The one adjective that everyone seems to describe Jessie J's reintroduction to the American audience is "loud".
Jessie J, Ariana Grande, and Nicki Minaj all offer their vocals over this Max Martin production. A good strategy on Republic's part, as it houses all three ladies. Jessie has been largely absent from the American scene since her 2011 pop hit 'Domino', so borrowing a little starpower from Nicki Minaj and Ariana Grande, 2014's "It Girl", isn't such a bad idea to warm Jessie back up with US audiences. 
Everything falls into place: Max Martin delivers a trendy production, in the lane of neosoul/pop fusion, a styling which was observable in Pharrell Williams's worldwide chart topper 'Happy'. Jessie J and Ariana offer strong vocals, and Nicki Minaj spits a rap verse to nab some Urban spins. Republic has ensured Jessie J has a high potential hit song with a Motown feel.
It works if you're reading the song information: so it works on paper. It falls apart when you hit play on the actual record.
"Bang Bang" indeed! The adjective that comes to the listener's mind is loud, like a gunshot. It's an apt description, because the song starts in an instant. Jessie comes right out of the gate with her belts, making the listener's hand yank down on every knob within reach to reduce the volume. Jessie is singing forcefully, and is very brassy. Many proponents of the song might say "oh nothing's wrong with belts. We've become accustomed to weaker vocals, so she should show what her momma gaaaaayaaaaa, right?" And that's true, nothing's wrong with belts.
It's how she's using the belts that makes it so offputting and unnecessary. When Whitney threw her head back and unhinged her jaw like a viper, you 
knew you were in for some serious volume. Houston wasn't just belting to belt, though. The music had surely built up to that point, and the lyrical content called for it. In 'I Will Always Love You', the tone was that of fervent passion. In Mariah's take on 'Without You', it was a tone of near suicidal depression, you could just SEE Carey flinging herself off a bridge on the final chorus's "I can't live if living is without you" line. Celine's 'My Heart Will Go On' indulges in all its melodrama and its impossibly fantastical story when she belts for her life in the final chorus, turning the formulaic key change and sappy lyrics all the way from an irredeemable cheesfest to a guilty, 
guilty pleasure. The point I'm making is, the music 
called for belts. The lyrics 
required belting to work.
Now, what are Bang Bang's lyrics about?
Jessie J is a bad, bad, 
bad girl. That's it. Oh, and Ariana too. 
(lol)
Jessie J... likes to have sex! In a plethora of places! Like the back seat of her car! Ooh! (OK, so 'Drunk In Love' this is not. Bang Bang-ing in a car isn't the most novel of ideas. There was literally a song released in the past week that tackles this very subject. You think the writers would've tried to up Jayonce, no? Sex in the attic, sex on the roof, sex in the chimney. Would've made for a more interesting song.)
What makes Jessie and Ari 
(lol) so bad that this guy just HAS to get with them? Because there's nothing in their character I can find that screams "I'm a bad girl". I can't find a reason in the lyrics, either. If anything, Jessie ADMITS in the first two lines that the other girl is more attractive than her. The only thing Jessie has over this other girl, and the only thing resembling a message in the song, is that Jessie is willing and able to bang bang this guy, anytime. Charming.
Let's take a reprieve to talk about the man behind the music and lyrics, Max Martin. Max Martin is the granddaddy of modern pop. (He's on the younger side of forty, but that's just how fast pop music and the industry moves.) He's been the mastermind behind a crap-ton of pop hits, so it's impressive to see him stick around and consistently see success in such a fickle industry. That doesn't change the fact he has been off the ball lately.
We can backtrack to one of the artists featured on Bang Bang, Ariana Grande. Grande released the song 'Problem' to great success in the late spring. Was a Pop radio hit, has been in the Top5 on the charts since its release 12 weeks ago, an all around success. 
Now, 
stop and realize that Max Martin literally wrote 10 lines of lyrics to make Problem. I'm not going to even count the 'one less problem without you' line because that's not an original lyric, that line has existed before the song's creation. Realize that Iggy Azalea wrote the same amount of lines with her 
feature that Max Martin did for this song. Martin doesn't even bother to write a bridge, or much of a chorus, even.
The production tries to be "Urban", or whatever some listeners claim it is, but is ultimately pop (I should also point out how Martin recycles the arrangement of the chorus from Christina Aguilera's 'Your Body' and the instruments of J.Lo's 'First Love' and puts them into Problem's verses.) up until the "chorus" (If you could even call it that.) where it tries one last time to draw some HipHop radio spins. What experimental, crazy pop-urban fusion does Max Martin offer?
An 808 kick with a sax loop over it. Oh and a Ying-Yang twin feature.
Bear with me, because this ties back into Bang Bang, because the same problem, no pun intended, is present in his latest offering.
Max Martin is trend savvy. He anticipated the trap trend with 'Dark Horse', he anticipated the rise of horn sampling with 'Problem'. Now he sees Pharrell Williams's 'Happy' in charts worldwide, and naturally goes to make a neosoul piece.
And that's where the song fails. Listen to Happy, this 
song, and Bang Bang. Happy and Lee Hi's motown influenced '1.2.3.4' are both soul songs that experienced crazy success in two different corners of the world. Besides the physical distance between their places of origin, I want you to see what makes them different from Bang Bang.
1 2 3 4 and Happy 
literally  have three instruments playing at once. A drumset, a bass guitar, and an electric piano, with some light clapping during the choruses. 
Bang Bang has a marching band drumline, a hornline, a synth on bass, another bass guitar, an 808 drum, an electric piano, a team of handclaps (Which unlike the other two songs are obviously synthesized and sound awful.) and a 
goddamn choir on the chorus. It misses the point that soul songs in the vein of Motown are supposed to be simple music. A production like Bang Bang's would be inconceivable in the 60s, and would sound terrible. It's inauthentic, and is transparent as a dash for a hit song.
Max Martin doesn't write a bridge for this song either, the song has 14 lines in it. Nicki, yet again, matches this amount with her feature. The lyrics tell nothing resembling a story nor coherency, and offer nothing that actually makes you feel like the bad bitch Jessie claims she is while singing it.
On the vocals side, the song does not call for belting, like I said. Jessie J sounds forced and brassy, Ariana is WAY past her dynamic range and sounds incomprehensible. Barring harmonization, her feature is literally two lines long and is irrelevant in the grand scheme of the song. The only reason she is on this record is to shift copies, not to work with the main artist and elevate the song to the next level. (A good example of this is Kimbra in 'Somebody That I Used to Know.) Nicki phones in her verse, but still ends up being the best just because she doesn't shout at us.
 
'Bang Bang' misfires in the faces of all who were involved. It's overproduced, noisy, and a wannabe soul-pop fusion that sounds more Tinseltown than Motown. Max Martin misses the mark, Jessie J is doing too much, Ariana sounds bad and is unnecessary, and Nicki is on autopilot.