We have a safety rule at my gun club, that says a gun is only capped at the firing line, and then only when the line is open and you are ready to shoot. Loading a gun in your kitchen to get out of the weather is one thing. Capping it is another. That should always be done outside, with the gun pointed down range. At revolver shoots at our club, only one chamber is capped at a time. I am not a big fan of this restriction, but it happened in response to prior bad safety practices by some shooters, and the club officers decided the club could not afford a lawsuit of some guy shot himself or someone else loading the gun any other way.
There are reasons for safety rules. Most " accidents" occur during informal shooting events, and not at ranges with range officers present enforcing safety rules.
My father, years ago, was using some Foreign-made caps he bought at Friendship, to shoot blanks in our back yard on the 4th of July, and found a cap where the explosive mixture had been dribbled down the inside of the cap. He was very careful to put that cap on the nipple, and he had us stand to one side to see if there was any extra flames coming out where that compound was on the side. Of course, there was a rather large flame, but mostly it blew the side of the cap off, and it barely missed us going by.
From that day on, we have done three things in shooting cap locks:
One: we use only American made percussion caps.
Two: we now visually inspect each cap before putting it on the nipple.( This is not as difficult a task as you might think. I use a Tedd Cash Capper, and put 100 caps in it at a time. I jiggle the capper until all 100 caps are turned facing up, and then visually inspect all of them at a glance. In fact, its much easier to spot something " Different " when you are looking at a whole group of similar objects. )
Three: We do not put caps on nipples with our fingers, anymore.
I have found several caps that had no compound in them at all, and finding them before capping my gun has saved me considerable inconvenience. Its not been restricted to one company, I might add, So I won't identify the names of the companies whose caps have been defective, here.
We did receive some foreign-made caps from a relative who wanted them destroyed properly, and among those caps, we found another cap with the compound dripped down the inside of the cap, just like the cap Dad had those many years ago.