The following information on handgun barrel obstructions comes from Section 24 of Shooting by J. Henry FitzGerald. Shooting is also available to purchase in print.
Many shooters have wondered after firing a shot why the barrel has collected an egg-shaped swelling. The revolver has usually been blamed when really it has nothing at all to do with it. It is caused by some obstruction in the barrel when the shot was fired.
It may be caused from a bullet with an oil-soaked cartridge with only power enough to carry the bullet part way through the barrel; or it may be a piece of tobacco, candy or pocket lint, in fact, any small article carried in the pocket or fallen into the holster that has worked into the barrel. However, the bullet lodged in the barrel will account for ninety-five per cent of this trouble.
I remember a police department which I visited some time ago where the men showed me eight revolvers, each with a bullet lodged in the barrel, and they told me the arms were defective. I pushed the bullets out and with fresh ammunition fired several hundred cartridges in the eight revolvers without any trouble whatever. Bullets have been known to lodge in the barrel from fresh ammunition because the loading machine did not throw the proper charge. This is one of the reasons why the arms companies will not guarantee firearms when hand-loaded ammunition is used. They know that hand-loaded ammunition if properly loaded is not going to cause trouble, but if loaded carelessly is bound to wreck the finest of arms.
If a weak explosion is heard when the shot is fired, investigate to see if the bullet has left the barrel before firing another shot. See that the barrel is clear before firing the first shot.
I have found barrels with the entire six bullets lodged in the barrel and, of course, it was ruined. When a bullet or other obstruction is lodged in a barrel and another bullet is fired, the compressed air between the two bullets causes a swelling or ring in the barrel destroying the accuracy of the arm. If the first cartridge fired is so weakened that the bullet stops at rear end of barrel and another shot is fired, the second bullet would stop before it was clear of the cylinder; thus the powder and gases would form a blasting charge instead of an explosive charge and the revolver might be ruined. If the last bullet should enter the barrel before striking the second bullet, then the gas pressure would be relieved by the escape of gas between cylinder and barrel and only the barrel would be ruined.
Care should be taken to wipe thoroughly cartridges that have been dropped on the ground before placing them in a revolver or pistol, because particles of stone may imbed themselves in the grease or lead and scratch the barrel as they pass through.
An obstruction in the cylinder may be carried into the barrel by the bullet and cause the barrel to swell or be forced out of round, but this does not always destroy the accuracy of the barrel.