It’s impossible to find fight’n ammo but for fun this low recoil low noise ammo is great . Especially in a Raptor/ pistol grip set up Its available at Walmart for $7.87 a box. Nice set up there- Lets buy your car!
There are plenty websites that show how to properly measure a shotgun barrel. Their are zillions of 18 in barrels out there.
I cut mine pretty close to 18 3/4 due to the vent rib. I know it’s legal.
I used a hacksaw and file and polished up with a work sharp on this vent rib barrel. I added a hi viz sight. Not that difficult. Do not use a Raptor or any type pistol grip unless your a seasoned shooter. It’s fun and devastating but also a learning curve.