The modern duty handgun is safe, dependable and durable. But for good measure, here are a dozen rule violations which can make a handgun less reliable:
1) Get bug repellent on a polymer gun
The rumors are true. DEET (N,N-Diethyl-meta-toluamide) is one of the most effective bug repellents in the business. Unfortunately, it is also an extremely effective solvent. Some manufacturers have warnings about getting DEET (and other solvents) on the magazine feed lips. It's actually best to avoid overspraying DEET-based products on any part of the gun.
When shooting in a mosquito-prone area, apply DEET-based products to the back of the hand, not the palm. This will also keep the palms from becoming slippery.
Solvents designed for cleaning guns, like Hoppe's No. 9, are safe for polymer. Clean the working areas of a handgun normally and use mild soap and water for where the hand connects to prevent a slippery grip.
2) Use steel case cartridges
There has been a lot of discussion about the detrimental effects of using steel cased cartridges for practice. One would think that steel cartridges would score or break the extractor. Generally, they won't, as the cartridges are usually made of mild steel.
Another theory says untreated steel will rust. Although there certainly is a risk of corrosion, almost all steel cartridges have a protective lacquer. There really isn't any usable data about galvanic corrosion, but there may be some steel products out there where this may be an issue.
Steel cartridges are not recommended for use on a regular basis, as steel lacks the inherent elasticity of brass. It may be soft enough to feed in a chamber, but it does not seal the chamber like brass does. The bottom line: Using steel cartridges occasionally will not harm the gun. Use it sparingly and clean the gun well afterward.
3) Repeatedly chamber the same round
A cartridge is composed of a brass case, primer, powder and bullet. When loaded, the case mouth must pinch on the bullet hard enough so it does not move prior to firing. Repeatedly chambering the same cartridge can shorten the length of some cartridges, to the point where the firing pin may not get a decisive strike; and it can cause the bullet to work its way out of the cartridge, causing it to change the characteristics of the shot or (in extreme cases) get it lodged in the barrel.
The Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) explains if a cartridge is too long, it will cause the firearm to feed incorrectly or, more likely, fail to fit in the magazine. If it is too short, it can increase the pressure inside the cartridge to dangerously high levels.
4) Repeatedly chambering a shouldered cartridge
The headspace of any cartridge is the area which indexes it in the chamber, preventing its excessive forward movement. The SAAMI dimensions of a case allow for the variations of case lengths. For a straight walled case where the rim is the same diameter as the case mouth like a .45 ACP, .40 S&W or 9mm, the cartridge headspace is on the case mouth. For the .357 SIG, headspacing is on the case shoulder, the area where the wide base of the cartridge tapers to its smaller diameter.
SIG SAUER has capitalized on the law enforcement utility of the .357 SIG by making their P250 Subcompact Nitron models in this caliber. When the .357 SIG case shoulder stops the forward motion of the cartridge on the case shoulder, the brass shoulder slams against the steel walls of the chamber. Do this too many times and the case gets shorter. If the case is too short, the firing pin will not contact the primer reliably.
The solution is simple: Rotate cartridges in the magazines by emptying the magazines before a shift. Reload and use a different bullet in the chamber that day. Once a month, rotate the bullets out by shooting them. Fill up with fresh ones to ensure that the magazines are inspected daily, the bullets are visually inspected and the gun goes bang when the trigger is pulled.