Best way to pattern a shotgun
 

If I'm not concerned about the pattern, but rather the point of impact, would it be better to shoot slugs or something like turkey shot?

I have a franchi-I-12 that came with shims, lost all of the shims but one, but I have a feeling it is shooting to the left or right. just need to be sure. whenever i go to pattern and use turkey shot, i can never really tell for sure. But i do notice that my percentage while trap shooting is much better with my remington 870, and i'm not sure why. (perhaps just because my barrel is an inch or so shorter on my franchi?)

A large piece (36"x36") of white paper or cardboard with an aiming point drawn in the middle will give you a good indication where your pattern is in relation to your target. No need for turkey loads. Trap or game loads, particularly the ones you plan on hunting with, will be easier on your wallet and your shoulder. Shoot at about 30 yd. depending on how tight your choke is.

There's an excellent post over in the turkey hunting forum on how to pattern a shotgun, it's a sticky so it's at the top. Use something like posterboard, 24x36, and use the loads that you're going to hunt with. Since you're worried about point of impact shoot from a distance where your pattern puts all of the pellets in a 12" circle. The barrel length has nothing to do with the pattern, the choke determines the pattern. The 870 might just fit you better so it hits where your pointing.

As was said, pattern with the shells that you're going to use. Don't try to use slugs because they won't necassarily show you what you're wanting to know.

If this is just a one time deal then the poster board or a big cardboard box will work. I test my POI vs. POA at 16yrds with a full choke. I shoot while "aiming" as best you can with a shotgun at a spot on the board about 3-4 times. I just use birdshot for this. This will show you the average impact and help eliminate errors with just a single shot. I like a field gun to have the pattern average centered left to right and what is called a 50/50 pattern. Half my pattern hits where I'm aiming and half hits above the point of aim. If your going to testing differnt load and choke combo's I went to lowes and bought a roll of brown paper from the painting section for $10 and just staple it to a wooden pallet.