I only have some experience with shotshell ammo, but here goes my opinion:
1- The wholesale network is layered and where you buy depends upon on how much and the connections you have.
2- Most foreign ammunition manufactures have exclusivity with a US importer. If you want to purchase from those, you are talking about purchasing by the container in terms like full or half.
3- These importers will have a network of wholesalers who purchase in very large quantities, specialize in freight to get pallets to the regional wholesalers.
4- Most gun shops purchase from those regional wholesalers. Examples are Reed's and Coyote Valley Sporting Clays who uses the same regional wholesaler out in the central valley of CA.
5- Surplus sellers often have an importer license and bid for foreign countries surplus military ammo much like we do here. Most of the time, though, they are dealing with a local company in that country who handles the transition of ammo from the military into sea-going containers and ship to the surplus seller here.
There is good money reselling from sources listed in #2 and #5. If you have the cash, you can expect at least a 2x margin up for starters. As you build up a relationship, the margin will grow. Of course, this is skipping over the middle layers of wholesalers. You will need serious capital investment this way. We are talking in the six figures and you are getting a severe restriction in ammo type. So if you wanted to offer shotsells, .223, .308, 40sw, 9mm, .45auto, you are getting upwards of $1m invested. They will not fill up a container to your specs of a wide range of ammunition.
We were running a club buy of shotshells for a very large club with skeet, trap, and sporting clays members. We would purchase anywhere from a half container to a full one per year from the US importer of a European company investing anywhere between $100K and $200k. The importer was willing to load the container with a mixture of 12, 20, 28 and .410 that we wanted (actually they would have the European company load the container that way for them). The last run we had was 2007 and we were bringing 25 round boxes in at just under $3/box. Easily could have sold those for 6/box and I think some local dealers must have slipped in. Around the time our container would arrive, some local stores would have this particularly brand in stock.
Keep in mind, we had a very rich dotcommer front the purchase for us. He also had access to receiving docks and storage which is an absolute must for a freight company to deliver. He had excellent contacts in the freight/rail industry.
But there are opportunities in this business. I do not believe gun shops have not adjusted to the new business model demanded by today's younger shooters. They still prefer to stock individual 20/round 50/round retail boxes in various calibers. However, the younger shooters today have consolidated to where they are looking to purchase specific calibers in bulk quantities. This fact has not gone lost on WalMart and is probably why local gun shops here raid WalMart to resell.
I think you can base an ammo business on bulk selling of .22lr, .223/55.5, .308, 9mm, 40sw, .45auto, 7.62x29, and the 12/20/28/410 shotshells. You can do bulk brick&mortar out of a warehousing operations, internet sales, and the gun show circuit as well.
Right now, markup is running slightly under 2x from local wholesaler rates for Winchester AA shotshells. The issue dealers had is getting a reliable stream of quantity in though. In order to maintain a sufficient quantity on their shelves, they are willing to hunt for wholesale purchases with a markup less than 2x for now. Sometimes, they get so worried about the replacement cost being much higher, they do stupid things like 3x markups which only result in the product collecting dust and eating their capital. We have one store out here that is selling WinAA 28ga. for $16/box right now.
ETA your answers:
Is it possible to contact some manufacturer, buy a pallet or two from the factory, and become a reseller? Yes, the US importer/rep of the foreign ammo companies are more receptive.
Or is this stuff sold from manufacturers only to recognized wholesalers? They will try and tell you that, but like everything else... money talks, others walk to the regional/local wholesalers.
If so, how do I find the local wholesalers? Call the manufacturers and ask. They will actually help you get in touch with the local wholesaler who will try and take you to the cleaners if you are a new upstart business with big bucks to invest. Your local wholesaler will want to qualify that you are a true dealer and not putting together a group buy. They will want your FFL, business license info, and sales tax permit info.
Kemen is one of the best to deal with, imo. Search on Kemen and container to get their price quote request webpage.