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I'm a bit confused by the Big Mek's options. He has two sections that say "Replace Slugga with...", although one of them has Mega Armor. This seems weird, because when the Warboss takes mega armor, it replaces his slugga and choppa. Is this a typo or something? Because I've checked the FAQ/errata, and it doesn't seem to address this.
Mega armor comes with a shoota and a powerklaw, so it's safe to assume he doesn't have the choppa either. I'm assuming it's a typo, because mega armor doesn't give the option to choose between using a close combat weapon and a powerklaw. They only have the powerklaw.
I wouldn't give a big mek mega armor though. It'll work a lot better on a warboss (better toughness, strength, and weapon skill) for only a few more points.
"Any job worth doing, is worth doing with a powerklaw."
I wasn't going, honest! Actually, i ran my Big Mek with a KFF and a Kustom Mega Blasta for like, three games before I realized i couldn't <.<