I bought a boneyard CYMA MP5K, that came with a few mags, just for this reason. It works beautifully now with a GATE Titan v3, XT brushless motor and 7.4v LiPo's, however it's become an ornamental pew pew that I use rarely just as a novelty.
It's just so HEAVY as a sidearm and to be honest gets in the way more than a pistol does if I'm crawling through bushes and undergrowth So, 100% a pistol all the way for me (although using an AAP-01 it's not the quietest).