From the OK Corral to Afghanistan
From the early days of cartridge firing weapons, there has been a school of thought that promoted sharing the same round in your handgun and long gun.
Some classic examples are the Colt Peacemaker revolver and Winchester 1873 carbine, both in .44-40. Other possible combos were in .32-20 and .25-20. In The Rifleman TV show, Chuck Connors spun a model 1892 Winchester in .32.20. Moving up to WWII, we can't forget the venerable 1911 .45 automatic pistol matched with a M3 "Grease Gun" submachine gun in the same caliber. Across the pond, a British commando might have carried a Browning Hi Power pistol and Sterling subgun in 9mm, while German soldier could be found with a Walther P38 pistol and MP40 subgun, again in 9mm.
In modern times, Cowboy Action Shooting competitors frequently use recreations of those old guns in .44 special/.44 magnum, .38 special/.357 magnum, and .45 Colt. Child competitors can use revolvers and lever guns matched up in .22 long rifle caliber. You local police officer might carry the Beretta Storm carbine/pistol combo that not only uses the same ammo, but the same magazine. The Kel-Tec P2000 carbine can be had with lower receivers that use magazines from Glock, Beretta or Smith & Wesson for complete compatibility with the corresponding handguns. A camper or squirrel hunter might go to the field with a Ruger 10/22 rifle and Mark III pistol, both in .22 long rifle caliber. A unique combo is the WWII era M1 carbine matched up with the discontinued Ruger Blackhawk single action revolver in the same .30 caliber. That combo is deadly on coyote sized game and would be great for a rancher's holster and pickup gun rack.
Personally, I have three different combos. Each has it's own job function, and I find the ammo compatibility to be a true benefit.
Competition: I occasionally shoot in Cowboy Action matches. I use a Winchester Model 94 Trapper carbine and a pair of Ruger Bisley Vaquero revolvers, all in .44 special/magnum. I load my own .44 special ammo for matches, and have a fairly mild load, pushing big slow bullets. The 94 was originally made for rifle length cartridges, and many say it won't function well in pistol calibers, so they use a 92, 73, or a Marlin. I seat my bullets a hair longer than a standard .44 special round, and I have never had a lick of trouble with my Trapper feeding or failing to function. Maybe I'm just got an unusually good one, but I've put several thousand rounds through it and would trust my life on it if I had to. A competent shooter can quickly learn to get off 10 rounds from a lever action in 10-12 seconds. A champion class shooter, using a race tuned carbine, can get off rounds faster than the cyclic rate of some full automatic weapons. Same thing with a pair of single action revolvers. I shoot a bit slower with mine than I do with a semi-auto pistol, but there are plenty of shooters that can make the single action go faster than a 1911 can action it's slide.
Sport: I have several .22 rifles. My favorite is the venerable Ruger 10/22. I am currently without a .22 pistol (other than my NAA mini-revolver) but plan soon to buy a Walther .22 that I can later get a suppressor for. I also have a Henry lever action .22 and plan to get a Ruger Single Six .22 single action revolver for inexpensive cowboy action practice.
Defense: A few Christmases ago, my wonderful wife surprised me with an Olympic Arms M4 style carbine in 9mm that takes Glock magazines. I also have a Glock 17, so my standard 17 round magazines can go in either one. I also have plenty of the 31 round G18 mags that will function flawlessly in either one as well.
Especially in the defensive realm, there are those who say that a pistol caliber carbine is of no use... that a .223 or 7.62x39 round will be much more effective. Depending on the situation, those people are right. But out here living in the woods, I would never have a clear shot over 35-40 yards, and the pistol caliber is great at that range. If my lanes of fire were over 75 yards, I'd probably rethink things.
Is anyone else a fan of sharing their ammo betwixt their handguns and long guns?