Patterning is a great idea...and do it at various ranges out to 40 yards...
I have an 870 that has had some 'work' done to it...
The barrel has been taper bored to IC (improved cylinder). Taper boring is a machining process where the entire length of the barrel is tapered to the ultimate choke diameter at the muzzle. Factory or stock choking typically chokes the last few inches of the barrel. Factory choking puts a lot of stress on the shot column and typically will result in wider dispersion along with 'flyers'. Taper boring on the other hand gradually constricts the shot column and produces night and day differences in shot patterning.
In the case of waterfowl guns...no other modification will increase the range and down range density of a shot column like a taper bore...easily can add 10-20 yards of lethal range to your favorite goose gun...
In addition to taper boring, the 870 has been ported at the muzzle. Porting is the process of drilling a series of holes, angled back, on the top of the barrel. Porting reduces recoil, muzzle rise and tightens the pattern further still.
In addition to the boring and porting, the trigger shear was lightened/re-machined, ghost ring rear and tritium front sight installed, magazine extension installed (7+1), youth butt stock installed, Sure Fire illuminated front slide installed, over sized safety button installed, over size slide release installed and a six pack side saddle...
This particular 870 will print all 9 buck shot pellets well inside of a standard pie plate at 40 yards, and produces holes the size of your fist at 25+ yards in plywood...
Extremely lethal...
Oneway