This sounds like a really interesting idea! I have never tried it, but in theory:
- Can be firing from turn 1
- Will insta-kill anything with a toughness 4 or below
- Very strong vs. infantry spam armies (tyranids, orks, other imperial guard, tau)
- Will do well against campy armies due to the lack of the armies mobility not being taken advantage of.
- Low model count for your army, if the enemy gets in small-arms fine range (24") then a lot of expensive models will be lost
- The extra scatter may cause the blast to scatter onto your units on a small map, which is compounded by the low model count, or out of the board.
- Will do very poorly against anything with a 2+ armour save (terminators etc) due to blast only being AP3
- Enemies will likely get a cover save on a small map, meaning AP3 isn't too relevant against infantry spam. Barrage mitigates this somewhat, but the lack of accuracy means the blast will be very hard to target
So it depends on your local meta as to whether this kind of army will be effective. You want a high-model count, low-mobility army to take maximum fire from your shells before mopping anything else up with your BS4 lasguns. This is probably Orks, Tau, Imperial Guard, and will probably suck against Dark Eldar unless you get some great luck. Even then you are relying on your blasts not to scatter off the map, but with 6 that shouldn't be too much of an issue
This won't be a competitive build.