Mossberg makes a 20" barrel with choke tubes to fit the 500 and 88. Wholesale lists them for $160, including 3 chokes. This might be cheaper than having the 19" barrel threaded, assuming it is thick enough to be done. Since the Mossberg chokes interchange with Win-choke, Browning Invector and others, the companies Waterfowler mentioned probably make chokes that will fit.