I have the Double Shot perk for Zer0, which should help even more with the Bekah, since it itself is a double shot.
http://steamcommunity.com/sharedfiles/filedetails/?id=480095589 That’s my Hammer Buster
http://steamcommunity.com/sharedfiles/filedetails/?id=480095548 And that’s my Bekah.
In theory, the Bekah should do more right? Nope. Against Big Bertha the Bekah does 332K to 480K. The Hammer Buster goes all the way to 678K.
Can someone explain to me why this is?
The Bekah splits into extra projectiles after a certain distance, so first you need to know how far to stand away. Second, you need critical hits to get the most from the Bekah. If you are only getting body shots, it won’t be anywhere near as effective as it could be.
So if I’m usually getting body shots, Hammer Buster is better, if I get crits, then Bekah is… And how far away should I stand? I;m usually pretty close.
I don’t know, RNG has never given me a Bekah. Go to the test dummy and experiment.
No the bekah is better because you get 4 bullets per trigger pull
Also if you’re playing zer0 I would learn to get more crits, that’s where most his damage potential comes from.
Also note that if you’re specced into the skill that boosts projectile speed, you will need to be further away for the split to aoccur (applies to any weapon with split projectiles, not just the Bekah).
If you’re specifically looking at Big Bertha, I’d go Hammer Buster just because you need to fire and take cover all the time, so most hits will be body shots. (Corrosive is even better in that case though). Generally, I’d take the Bekah if I’m in a situation where I can land the crits, but the Hammer Buster if I’m running around frantically like a headless chicken trying to avoid FFYL (see @Derch’s reply.) Of course, I’d be even better off if I learned to play better… 
The bekah still does more on body shots, not just crits.
Just follow a simple rule: Don’t get too close to the enemy while using the Bekah. As long as you can see the “white orb” appearing after you fire each shot, you are using it correctly. It is a very powerful assault rifle and I would say underrated as well. Even body shots can deal a lot of damage and critical hits will simply decimate things.
The only time where the Hammer Buster slightly outperforms the Bekah is during close range situation. Outside of that range, the Bekah completely outperforms the Hammer Buster, assuming you use it correctly.
I’ve just done a run on Maya using Bekah. And even if I haven’t quite figured out how to get the most out of it yet, it’s one of the most impressive guns in the game. I haven’t used it against raids yet, but badass enemies and minibosses get humiliated by it. There is absolutely no competition whatsoever.