Free: Contests & Raffles.
My tip is be absolutely POSITIVE that when it is finished it measures greater than 18"
I did it to a 20ga shotgun, cut about 3 inches when some jackwad tried to shoot the gun with a glob of mud in the end of the barrell (wasn't me) anyways, it shoots really really bad. The pattern is terrible, not even somewhat round but it makes a big oblong pattern with gobs of strays. buy a new barrel