Brownells has a longer mag tube and barrel set, though it has a 20 inch barrel for the 500
http://www.brownells.com/.aspx/pid=2512 ... HOTGUN-KIT
But what would be so wrong about keeping the mag tube you have now and having your 18.5" barrel cut (lathed) to the shorter length you want?
Most machinists would shorten the barrel and true the face for way less than $50. Matter of fact, I just had a 22 barrel shortened, recrowned and threaded for $40.
Just cutting the barrel and truing the face should be a piece of cake that even a hobbyist could do.
http://www.brownells.com/.aspx/pid=2512 ... HOTGUN-KIT
But what would be so wrong about keeping the mag tube you have now and having your 18.5" barrel cut (lathed) to the shorter length you want?
Most machinists would shorten the barrel and true the face for way less than $50. Matter of fact, I just had a 22 barrel shortened, recrowned and threaded for $40.
Just cutting the barrel and truing the face should be a piece of cake that even a hobbyist could do.