Recently someone had a failure with their sidearm that required the services of a gunsmith to fix. The cause was attributed to a bad round from a major manufacturer with a reputation for producing quality ammunition. As the description of the failure was such that I cannot corroborate or debunk the diagnosis I will not name the manufacturer. However, I have seen enough failures with factory loaded ammunition that I feel safe saying checking your ammo is a good thing. Triply so if you handload your own (though like Mas Ayoob I recommend factory ammunition for serious social purposes).
Inspecting your ammunition isn’t a super tedious process, you can do most of the steps while loading your magazines. First check to be sure the cases all have primers and that none of them are inserted backwards (yes I have seen this), crushed or indented. Next check the case to be sure there are no suspicious bulges and that the case mouth is undamaged (I have seen a case where the bullet was off-center and crushed the case wall). Finally, examine the bullet to ensure that it is not set back in the case or protruding too far.
Once your ammunition has passed the visual inspection you might just decide it’s good to go and have at it. However, there is one final way to verify that your ammunition meets standards. The first is to use a cartridge gage to verify your ammunition meets SAAMI specifications. This is all well and good but some guns are not chambered to SAAMI specification, so we need another way. Fortunately, you already have what you need to do this.
Disassemble your pistol and stand the barrel on end. Most likely you will see something having at least a casual similarity to this: In the circle you see what is referred to as the barrel hood. This is a critical dimension for ensuring the breech has fully closed and is safely ready to fire. It’s also a way to see if your ammo will safely chamber. As you can see in the picture the bottom of the case is flush with the end of the barrel hood. This is what you want to see when you drop a live round into the chamber of your disassembled pistol. If the case is too short, say for instance you have a .380 ACP mixed in with your 9mm the bottom of the case will be below the end of the barrel hood. If the case is too long the back of the round will protrude above the end of the barrel hood. I would not attempt to fire these rounds and if each round I was checking was too long or too short I would take the gun to a ‘smith and have them verify the chamber with the appropriate gage. I have on occasion seen guns brand new out of the box with out of spec chambers.
If your ammunition passes all of these it should chamber just fine assuming there is nothing else wrong with the gun. Notice I didn’t say it would fire. One year a bad round cost champion shooter Mickey Fowler a rather large match. During the manufacturing process the case had slipped through without getting a flash hole punched so the primer could ignite the powder. Short of taking the round apart there was no way to detect this flaw before the round was chambered and the hammer dropped. It’s an extremely rare flaw, but it can happen.
One final note. Do not assume you can just cycle the ammunition through your gun rather than take it apart. Besides the fact you are creating an unsafe condition that could lead to a negligent discharge, you could also start the bullet in the process of setting back in the case which can lead to unsafe pressures if it gets set too far back.