I'm very sorry if this is vague. I barely remember much so the curiosity is the biggest thing killing me.
I just remember this old anime (nothing new, not PreCure type anime either) where both girls and boys (though I think it's just 2 or 3 boys? Rest girls) being able to transform with their weapons. Hyperdimension Neptunia: The Animation sparked it and I just can't remember that old anime but it reminded me of it. I just remember some of them transformed with cards? Some with.. guns? I hope it's not too hard to find since very little anime has a transformation sequence for both genders. I also remember that it's not very sparkly/cute if that helps. I don't think it's a "magical girl" genre like Sailor Moon but I honestly could be wrong at this point. It's not Cardcaptor Sakura.
So main points again: Boys could and had their own transformation sequences
Mostly girls, but I remember them transforming with weapons (not into weapons, only with). I think there's guns, cards, rifles? Not sure.
It's nothing within the past 7 years. I'm thinking it might be 10+ years old? If that helps? The theme. Not sure? They were all fighting something? I don't think it was anything mech like 86, but again I could be wrong.
Sorry for the vague info... I just remember it had really cool music and sound effects when they transformed. Hope someone knows it.