I finally got out to where I could have it with buck and slugs. It not only went fine, but I only had to drive about 35 minutes to get to a place called Mike Raahauges Pheasant Club instead of well over an hour from southern Orange County to get Burro Canyon.
They set me up all by my lonesome on the 100 yard rifle range, but had me firing at 10 yards to simulate lilkely maximum home defense distances. When I say they set me up, the rangemaster spent about half an hounr showing me how to load and use the 870 for defense as opposed to breaking clays. This was all with the open cylinder 18 1/2 inch defense barrel.
The range also bosts several sporting clay fields.
Anyway, while the buckshot and slugs definitely packed more of a wallop than the target loads from WalMart, it was doable. Two hours have passed and I have yet to feel any discomfort.
I also feel more confident after having loaded up my beast with 7 rounds and fired them as quickly as I could without rushing or aiming. He had me concetrating on the center of the man size target, bring the gun to my shoulder and firing without aiming with the bead, Besically pointing like shooting clays.
One ammunition type I was curious about surpised me. Its marketed by Remington for LE. I thing it is moderately low recoil, muzzle velocity being around 125 fps, and has a load consisting of a 1 oz slug and 3 pieces of 00 buck. I tested it once by taking more of an aimed shot. The slug was cenetered 1" to the right of the X and the 3 shot were alomost perfectly spaced around it in the 9 ring with each shot roughly at 120 degrees. The spread at 10 yards, which I haven't actually measured, is around 8" with that barrel.
Unfortunately, it may not be legal ammo for a civilian.