Issue is with an AK, an AR, an SKS, or any other "who knows who made it" gun would be analogous to asking, "What is the best selling vehicle in the US?" and answering with something like sedan or pickup truck. Way too vague and way too open.
Like was stated earlier as precise of an answer would be "mauser type bolt action".
Actually, saying "Mauser 98" or even "98K" would be a better analogy, since the the Mauser 98 was one design that was produced by a great many companies/facilities (not just Mauser), but the parts were built to a single spec and are functionally interchangeable like the AR-15's.
You have to affix a brand to the model in order to get a real idea of just how much impact a particular company has on the firearms market.
In today's gun market, the name stamped on the product is less important than the product itself. Talking about the AR-15 is like talking about the M1 Garand; it was made at multiple facilities by multiple companies, but it is one interoperable design with interchangeable parts. And not many people realize that, say, a Rock River AR and a Colt AR are often built off receivers from the same forge (e.g., Cerro), just with different grades of parts and with final machining and heat treating done by people wearing different company hats.
Best selling commercial rifle I'd go with the Winchester 94, it was around forever and really, who never owned one? I'm willing to bet it was even better selling than the generic category of AR variants. Sure, a bunch of people have AR variants but a BUNCH more have a Winchester 94.
SKS as best selling military rifle because they were so damn cheap along with the ammunition.
Mostly agree on the cumulative totals. Winchester says 6 million Winchester 94's were sold in 112 years (and some others say 7 million) before production ceased in 2006, which works out to 54,000-63,000 a year on average, though sales were higher in the 1950's than before thanks to rising incomes. I'll bet at least half of those still exist, probably far more, so it's fair to say there are at least 4 or 5 million still in U.S. homes. And best I can tell, 3.5 million Remington Model 700's have been sold since it was introduced.
IF the 7-million-SKS's-in-the-USA estimate is correct, then the SKS would just barely edge out the 94 (because, as you state, it was so darn cheap until a few years ago). I am still kicking myself for not picking up some of those $69.95 16" barreled models back in the '90s. But again, I have not been able to verify that number.
As to AR-15 sales, in the mid-2000's when I ran the numbers, AR-15 sales were running between 200,000 and 250,000 a year total from ~30 different assemblers, so say a million every four or five years (with the pace having picked up significantly since 2007 or so). It's been the top selling centerfire rifle since probably 2004, and since total sales have almost certainly exceeded the two million mark by now, I dare say it won't be all that much longer before the AR passes the Remington M700's cumulative 3.5 million mark.
As to whether it will reach the Winchester 94's 6 million mark, that's hard to say. In the Winchester 94's heyday, there weren't all that many rifles to compete with the 94 in the "does everything" role, but there are a lot more choices out there today, and it's a lot easier to get them to market. Given the AR-15's continued popularity with my generation and the next, though, I'd say its chances at equaling that mark are pretty good.
Shotgun is definitely the 870, self loading the 1100.
Agreed. From what I can tell, the number of 870's sold over the decades stands between 8 and 10 million, which probably makes it the single most produced centerfire firearm in the nation.