To prove this, I will be compare the Mk VIIa to the Mk VId.
The two guns seem quite similar at first, having the same Ammo and Collateral stats, the only noticable difference is the Mk VI’s relatively large advantage in Mobility and the Mk VII’s slight cleaving ability. The two guns even have identical flat damage rolls (range of 47.5 to 62.5). However, the Mk VII uses two ammo per shot, while only dealing 80% more damage per shot, meaning that the Mk VII has a lower damage/ammo ratio than the Mk VI, which is particularly bad considering one of the weaknesses of this family of guns is their ammo effeciency.
As for specific targets, both guns suffer from drastically reduced damage against many enemy types (one of the main reasons that Recon Lasguns suck), especially against Maniac and obviously Carapace, though the Mk VII does have a slightly higher damage multiplier against Maniac and does not suffer against Flak or Unyielding, but ultimately, it doesn’t matter for one simple reason.
The fire rate of the Mk VII is 7.69/s, but since it fires two shots at once, we can say it is an effective rate of 15.38/s, but with half the critical hit chance of a standard weapon. The Mk VI however, is up at 16.67/s, and has no such drawback. Therefore, with the same damage rolls, the Mk VI deals more damage over time than the Mk VII does. As for the Mk VII’s advantage against certain armor types, the Mk VI counteracts this weakness with its overwhelming advatage in producing critical hits. You are more than twice as likely to score a critical hit with the Mk VI in any given amount of firing time and with the way critical hits work in this game (that being, proccing a critical hit causes all of your subsequent hits within 1 second to gain the critical damage multiplier), the Mk VI is capable of sustaining the critical damage multiplier for a substantial amount of time (upwards of 75% uptime with proper builds), allowing it to deal much more damage in the same amount of time to all enemy types regardless of armor type multipliers. This disparity is even further increased when we consider perks and talents, which disproportionately benefit the increased critical chance of the Mk VI as well as offering Rending, which closes the armor damage multiplier gap between the two guns.
All in all, Recon Lasguns suck, but the Mk VIIa sucks especially hard.