The offbrand UMP45 KSP45 will appear to others as the MPX/Signature SMG and is unlocked at level 32.
The KSP45 is a burst fire SMG designed to take out a target with a single pull of the trigger.
It features enough base damage to score one burst kill on a tan SWAT if all 3 shots land to the head, 2-5 ammo pickup, and above average accuracy/stability to help keep your bursts on target. The KSP45 notably features a lot more attachment options that gear it for straight combat rather than concealment, like the goofy box mags and a large assortment of longer barrels. You might be able to make it work in a stealth build if you purposely make a ridiculously cursed build, but the KSP45 probably isn't the best weapon to pick for stealth.
There is also an Akimbo variant which keeps the 3 round bursts and plays like a very bizarre shotgun, minus the shotgun skills.
Known Issues:
Yes we know the player's left wrist is going through the drum mag/"xmag"s in the hold animation. I originally planned to scrap those goofy box mags because of that, but Nya was alright with that error existing.