I just bought the mossberg 500 combo from Big 5 and the gun came with an 18.5 inch security barrel with cylinder bore choke and a 28" vented rib barrel with an accu-set choke. I'm currious if this accu-set choke barrel is an interchangeable choke barrel or not, because the end of the barrel has no threads on it so how would the choke be installed/attached when there is no threads. Thanks for any help you can give me.