For a long time, I was of Twodollarpistol's school of thought. I figured at inside-the-house ranges, No. 4 or No. 6 shot would be effective without over penetrating.
A crusty old Marine working in a gun store told me that for close range work the Marines used No. 2 buckshot because that provided the most pellets with enough mass to achieve adequate penetration.
More recently, I have seen a video in which Massad Ayoob recommended No. 4 buck for the same reason.
If you haven't seen the tests and discussion about shotgun rounds on the Box O' Truth, check it out:
The Box O' Truth #3 - The Shotgun Meets the Box O' Truth - Page 1
The Box O' Truth dude concluded that birdshot, including heavy shot like No. 6 and No. 4, is for birds. He was not satisfied even with the lighter buckshot loads. He says any shot that will be effective against a human will penetrate numerous layers of sheetrock.
Remington's website provides some helpful information, follow this link and click on "Specifications":
Shotgun Ammunition - Shotshell - Express Buckshot - Buckshot Ammunition
I emailed Remington and asked about the weight of individual pellets in their buckshot loads and this is what they told me:
Weight per pellet:
#4 buck-20.83 gr
#3 buck-25.59 gr
#1 buck-21.17 gr
0 buck- 39.77 gr
00 buck-48.61 gr
000 buck- 51.47
Everybody won't come to the same conclusion and what Twodollarpistol said about determining for yourself has a lot of merit. Based on everything discussed, including the information provided by Remington, I conclude that Ayoob is right. 4 buck is 27 .25 caliber, 21 gr pellets, traveling 1325 fps. If that doesn't "git 'r done," shoot twice. I wouldn't go any lighter than that, but even considering the Box O' Truth test, I am satisfied that it is adequate. Of course, YMMV.