On more than one occasion, I've suggested that it's not a bad idea to have an upper receiver group for your AR-15 chambered in something other than the "traditional" 5.56x45mm NATO. I've usually recommended 7.62x39mm Soviet/Russian, because of its significantly lower price compared to cartridges like the 300 AAC Blackout.
Here's more evidence that, even in this age of drastic ammo shortages and soaring prices, the cost differential can still be very much in your favor. SG Ammo, my favorite online dealer, sent out its latest newsletter yesterday. In it, the following rounds were advertised:
500 Round Case - 5.56mm 62 Grain Green Tip FMJ M855 IMI Ammo Made by Israel Military Industries - $399.00. This translates to a cost per round of 79.8 cents - let's round it to 80 cents.
1000 Round Case - Wolf 7.62x39 122 Grain FMJ Ammo Made by UCW - $439.50. This translates to a cost per round of 43.95 cents - let's round it to 44 cents.
(There's cheaper 7.62x39mm available from SG, but it has corrosive primers [not a problem if you clean your rifle after every range session, but a complication I'd prefer to avoid if possible]. Also, unlike this type of Wolf ammo, it's coated in lacquer, a Soviet and Russian standard for mil-spec steel-cased ammunition. Their weapons are designed around it, so it doesn't cause problems for them, but the AR-15 has tighter tolerances, so it can experience some difficulties feeding and functioning with steel-cased ammo in general, and lacquer-coated ammo in particular. I don't mind that for training purposes - it gives me a chance to practice failure drills, something we should all be doing regularly - and anyway, the malfunctions are few and far between.)
Thus, even at today's appallingly inflated ammo prices, decent-quality 7.62x39mm can be had for only slightly more than half the cost per round of decent-quality 5.56x45mm. That's a very compelling saving. What's more, if you know what you're doing and can get the parts, you can build an AR-15 upper receiver group in 7.62x39mm for about $450, even today. (I just priced the components to confirm that. It's a realistic total. If you prefer, you can buy a pre-built one, in carbine or pistol configuration, for $499.99.) That means you could buy 1,000 rounds of Wolf 7.62x39mm, and an upper receiver group to fire it, for $939.49 - not much more than the cost ($798.00) of 1,000 rounds of IMI 5.56x45mm (it's a difference of only $141.49). After that initial expense, every time you bought ammo for training purposes, you'd pay about half the cost per round for 7.62x39mm as opposed to what you'd have had to spend on 5.56x45mm.
Savings like that are not to be sneezed at. If you're finding it difficult to afford (or to justify the cost of) 5.56x45mm training ammo, you might want to consider that alternative.