http://tvtropes.org/pmwiki/pmwiki.php/Main/FridgeLogic time...

If the Q35 was a pre-war prototype and the rest of the "true rifle" plasma rifles that descend from it are Enclave designed/made (or do the Van Graffs have production capacity?), where did the Multiplas Rifle design come from? Is it an Enclave-made weapon too? Its design, albiet similar, is still distinct enough to be different.

pretty sure it's a ROBCO made weapon

Well I as understand it the Q-35 wasn't the prototype of all the Enclave Plasma Rifles but was a prototype for the latest model that would probably replace the Enclave one. In an average locked room on the second floor of the REPCONN Headquarters there is a terminal with a file on many failed prototypes.

Since I don't recall it in any prior game, it might have been a top secret Repconn project never finished like the Q-35.

I think it's safe to assume that it was in service at similar times to the regular Enclave Plasma Rifle, which itself didn't appear until Fallout 3 but the repconned it into Fallout Lore as something developed before the war for the Enclave, if in-game evidence is needed well their is a dead prospector in Remnants Power Armour who was wielding one at the Death Pronimitory.

Huh, you raise a good point, all I remember when I get there is 'ARMOR ARMOR ARMOR' :laugh:

Really, all i remember when i get there is 'DEATHCLAWS!' (going there at low levels with 7 stimpacks is quite a challenge'

Go there with a single Stealthboy and you can nab it all at any level, took some time to find the right route but if you climb up some rocks on your right after going the slope, run across that and go north at about where the prospectors are you can stand on certain rocks and the deathclaws path-finding makes them all run to the otherside and out of sight, I dropped down grabbed all the stuff and fast-travelled away (actually you don't even need a stealthboy)!

Where's the fun in that? Besides Deathclaws aren't too tough if you got a sniper rifle of some sort.

Robco and General Atomics were responsible for a lot of energy weapons. The Enclave picked up where they left off. Winchester made the P94 plasma rifle, though.

I haven't killed a single deathclaw, then again I don't stick with a char long enough to reach level 20, all I want is the armour, nothing more, killing deathclaws has never been my bag; I hunt NCR pigs from out of sight through the scope of a Gauss Rifle for glorious vengeance for my colleagues who died at Navarro, however I need the appropriate uniform and as an Enclave Officer Uniform never appears I go for the armour.

officers armour:

on topic, if I didn't slaughter them at earliest convience I'd go do the quests but I hate them so meh,

The best explanation is they began mass production before the war just in case and had mass blueprints that have been found in ruins since they're defeat, so silver rush found these and improved upon them.