Getting consistent RPM for guns in an FPS?

So I'm currently making an FPS game, it's going great but I've run into a bit of a problem, I can't seem to get the rate of fire of a weapon near-exactly the rate of fire it is supposed to be in testing. So far I've tried two methods, first was checking every frame to see if you are able to fire again using tick()-lastFire > 60/fireRate, which produced about ~620 RPM on a weapon which is supposed to fire at 700 RPM. Next method I tried was using repeat wait(60/fireRate), which produced relatively the same results. Not exactly sure how I would fix this, I assume I could use delta time from RenderStep, but I can't think of something I could do to utilise it. Any help on this is appreciated.