It seems the original author intends to create a sentry gun that replaces missiles with player piercing shots of beam particles. For one, this is a brilliant weapon for Valve to introduce hit-scan like projectiles into the game (for that is what the laser weaponry aspect of this item would certainly be considered, an incredibly fast projectile, almost as fast as a hitscan weapon, one or two tenths of a second less.), and for two the engineer certainly could use new things.

I would presume since the beam pierces players, it will fire less quickly than a normal missile from a sentry, though perhaps the engineer need not refuel the missiles for the sentry, as most beam weaponry in TF2 has infinite ammo.

Other than that, I would suggest the 1st sentry tier to have 85 HP, the second to have 195 and the last tier to have 330. This would create variety in sentry HPs, and while the first sentry gun might have less HP than normal, perhaps it could have a very small increase in it's 'head movement' turning speed.

The third tier's laser shot could also, perhaps, have a small fire explosion on impact with the first person to be hit, kinda like a pyro's detonator explosion.

Further more, now that I consider the model more, it is obvious that tiers one and two would also have auxiliary firing mechanisms inplace, thus increasing their overall damage output at earlier levels, which is interesting. For tier one I would suggest making the laser shot to be weaker than the Righteous Bison but function similiarly in most everything except speed, as the laser shot from the Bison is terribly sluggish in it's travel movement. Tier two's laser should be a small improvement on the Bison's damage output and Tier 3 should see a significiant change in the way the damage is output all together.