Unrealistic practice or training will not help you in a violent confrontation.
You can practice with .38 Special in a .357 Magnum revolver, but the muzzle blast/flash/recoil from .357 Magnum is not the same as .38 Special in the same revolver. In the dark of night, can you acquire and repeatedly hit the target with .357 Magnum as fast as you can with .38 Special?
Can you hit the target with the second .357 Magnum just as fast and accurately as you can after you light off the first round as compared to .38 Special?
If not, would that indicate that perhaps that's not the best round for defense use?
Is .357 Magnum so much more effective at poking a hole in the target that .38 Special will not do?
If you can't or won't train with a certain type of firearm or ammunition because it is painful or unrealistically expensive to do so, would that indicate that perhaps you should evaluate alternatives?
I carry and use a 9MM pistol because I can hit the target faster and with greater accuracy than I can with .45 ACP, .38 Special, or .357 Magnum. No amount of training is going to change physics and nothing is going to change the fact that a round of ammunition with twice as much weight in materials costs more and a cartridge that's twice as big reduces your magazine capacity in an equivalently sized magazine.
There's a point where having the best firearm, the biggest bullets, or the most bullets is a moot point, but there's also a purpose to marksmanship training, realistic defense training, and carrying what you train with.
If I could not afford to shoot 556MM through my AR, I would opt for a different type of ammunition or different weapon for training and carry/use. A 556MM AR may be the best home defense weapon on the planet, but if I can't afford to shoot and maintain the weapon and an AK74 or .223 ammunition in the AR is almost as effective, I think I would make the decision to use the weapon and ammunition that I could afford to maintain and shoot.
Apart from experimentation to determine which type of ammunition is most reliable in my weapons of choice, I see no upside, apart from economy, to practice or training that is not consistent and realistic. Is defense something you really want to apply serious bean counting to? I'm not suggesting that you need gold plated bullets or anything like that, but you can't achieve much more realism than by practicing and training with what you actually intend to use to defend yourself with.
A Mosin Nagant may be a perfectly acceptable infantry rifle, assuming all the other infantrymen are carrying something similar, but to imply that it is a sniping rifle equivalent to a M24 or M40 because both are 5 shot bolt action rifles, have scopes attached, and shoot cartridges of equivalent power is just laughably absurd.
If the only practice you can afford is the use of snap caps, then it's better than nothing. To imply that using .357 Magnum snap caps is equivalent to shooting .357 Magnum ammunition is silly.
If cost was not a consideration, and I had a choice between a .357 Magnum revolver and a 9MM pistol for carry, I would choose the 9MM pistol every day of the week. The fact that the revolver weighs more, carries 1/3 as much ammunition ready to use, takes longer to reload, is more difficult to fire quickly and accurately, and both the revolver and revolver ammunition cost more than the pistol and pistol ammunition makes the choice really easy (for me).
I don't own a single .22LR rifle or pistol because I'm never going to take a .22LR to a gunfight and I don't hunt. I have .357 Magnum revolvers that I shoot for nostalgia, but I shoot .38 Special through them. If I were to ever carry one of my revolvers, it would be loaded with .38 Special because that's what I can afford to shoot the most through it, less recoil, less wear and tear on the gun, and that's what I can shoot most accurately with at any appreciable speed. I'm sure .357 Magnum would work just dandy in a gunfight, but after thousands of rounds of .38 Special I see no compelling reason to switch to something I am less practiced at shooting when my life depends on my shooting skills.