I think the best bands for a rifle have got to be hands down Snipersling Black.
Sniper Black has some interesting properties. It's got the fastest retraction out there (correct me if I'm wrong, I haven't seen anything beat it). It's also got a low maximum elongation factor, as it doesn't want to go past 500-550% elongation, it starts to really stack around there. This means that you'll need a longer ABL for the same draw. For example, if you were maxing out something like Precise rubber at a 30" draw, you'd need somewhere around 4.5" or 5" of ABL to get 600-650% elongation. For Sniper Black, you'd need 6" of ABL to get to the same draw. This means you're getting an inch less of acceleration, but it more than makes up for it imo.
My thoughts on what makes Sniper Black so fast has something to do with how much weight it draws at. Let's say, if you have a 20:15 taper cut of 0.5mm precise at 5", and you have the exact same cut and length for Sniper Black, and draw them both to ~550%, you're going to get like 60% more draw weight from the Sniper Black, and your going to get around 20% more FPS. For a lot of people, the extra draw weight doesn't seem worth it, and it doesn't make for the most comfortable shooting. On paper though, for the same volume of latex, sniper black has the highest draw weight, which also translates to the mass it's able to chuck. This means you'd need a fair bit less latex to get the same FPS on the same ammo as any other latex. The reason it retracts so fast is because latex has to pull its own mass as well as the ammo and pouch, which means sniper black is more efficient at retracting its own mass over any other latex (not more efficient in terms of draw weight : ammo fps, this is different).
What does this mean for slingshot rifles?
First of all, comfort isn't an issue if you're not having to pinch the ammo by hand. Heavy draws can have negative impact on accuracy, but if you're using a built in release mechanism, this isn't a problem.
Second, if you're using a slingshot rifle, you're probably looking for max fps in your ammo. Sniper Black is the obvious choice.
Third, if you're using a slingshot rifle, I'm assuming you're looking to throw some of the heavier calibers of ammo, things that are at minimum 3 grams up to 10 grams and on. Sniper Black pulls above its weight class compared to other rubber, so it makes sense to go with that.
Fourth is that SniperSling is an all around awesome company to deal with and has great customer service and report with the community. So you can't really go wrong there lol.