I feel like you'd be unlikely to notice the difference between the two steels in practical application as far as AR-15 bolts are concerned,
I'm sure you wouldn't. They aren't exactly the same but they are very similar. I was just disputing whoever said they were the same, because they aren't. That said, I'd still go with the tried & true like you.
What I was getting at was this: AISI 9310 almost certainly isn't really cheaper to work with in the sense of the difficulty of machining, just that it's more available. In other words, if a machinist wants to make one of these bolts he can go online and buy 9310 rod stock in whatever footage/diameter is appropriate. If he wants to make, say, ten, he can literally buy almost exactly that amount of steel, easily, with just one phone call. Hell, they might even list the prices direct on a website. It's that common because AISI grades like that are WIDELY available.
On the other hand, proprietary internal grades (like, I presume, this carpenter-158)are not very available at all. That same machinist would have to call a whole bunch of suppliers to see if they have any available and in the appropriate dimensions, and they very probably don't.
Which means he is going to have to go up the chain where they quote you dollar per pound, but the order minimum is in tons.
That's what I was getting at.
I don't know know a whole of lot about tool steel like I said, but I understand a bit about the industry. I suspect this situation is exactly why there are so many 9310 bolts available.