A lot of ranged weapons have a decreased RoF in a seeming attempt to curb their use with regards to multiple hits. A RoF 1/3 weapon has no chance of generating more than one hit in a turn, and even then, can barely be aimed (it takes 3 AP to shoot, and the Aim action now only benefits attacks made that turn).
The Single Shot quality exists, but is worded to reference very low tech/simple weapons. It forces the RoA of a weapon to 1 no matter its RoF.
I feel alot of the weapons listed in this book should have RoF of 1/3 altered to be RoF 1/2, or even 1, but gain the Single Shot quality.
This is mainly looking at sniper rifles, but would even make sense for melta weapons, and the long-las.