Technically its called shot, and their not pellets. Pellets are nothing like what a shot shell shoots. Shot shells are loaded what are known as BB`s or "Ball Bearings" vairying in size. Bird shot has many "BB`s" or shot compared to 00 Buck which has only 6 - 7 large shot or "BB's"
Not to be insanely accurate but it is nice if the correct terms when possible can be used or stated. Just like how its upsetting to hear ever kid refer to a magazine as a clip.
You are, of course, absolutely right, and I was grossly simplifying things to explain to a user that might not be familiar with firearms terminology.
a #00 buckshot shooting 12ga. shells fires usually 7-9 (depending on the shell's manufacturer and brand) shot, each #00 in size, which is 8.4mm in diameter, which is within an error of margin of what I said, 9mm. Shells of corrispondingly higher gauges shoot correspondingly smaller shot, up to the minuscule #FF "birdshot" at .23in (5.8mm) a shot and barely able to penetrate a leather jacket.
Obviously it would be a burden on the engine of FO:NV to represent every shotshell from Triball or even #0000 to #FF, so I assume that shotshell represents the smallest round that you'd use against a human, a #00 buck, and slug represents a 440-grain (.50cal/12.7mm) "American" or "Foster" slug design.
Oh, and what most people call a "clip", you can get away with calling it a "box mag" or "box magazine" or even a "box clip".
Very few pedantic arms enthusiasts (like myself) will get on your case for calling a box mag. a "clip" though, since arms that use a moon clip or stripper clip are always at least older than the M24, which used a box mag even though it was a variant of the moon-clip-fed M1A1 Garand of WWII.
Though it should be noted the M1A1 Garand (not to be confused with the M1A1 Abrahms Main Battle Tank) is in FO:NV!
I actually own a moon-clip-fed M1908 Springfield. A better deer rifle you could not ask for. Hell, in a post-apocalyptic world I would swear by her if I could loot enough .30-08/7.62x65 shells from either sporting goods stores of the soviet military. In my opinion there has never been so fine a target rifle. PS. look for my "Uncle" Stuart fallout mod, based on my uncle that has killed more deer with that rifle than Boone ever killed Legionmen.