When I was a tank commander with a National Guard unit, it was difficult to participate in live-fire training on a regular basis because, well, they're just aren't any tank ranges in West Virginia. The Army did have a tool that allowed the mounting of an M16 coaxially with the tank's main gun and we also had .22 LR conversion kits for those M16s. With a few miniature armored vehicle targets, we were able to conduct fairly realistic live-fire gunnery training almost anywhere at very little expense.
When I was competing in police and practical pistol matches on a regular basis, I employed the same logic by using a .22 LR conversion kit for my 1911. No, it wasn't exactly the same as using the "real" thing but it was darn close. The operation was the same and there was no difference in sight alignment and trigger pull; the two most important aspects of accurate shooting.
Rimfire conversion kits are available for a wide selection of defensive handguns and many manufacturers like Kimber, SIG Sauer and CZ manufacture them for their own handguns. These same manufacturers also offer dedicated rimfire pistols that are, for all practical purposes, identical to their centerfire counterparts.
This concept of understudy guns is not limited to main battle tanks and defensive handguns. Hunters can enjoy the benefits of sub-caliber practice too. Savvy hunters will pick up a .22 LR similar or identical in design and function to their primary hunting rifle and use that rifle on a regular basis for practice, plinking and small-game hunting. A perfect example of this understudy relationship is two bolt action rifles from Ruger; the Model 77 Hawkeye centerfire and the Ruger 77/22 rimfire.
My primary deer hunting rifle is a New Ultra Light Arms model 20 in .243 Win., but I also own a New Ultra Light Arms model 20 RF in .22 LR. Both are bolt actions, both have the same stock configuration and length of pull, both are fitted with similar scopes, both Timney triggers are set to the same weight of pull and both rifles weigh within an ounce of each other.
I can pick up a brick (500 rounds) of .22 LR ammunition for around $25; about what a box (20 rounds) of .243 Win. costs. I use the model 20 RF for plinking, squirrel hunting in the early fall and routinely practice with it from various field shooting positions to keep myself familiar with its .243 Win. twin. Last year, when a nice whitetail presented itself for a brief moment at 188 yards, that .243 instinctively found my shoulder and my finger did the same with the trigger.
Don't let a thin wallet or limited access to a 100-300 yard range infringe on maintaining or improving your skill with your favorite firearm. Invest in an understudy gun and improve your marksmanship, save money and have fun, all at the same time.