OO buckshot is plenty capable of killing a deer. Just pattern the gun to see if it throws a good pattern. And since you have iron sights, you can dial in your pattern to make sure your point of impact is the same as where you are pointing the gun.
In 1990 I shot a realy nice 9pt buck at 15 yards with 3 inch Magnum 00 buckshot...First shot hit him right in the chest, he turned sideways and I hit him broadside again, knocking him down. He then got up and ran right into a big oak tree and I blasted him again. He ran over 500 yards and another hunter shot him. I swore I would never use 00buck again.
It did. There was blood all over the place. He was a very large body 3 1/2 year old deer. Christ I even knocked him down with the second shot. You can't blame me for not using it again after that fiasco.
Im shooting Winchester 3 inch 00Buck 15 pellets. I shot my deer last week at approx 25 yds with Improved Cycl choke. I counted 11 holes when I gathered him up. I've never lost a deer with this load. I just keep'em close. No further than 40 yds.
We are limited to Buckshot when we do are deer drives with our Club, so we have to make the best of what I consider a POOR killer. Very rarely do you get a good blood trail if you don't kill the deer right away.
In our experience we have found that th PATTERN MASTER CHOKE with Federal 00 BK loads seems to hold great tight patterns.
The Pattern Master Choke was developed along with Federal Cart. Co. And it seems to work best with their 00Bk. loads. There in a very fine lip inside the choke tube that holds the wad for a milisecond allowing the pellets to exit the barrel before the wad. That way the wad never contacts the pellets after they leave the barrel. About 8 or 10 of my Club members now use this combo, and are shooting phenomanal ( spelling ? ) patterns.
Last year I dropped a 8 pointer dead in his tracks with my 10ga. at 57 yds. There was 11 pellets in his chest.
These chokes are available through Cabela's. The only draw back is the cost $89 [sad]
If you read all the above posts you will clearly see why everyone needs to pattern their shotguns. What works in one gun may shoot terrible patterns in someone elses. So I tell everybody to invest in a variety of buckshot and try as many different types as you can, till you find the load that your gun likes the best.