A 12 gauge with buckshot can be very effective on deer...But you must do your homework....I have a Browning B-80 that will put 5-00 pellets into a 8 1/2 x 11 piece of paper (on the horizontal) every time, out to 40 yards...( I know that I need the Improved choke tube and use only 3 inch Winchester Supreme 00 buck)....That's the key...You need to pattern your gun with several brands and sizes of buckshot and with each choke tube to see what it is most effective with....I also have a Remington 1100 that I bought in 1975 that has a 21 inch barrel with rifle sights, it will group inside 1 1/2 inches at 50 yards with Remington Foster type slugs...and put 4-5 pellets of #1 into the same 8 1/2 x 11 paper....

You will probabably have best patterns with an Improved or Modified choke tube...You also need to try #1 buck as well as 00...I would start with Federal Premium and Winchester Supreme buckshot over Remingtons...I don't shoot buckshot over 40 yards...That's where most guys get in trouble (in duck hunting they call it sky blasting)....

I have killed 35-40 deer with buckshot, both 00 in the Browning B-80 (3 inch chamber)and #1 in the Remington 1100 (2 3/4 inch chamber)...If you are able to put 4-5 pellets into the lungs of a deer, and the range is 40 yards or closer, you will be fine....All this being said, I haven't killed a deer with buckshot in several years....A .24 diameter bullet movng 2800 fps or so is much more effective...Unless you are dog hunting, take the .270...

If your shotgun has a vent rib, you could install an aftermarket sight and use foster type slugs, this also helps center the pattern (much like a turkey hunter's shotgun with sights) of the buckshot...

Used properly buckshot is very effective....We have a huge problem with dog hunters dropping their deer dogs off beside a couple of our farms and trying to run deer off our property and onto the property they have permission to hunt....Many times I have killed deer running through the woods that they have pushed by me with buckshot...Last time I did this I dropped two does with two shots...But the range was about 25-30 yards....Good Luck