EM-33 Plasma Pistol
Plasma-based hand weapon in use by Earth's Starfleet in the mid-22nd century until its replacement by the Phase Pistol. Also used by civilian starship crewmen for personal defence.
designation "EM-33" comes from the episode "Fight or Flight"
where Ensign Hoshi Sato says she's been trained on EM sidearms; Archer
then tells her the Phase Pistol handles "pretty much like an EM-33."
It also seems that, in Star Trek, "33" is the new "47" - Reed refers to a projectile-firing machine-pistol in the episode "Terra Nova" as a MK-33. Since we know that the Terra Nova colonisation expedition left Earth about 80 years previously, it seems that either plasma-based hand weaponry hadn't been developed at that time, or wasn't included in the mission's manifest, or hadn't survived the intervening 80 years.
Also, in the episode "Sleeping Dogs" Sato says she found the EM-33 easier to aim because of its "particle drift." My initial theory was that this was some sort of dispersal of the plasma charge, meaning it impacted on a wider area; however, Tom Aylward-Nally's suggestion is: "Presumably the EM-33 creates a magnetic pocket inside which is a lot of hot, ionized and all-around 'OW!'-causing gas. However, presumably this plasma still obeys Newtonian physics and drifts behind the magnetic packet, and thus the plasma ends up denser towards the back of the actual bolt. Thus, the actual smack of a plasma pulse weapon would hit a little after the actual bolt does, making hitting a moving target a little more complicated." Jeff Benson says: "I was thinking more along the lines of a recoil, similar to a semi-automatic handgun. Those who used the previous weapon were used to having to adjust to the recoil, and this is backed up by what Reed says: 'just point and shoot.'"
I'm going under the assumption that 'EM' stands for 'electromagnetic' and that the weapon was introduced in 2133. It needn't necessarily be so - Randolph King's email reminded me that today's M-16 rifle wasn't invented in 1916! - but it's a working theory at any rate. . . Bob Karo, on the other hand, suggests it's an acronym for "Earth Military."
In a particulartly inept bit of continuity, the EM-33 was seen in season 2 episode "Canamar" as an alien weapon. . .
In seasons 3 and 4 they're back in Earth hands, albeit with a new paint scheme, as standard-issue MACO sidearms! These fire the same blue-white pulses as the MACO rifle, and obviously have a similar ability to stun given we see people incapacitated by them in "Chosen Realm" and "Hatchery." Can this still be considered an EM-33 if it's been modified? Should we be calling it an EM-33A1 or an EM-34?
The plot thickens in "United," the second part of season 4's Andorian mini-arc. After being shot by a MACO EM-33 (in the previous episode, "Babel One"), Shran's consort Talas is told the 'phase pistol' was set to kill. On its own, I'd write this off as another case of one hand not knowing what the other is doing - in this case, the writers (the Reeves-Stevenses) not knowing that a pulse pistol (and not a phase pistol) was used in the scene. But then, Talas reveals she has seen how fatal a "phased-pulse infection" can be to Andorians! Jason M contacted me to highlight this fact (that I'd missed myself when watching the show). I don;t see how anyone could call a phase pistol-proper a pulse-phaser. . . Is this then conclusive proof that MACO weapons are based on phaser technology?
Craig Binkley, Enterprise's Prop Master, recently revealed in an interview that, when designing the prop, for sizing purposes they studied the Israeli "Desert Eagle" handgun. Kevin Jones, who provided me with the prop images below, has also confirmed the handgun (he refers to it as a .45) origin. Zed Snardbody, on the other hand, has commented on the resemblance to a pistol called a Jericho.