an option for 'laser autofire' mode which just cools down quicker between bursts. I was hoping to add in the add-on slot for the cooler on the scientific laser later.
Laser rifle comes earlier and does laser beam damage only (to begin with. I'm hoping to add an add on to make it use the 'ret power shot' mode to fire normal pulse lasers). It doesn't use crystals to build it like the laser cannon.
Laser cannon has laser beam in it's 'laser autofire' mode, but other wise is similar to normal. I have adjusted the energy consumption vs energy in batteries so when it uses a battery up it actually is used completely. I've had too many games where I kept glancing in my inventory and thought I had tons of ammo... but they all had one shot left! Frustrating, and pretty pointless.
All the old earth guns now can have both long range and AP rounds. An early research topic allows you to build ammo for these (first just normal ammo, advanced firearms gives the others), not building the guns themselves, but this leads to the normal 'firearms' topic. I'm considering allowing them to all to have a few more add ons too, though not all three. You need some other reason to make the weapons you make later be more adaptable than the old ones.
I removed the prerequisites of 'martian language' and 'magnetron' from EM handling and plasma handling (but not from EM theory and Plasma theory), meaning you can USE these weapons earlier, but still not research any human designed weapons etc. I also took out the Plasma handling skill from the plasma throw (flamethrower) mode of the martian plasma cannon so anyone can use it. I also made plasma martian weapons more effective. Beware Matriarchs with these now! The flamethrower is slightly longer ranged and a bit quicker and the plasma shot (big flashy glowing ball thingy) travel faster and quicker to ready. This is true for ALL plasma weapons, including robot ones

I added advanced robot battery that has twice as much storage as before (and costs more to make) coming in at the same time as adv battery. Also added in AP and Long ranged robot bullets.
All batteries have ten times as many 'rounds' in them, but normal shots cost more 'rounds' too. Only beam lasers cost 1 or 2 shots every time they fire. They fire so rapidly it was neccessary or beam lasers would use one battery every few seconds!
(Edit: Almost forgot!) Altered the shotgun and the beastmen cannon. The Beast cannon is now more damaging, but closer ranged, plus uses 3 rounds, not 2. Now it uses up the ammo mag completely. The shotgun now has three ammo types.
Shell: a slug that does normal damage but NOT shotgun style. It's a bullet, there is a chance for it too miss! It doesn't just go downn in damage at range. Modes aimed and snap only.
Buckshot: has scattershot mode only, but has increased range, but reduced damage. It only uses one shot when fired but the mag hold 6 shots rather than the 8 in the shell mag.
Flechette: basically scattershot but it does AP damage, fires multiple small tungsten, spin stabilized darts. Just like AP ammo it's good vs armored foes, not so good vs unarmored. This is slightly more accurate than buckshot
I think that covers it for the moment. I hope I can get this thing working right soon! Later I hope to add in some mods to one or two armor types and (if I can figure it out) change some of the graphics, at least the icons on the ammos. Right now the FN-2000 and others have ammo that looks identical if it's normal, AP or LR. I'd also like to add in some more high energy weapons late game, like a rapid fire Pulse laser (definitely be a heavy weapon) and perhaps some form of particle weapon... not sure about that, it would have to have some kind of damage varience over different targets that would differentiate it from other types of damage. Otherwise it'd be a 'big laser' in all but name. Anti matter particle gun? Meson gun that skips objects and walls etc? (though I don't think the 'shoot through walls' mode works for any weapon but psi, and that's hard limited to the ret Psiweapon.

) One thing I thought of was some form of autocannon, basically a heavy calibre machine gun with varying munition. Not sure if you can get explosions to work with direct fire weapons though. Looking through the radius weapon layouts it doesn't look good. Can't seem to see a way to project the 'explosion' away from the firer and onto the target unless it's a missile.