Slower guns
If guns are to be seen as useless, finding a way of eliminating peasant armies would help. The early guns were less effective than the longbow--but a peasant could be trained to use them quite quickly.
Without guns, gunpowder would likely see much use anyways. A trebuchet could hurl a large powder cask, or mines would be much more effective if they were packed with powder instead of just collapsed.
But, sooner or later, someone's going to make them work better..though a significant delay is, IMHO, plausible.