This may be nearly impossible to pull off, but it's technically possible.

RIVEN: This riven has the highest possible value numbers for the stats chosen. Therefor, it will be just about impossible for anyone to ever obtain this riven, although possible. Fire Rate and Multishot are the stats you'd want if you were creating a Lead Laser, and -Zoom is a harmless negative.

[Ice Storm] and [Primed Slip Magazine] are there to bring the magazine capacity up to 70, making use of the 70.61 Fire Rate. (EDIT: I forgot about [Pressurized Magazine]!! In the spirit of actually having the fastest firing semi auto in the game, I just had to change the build. However, this now means you have a fire rate of 85.61 and a magazine of 56.

Steady Hands is there to make the gun more laser-like and less projectile-vomit-like.

And there you have it. The fastest firing semi-auto weapon you could possibly get in Warframe. It may be nearly impossible to obtain this riven, and it may require a rapid fire macro to actually use, but it sure would be fun as hell to wield a literal lead laser.