I’ll preface this by saying there’s a bit of brain dumping going on here, so a lot of what I’m saying here is not directly related to your last comment 
Maybe it’s just me, but I’d find instant cast mixed with non-instant cast very weird. While it would be semi-predictable (mainly if you take the Stack on Elite Kill upgrade), going back and forth between non-instant and instant would throw me off.
I get the argument of “Base speed is really slow, you shouldn’t need an upgrade to make it bearable”, I’m just uncertain if going all-in on speed is a good way to tune it.
Finding a nice spot for BB/BR in the big picture is kinda tricky, in a way - it’s a Blitz, so I don’t think it should be highly competitive with melee/ranged weaponry; on the other hand, considering how it’s pretty much always ready, I don’t think you can go wild and make it do crazy damage. If we break it down to pros and cons
Pros
- Fire-and-forget
- Affects enemies who break LOS
- Readily available (no effective cooldown outside of Peril management)
- Long range
Cons
- Slow
- All-or-nothing; ‘stolen’ kills will waste the cast
If you have any other additions, feel free to call them out
I’m not sure whether damage should be pro or con; I think its damage numbers are pretty good, but plenty of people think otherwise, so I’ll leave it out. I’ll just list off the numbers from the Psykhanium and determinations can be made from that
With no talents (other than Brain Rupture), it does the following
- one-shot Flamers, Bombers, Trappers, Snipers, Shotgunners, Scab Gunner
- two-shot Poxhounds (1350 damage per cast), Dreg Gunner (1350 damage per cast), Dreg Rager (1722 damage per cast), Scab Rager (1148 damage per cast), Mutants (2025 damage per cast)
- three-shot Scab Mauler (1350 damage per cast), Bulwarks (1688 damage per cast), Reapers (1688 damage per cast), Crushers (1350 damage per cast)
With EP, the changes are
- one-shot Poxhound, Dreg Gunner, Dreg Rager
- two-shot Scab Mauler (1 EP proc required), Bulwark (only 1 EP proc required), Crusher (2 EP proc required), Reaper (2 EP procs required; otherwise, they survive with <100HP)
leaving Scab Ragers and Mutants as the only enemies who do not see a change in breakpoints. I don’t have Creature Spawner installed, so I’m not sure what the numbers for Poxbursters are.
If it could reliably kill Dreg Gunners and Poxhounds in one shot without EP (which translates to +50 damage to Unarmored/Infested), I believe it fills a really solid niche as a sniping tool; with EP or Kinetic Resonance, the speed problem goes away (IMO), but you kinda need to pick at least 1; does that makes it “mandatory talent tax”* or “really strong synergy that would be a mistake to pass up on”?
Do you happen to know what the cast time of BB/BR is, by default? I saw someone mention 3 seconds, but I don’t think it feels like 3 seconds (more like 2 - 2.5 seconds, but I’ve never measured).
*I hate the term “talent tax”, time to go wash my mouth out with soap 