In general, no. Such devices are legal, as long as there is no "stored energy"... For example, those silly gatling gun 10/22 kits are ok, but if you attach a flywheel or a motor to the trigger, you've made a MG. Of course, you also need to be careful not to run afoul of the stupid "remote trigger" ATF letter, which basically states that certain remote triggering devices are MGs. Electronically controlled M-60s and the bicycle handles on M2HBs are considered MGs even if the host firearm is semi-auto...
The only way to be sure if your rigged up device is legal, is to send it off the ATF Technology Branch for classification. Just keep in mind that if they don't like it, you'll never see it again.