It was all impressionistic. They weren't really using muskets any more than the "real" Q Continuum was a dirt road as seen in "Death Wish". Just like we use technology to do things we can't, the Q did too - only on an infinitely larger scale. They were using weapons so powerful they were fired in their continuum and were destroying star systems in our universe as an unintended byproduct.
The Voyager crew were probably given Q powers (as Riker was in "Hide and Q") in order to compete with them and operate the Q weapons once they entered the Continuum.