In responce to your core question, the .270, .30-06, and .30-30 seem to be the most common calibers available from my observation. The .243 and the 7mm Mag come in pretty close to those mentioned above. My observation is watching the stockage levels at Wallie World, and the mom and pop stores in the rural communities that cater to hunters.
.223/5.56, 7.62X39, .357 SIG, .40, .45 ACP and .45 Colt, .380 ammunition don't stay on shelves very long but 9mm, .38 Special/.357 Mag, and 44 mag seem to always be on the shelf when everything else is gone.
Most use the bolt gun in their favorite flavor but I prefer a single shot.
I think some of that is supply too. They order VAST amounts of 9mm it seems. Even when it was scarce at Wallmart I noticed that when they got it in,they would get 15 boxes of 9mm to 1 or 2 boxes of some other calibers. The 9mm would be gone in a day (I dont use it,but I noticed what was there). They would get in a few boxes of other stuff like 32acp,which I did use (reload it now) and maybe 5 boxes of 45acp (which again I did use) and it would all sell out in a day or so. The 32acp would sit there for a few days and some stuff like a box or two of 25acp would stay there for a week.Now they are getting maybe 50-100 boxes of 9mm at a time and it stays for a while,but thats becuase they stock so much now. I guess it may also be becuase if your looking for cheap ammo,and you see 3 giant 250 round boxes of 45acp at Walmart,if its scarce you might grab them all just to make sure you have it next month. (Not to mention the people that try to resell it and the ammo hoarders waiting for Armageddon) If I know Im going to need 250 rounds next month and the month after that,and I see 750 sitting there,I'm going to buy it all,just so that I have it available in the short term future. On top of that add the hoarders who are stockpiling the stuff (which is a foolish thing to do,for the price you pay for 1000 rounds of loaded ammo you could buy the equipment to reload ammo and have 10s of thousands of rounds whenever you need it,and stock pile relatively cheap primers,bullets (or lead and cast your own) and powders. ) which puts even more strain on supply. Once supply increased I dont see that anymore. Most ammo is in stock at my local Walmart now. Before they had to have a 2 box maximum or if they got in 10 boxes of 45 someone would buy every last one. I spoke with one of the employees at that time and he told me that Walmart had just struck some sort of a deal with the suppliers to get more in. Apparently there was a real supply problem and they just could not get it. He told me that by the middle of the next month the supply problems would be fixed and sure enough,they were. Now that the supply is fixed,I think its a little more accurate to look at the shelf at Walmart and determine whats "common" as before it was too closely wrapped up with how many they ordered vs how many the supplier actually sent vs how much in demand it was.
I know I always see 30-06 and 30-30 and 308, none of which I use in my old military guns :-(
As a side note, ( I guess this whole post is one) I'm not too worried about stockpiling ammo in case its banned. First,if ammo and guns REALLY were banned,the reloading supplies I have would be enough to last me the rest of my life. After all,where could I shoot? Its like machine guns. I could easily get a semiautomatic and convert it to a machine gun if I really wanted to. I then would have no place to shoot it. Thats how people get caught,they make a machine gun,then go shoot it somewhere,someone hears and calls the police. The police come and investigate and I bet sometimes,they find its a perfectly legal machine gun with the proper tax stamp, and they go away (here,probably after saying "Thats a nice machine gun,wish I could afford something like that on my police officers salary",in other places after harassing the poor guy who had it. Similarly,if all guns were illegal,then shooting it,even in the middle of nowhere,is going to result in a call to the authorities and your arrest in most cases. In any event,if it really really came down to it,brass it CAN be home made. A lathe is a tool no one should be without and can cut and even form brass,if you have ever seen how they make the ends of brass musical instruments,its stretched and formed rather than cut. (Spend your ammo money on a lathe,rather than 5000 rounds of loaded 45acp ball from Walmart) Primers CAN be made too if you have the chemistry skills not to blow yourself up,in fact,just being able to make primers (I think berdan would be the way to go) and brass cases would be tradable on the black market for all the powder you need. In fact,even powder COULD be made,but would be much more difficult due to processing and requirements for uniform burn rates. The fact is though,none of that would be necessary as the black market would supply all the guns you need,in fact isnt the core of the argument against gun control,it does not work becuase once you decide to become a criminal,you no longer choose to obey the law.
