There's no real requirement for a new weapon slot, it's that your mech already has 2 Heavy Cannons, and has nowhere safe to mount a third. It'd need a bigass retrofit of some kind to attach a third arm or something. If you're talking something smaller with less recoil, that could be done much more easily.

Yea, I'm not really exclusively looking for a cannon. Just would like some more room for smaller caliber weaponry.

Could always try a under barrel kind of method.

I don't really have any conventional guns that could have an "underbarrel attachment." It's not like I just took an AR15 or something and just made it huge. They're built in weapons.

Rollhide!

HighEndNoob
Pierre Berguard [8]: You sit down at a table across from someone, and begin eating.
In between bites, you make conversation.
"So... What happened?"
"Fight. Sorta."
"Really?"
"Yeah. Big guy and scrawny dude. Little guy got his face punched in."
"Oh ♥♥♥♥, really?"
"Yeap. Short though, and he didn't fight back."
"Craaaaazy. Eggs are good today."
"Mmhmm."
Abilities:
Inventory:
- Field Medical Kit
- Field Repair Kit
- 9mm Glock
Conditions:

Tomaster
Vladimír Shurla [9-1=8]: Eh, the coffee will take a bit to finish, so you head back to your table where your acquaintances have already sat back down. You dig into breakfast, eating hungrily. Ahh, warm food in your belly. That'll wake ya up real good.
"So, you guys know about upgrading mechs, right? Like, adding new weapons?"
"Yeah, but no way in hell you're putting ANOTHER heavy cannon on."
"Yeah, yeah. I was thinking something smaller actually, for more...delicate situations, you know? Maybe a couple burst lasers or a machine gun."
"Hmm... That might be doable. Where were you thinking of mounting it?"
Abilities:
Inventory:
- Field Medical Kit
- Field Repair Kit
- 9mm Glock
Conditions:
- Sleep Deprived (-1 to rolls)
- Brewing Coffee (Ready in 1 roll)

Mecha
Name: Goliath Mk.V
Class: Heavy
Weaponry:
- Two Heavy Cannons
- Hellfire FaF Missile Pod
Inventory:
- 32 Depleted Uranium Heavy Cannon Shells
- 72 Hellfire Missiles
Abilities:
- Heavily Armored (+1 to defense rolls, -1 to dodging attacks)
- Depleted Uranium Slugs (+1 to attacks using the main guns)
- Morale Boost (+1 to all allied rolls at the beginning of the battle, or when the Goliath arrives on the battle scene)
- Ypsilon Strike [SW]
Conditions:
Status:
Arms: OK
Legs: OK
Torso: OK
Etc: OK

Roast Veg
Aark-han [4-1-1=2]: You knock hard twice, and the voices immediately stop. A second later, the door gets pulled wide open, and a man in a white doctor's coat looks down at you.
"Can I help you?"
You accentuate your bleeding face as much as possible.
"Yesh."
"Oh jesus, get in here."
He ushers you in, and you catch a glimpse of someone in the room in the back. The doctor walks over and pokes his head in for a second, whispering to the person in the room. Then he shuts the door and turns to you.
"Alright, if it involved rusty metal or dirty knives, tell me now. Otherwise, I don't want to know."
You say nothing, and the doctor nods.
"Good, that simplifies things a bit."
Suddenly, you feel something on your tongue. You gingerly spit it out. It's the half of your broken tooth that was still in the gum. Ow. So much ow.
Abilities:
Inventory:
- Field Medical Kit
- Field Repair Kit
- 9mm Glock
- Rough Map of the Exoneration
Conditions:
- Broken Nose (-1 to speech)
- Cut Lip (-1 to speech)
- Missing Tooth (-2 to rolls)

Mecha
Name: The Han Van
Class: Light
Weaponry:
- Innnardsplatter
- "Le Big Tweezers"
- Some Kinda Acid Sprayer [SW]
Inventory:
- 20 AP Grenades
Abilities:
- Scream Siren
Conditions:
Status:
Arms: OK
Legs: OK
Torso: OK
Etc: OK

Amazigh
"Amazigh" [2+1=3]: You look around the room for a place to sit down and enjoy your food, but find none. Damn. Well, I guess you could just stand and eat... No! A proper breakfast must be enjoyed with your ass planted in a chair!
Abilities:
- Cyborg Enhancements
Inventory:
- Field Medical Kit
- Field Repair Kit
- 9mm Glock
- Breakfast
Conditions:

JJA79
James Argus [8-1=7]: "So uh, Anything stupid happen in here?"
"Fight. Aark and Amazigh pissed off some big burly guy. He beat Aark's ass."
"Oof."
You then start in on your Delicious Breakfast. Oh god it's delicious. So good.
It takes a bit to work through all the food, but you do. You are comfortably stuffed, and think that a cup of coffee would hit the spot right now. You amble up over to the large coffee maker, but someone is already on it! Awesome, you don't have to use this bad tasting Instant Coffee.
Pilot
Abilities:
- Sniper expert (+1 when shooting from long range)
Inventory:
- Field Medical Kit
- Field Repair Kit
- 9mm Glock
- 3 Instant Coffee Packages
Conditions:
- Panic (-1 to actions when in close combat)

Mecha
Name: Stinger
Class: Medium
Weaponry:
- Machine Gun
- Light Rail Sniper
Inventory:
- 8 LRS Slugs
- 550 .50 rounds
Abilities:
- Energy Blast [SW]
Conditions:
Status:
Arms: OK
Legs: OK
Torso: OK
Etc: OK

Tokochiro
John Kosloswa [9]: "SQUICK."
One trip to the little boy's room and washing up later...
You step out of the bathroom, feeling much cleaner and much less violated. And to top it off, someone dropped a rather nice sports watch in there! Finders keepers, beeyotch!
Abilities:
Inventory:
- Field Medical Kit
- Field Repair Kit
- 9mm Glock
- Sports Watch
Conditions:

Game Events:

Head to the eating hall thingmabob
I am soooo unlucky,got sticky stinky cold stuff on me.

@Tomaster: A good anti personnel weapon would be an armpit (or similiar position) mounted chain gun, it has lots of ammunition, reliable, and can dish out some serious pain even at medium distances!

@Amazigh: that is asking to be beaten to a pulp/have revenge taken on your mech.