Whale Attacker breaks easily for me. It's very thin and fragile in two specific spots, near the end of Seaborg's "tail". The 2 main smash attack points (the whale heads) hit so hard that the AR can't handle the shock/recoil it gets.
It's an awesome AR for smash combos, but you need to have a good stock if you intend on using them. I've broke 2 before in just a couple of matches and I haven't touched the only I have right now because of this.
Also, there's another smash AR, which I don't remember the name of, that broke on the first match I used it in, then snapped in the next. It had 3 small wings facing right (yeah, that probably doesn't help lol). It wasn't something comonly used from what I remember, but it packed quite a punch.
First mold of Dranzer MS AR broke easily too since the metal frame part leading to the 2 "hammers" on each side was too thin to handle the shock. First they would bend. A few more matches and it'd snap off. My original DZMS, has one of them broken.
The newer mold is thicker around that part though.
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