Hey, you guys, T. rex is napping -- quick, tickle his ear hole and let's watch him try to slap it! And then we'd laugh and laugh, and he'd use those hilariously inadequate arms to rip us clean the fuck in two.
Though they were only slightly longer than an average man's, you've got to remember that a T. rex's arms were attached to nine salivating tons of pure, fabulous muscle. His biceps alone could curl 430 pounds apiece, and that's not even taking into account the fact that those guns had backup in the form of the friggin' nuclear weapons that were his chest and shoulder muscles. Paleontologists think that a T. rex's arms were powerful enough to help him push his enormous body up from the ground after sleeping, or to latch onto a female T. rex during mating. If they're strong enough to bone a T. rex with, those arms are more than strong enough to rip you apart like so much meaty Weetabix.
In case he gets bored of eating you whole and decides to make kebabs instead.