Elmerjfudd,
As a bullet moves down a barrel, the space the powder is burning in grows. This is called expansion. In a .223 Remington, the bullet has to move forward almost an inch and a half to double the volume the powder started burning in when the bullet was seated and the primer fired. In a .45 Auto firing hardball, it only has to move about half an inch to double that volume. So, despite being big and relatively slow, the .45 is able to double volume sooner, and so it needs a faster powder to make gas fast enough to do that. The rifle, on the other hand, must have a slower powder so as not to reach the peak pressure too soon and in a space too small, which would raise the pressure unacceptably.
One of the consequences of the above is that while you can theoretically load the faster powder to the same pressure as the rifle powder, the amount you can use would be smaller so it can't make too much gas for the smaller space it is peaking in. But that also means it makes less gas overall, so that when the rifle bullet was heading down the bore, expanding the space, pressure drops off faster. The results are going to be too little pressure at the gas port to operate an AR, and lower final velocity.
For example, in .223 Remington, even if you could safely load Tightgroup to the same peak pressure as H335, the muzzle velocity with Tightgroup would be about 25% lower. But you can't safely load it that high. The reason you can't is that the case fill is low (under 50%) and that leads to erratic pressure and pressure spikes in some circumstances. So you can probably load to around 5 grains and drive light bullets down the tube at pressures low enough to avoid getting a peak that exceeds the rated pressure should you get a double-pressure spike, which Dr. Lloyd Brownell demonstrated in the mid-60's can happen in 30-40% loading densities in bottleneck rifle cases sometimes. There will be no gas operation of an AR at that pressure level, though. Strictly single-shot.
I think, if you want rifle powder that is cheap, you'll need to look at the surplus sellers. They often have pull-down military powder or military surplus lots. You just need to be cognizant of the fact the military often gets rid of the ammunition and powder because it has passed their age limits, so you want to keep checking it for signs of aging and not load rounds for future years that you want to keep around. Load it and shoot it that same year.
The other thing to beware of with surplus sellers is they sometimes actually charge more than you can buy new powder for.
This place wants $199 for 8 lbs of WC844 surplus 5.56 NATO ball powder with shipping included, but Hodgdon H335 is canister grade (tighter burn rate spec than bulk) WC844 and you can get 8 lbs of new production at
Powder Valley for $158.50. Even with the hazmat fee and shipping, that should still beat $199, and you get the tighter spec powder.
This place is better on the surplus at $144 for 8 lbs, but the shipping and hazmat are on top of that, and not knowing the exact burn rate the advertised claim that H335 data is good with it has to be taken with a grain of salt. For $14.50 in price difference, personally, I'd stick with Hodgdon's version. The last time Jeff Bartlett had surplus WC844, it was like half the price of Hodgdon's product. That, to my thinking, is more like what I expect surplus to be priced at. But he doesn't currently have any.