A: COMMERCIAL, not handed out like lollipops by the Russkies
B: Why would you guess handgun? For that matter, why guess centerfire rifle? Both those items are very heavily controlled in many countries. For my money, the answer has got to be either .22 rifle or a shotgun.
I'm quite sure the Marlin 60 has outsold the Win94 (despite a 66yr late-start).
[thumbs through files]
I sold the book a while back, but I had a Guns Digest compilation from the 1970s which included an article on the subject.
IIRC, the most sold commercial firearm was the H&R/NEF single-shot shotgun.
I even found one in Iraq, which had been confiscated from (and then returned) to a Bedouin shepherd, along with an ornamented leather cartridge belt. We had a couple Baikal ones too, but when I saw the NEF, it really surprised me! Never new that some factory in New England was shipping cases of shotguns out to Baghdad.
-MV