ok, long winded translation ...
Most modern .22 caliber rimfire rifles and pistols are chambered in .22 long rifle. A fair number are in .22 magnum. Rimfire means the primer material is in the rim of the case. The .22 part is the diameter of the bullet; as in 22 thousandths of an inch in diameter. The rest of the cartridge name refers to the length of the chamber (with those lengths referred to by a name rather than a measurement). The chamber is the part of the gun that the cartridge is in when it is fired. The bullet is the part of the cartridge that flies. The case is the part of the cartridge left behind after the bullet flies. In the "case" of .22s (sorry, couldn't resist the pun) the longer the chamber(and corresponding case length), the larger the powder charge, and the more power/velocity the round has. The lengths, starting with the shortest are : BB "cap", Short, Long, Long Rifle, and Magnum. "Long" never was popular and I'm pretty sure it's no longer in production. BB is almost useless except for particularly large and aggressive cockroaches, and I'm not sure if it's still in production.
Hollowpoint and round nose refers to the tip of the bullet itself. Round nose has solid rounded tip. Hollowpoint has a cavity in the center of the tip. The cavity weakens the tip of the bullet. Upon impact with a soft wet object, it collects matter inside it and the forward motion of the bullet (as it packs more stuff in the cavity) creates pressure that expands the tip. Overexpansion from too much power/velocity causes the bullet to fragment rather than take on a mushroom shape. (When going through the air, the cavity also creates a pocket of pressurized air in front of the bullet to decrease resistance, but that's another subject, and applying to accuracy at a distance.)
I'm assuming he used a .22 Long Rifle (unless it was a .22 Magnum, if so OMG! no wonder!)
.22 Long Rifle is available in a fairly wide range of "powers" today ; ranging from the the lowest being Olympic grade hollowpoint match (exceedingly expensive and special order except for maybe 20 shops in the country) to cheap round nose ammo, and finally to very high velocity hollowpoints. (CCI stinger and yellowjacket are a couple of brand names of the latter). I was also assuming he would be using the cheap stuff from Walmart in the little cardboard box. That would be .22 lead round nose (specifying "lead" means the bullet does not have a jacket or metallic wash, just a dark gray oxidation layer).
If the box they came in was plastic with the cartridges individually hanging, they were probably overpowered for your purposes. Also, the longer the barrel, the higher the velocity/power (up to 20 or so inches). .22 LR is already a bit overpowered anyway, and another reason for recommending .22 short out of a pistol. .22 LR Subsonic would have been a better choice than the cheapest walmart stuff if it's still made. (It's also quieter because it doesn't break the sound barrier out of most barrels)
Also, it is perfectly safe to use .22 short in a "long rifle" chambered gun. Semi autos usually will not cycle properly so they usually have to be loaded and ejected by hand (effectively turning it into an irritating single shot). Often the shorter round will not feed properly from the magazine and they usually do not produce enough gas to work the action.
NEVER put anything in a .22 magnum except .22 magnum. The rim size and case diameter is larger than the rest of the .22 rimfire family.
