Same conglomerate, same design.
Hard to imagine them setting up two production lines.
Count the dollars. If it doesn't make a profit they won't do it. If there is a cheaper way to do it, they will change. If you can consolidate and work by mass rather than by multiple plants, you will do it.
It is all going to be the same thing. Same bullets from the same plants. Same 'brass' from the same plant. Same powder and primers.
If a company owns all of the component production, it forms a sort of monopoly that allows the maximum of profitability; all profits and expenses come from and go into a single asset group. If you can own a huge plot of land and run an entire campus full of manufacturing plants right in one area, there's no need to pay taxes and rent, or other extra expenses running buildings in several states or even towns.
If speer makes the bullets and primers, as well as the brass (i dunno, do they?) And they do it right on site, they don't have to depend on what Otis Industries is going to do. Go on strike, demand a price raise, contract with a transportation company that can't get the freight on time, doing it yourself is the best, most dependable way to do the work. Unless your combined efforts include a few morons, that is.
Take a couple of other examples. Walmart doesn't make anything that they sell, the brutal power that they have allows them to beat vendors into extreme cooperation. The holy grail for walmart would be to own the company that manufactures all of, or most of their generic medications with a renewable contract.
Who knows whether either speer or federal actually make the aluminum ammo in a plant that they own? It might even be manufactured under contract, so speer doesn't have to own or operate a line that does nothing but turn out millions of rounds of cheap ammo day after day and decade by decade. That product is the least important product line they own.
So, maybe, they have a contract with Otis Inc, and otis has an assembly line bought at auction from one of the defunct plants, and he has it set up to run blazer ammo 24/7/365. If his net profit is a penny a round, WOO HOO!
The polar opposite exists in some poorly developed countries. Many products are made by family members at home, and sold to dealers. Mostly hand made products, textiles, rugs, scarves, carved knick knacks, for example.
Cigars are a good example. Otis will own a cigar company that sells millions of cigars a year, cheap and junky.
Otis will buy tobacco from brokers at best commodity price. He will deliver this commodity to professional rollers who aren't working in a factory, and pay them a pittance for each one produced. Then, he packages and sells them in mass by the ton.
it's good. It keeps people working. It allows the little companies to prosper. It allows the peasants to have a cigar, one that wasn't made in an expensive factory, hence cheap. If moonshining was legal, you would see this system working all over the US as well.