The bayonets were designed to rest on the flash suppressor and with the front sight to only be so far back from the flash suppressor. So, it will require an adapter either to move the bayonet forward or a spacer to fill the gap between the ring and the barrel. If your's sits like the one above, it's not going to be very useful and you would be better off just getting a nice fighting knife.
You could also: (a) get a mid-length gas-sytem upper, or (b) apply for a Short-Barrel Rifle tax stamp and get a 14.5" barrel. Both options will give you the correct flash suppressor to bayonet lug distance for a bayonet.