narrow the bore. put a thinner nozzel in the end.

That will increase the velocity of the stream--good for sweeping, but it doesn't help a sprinkler that fizzles instead of sprays. For that, look for sprinkler supplies at the hardware store, and see if they have "water-saving" sprinkler heads. With the appropriate head, some short pieces of pipe, 4 T fittings, 4 pipe ends, a female hose thread fitting (preferably swivel), and a female pipe thread fitting, you can make your own low pressure sprinkler. Otherwise, you will need a pump and a large pressurized tank.