When buying a gun it is important to buy it from someone who has a good reputation for service and reliability. If you have any doubt in how to check a gun for its safety and serviceability take some one with you who is really knowledge and only knows a bit more than you do. If you are in any doubt as to the condition, do not buy it as there are plenty of good guns out there. It is also illegal for anyone to sell a firearm that is out of proof. If we take a shotgun as an example, this means that the diameter of the barrel has worn to a point that it is in excess of the wear limit that are set by the Proof House Regulations that tested the gun. It is really only practical to measure this wear with a proper gunsmiths bore gauge. Always ask what the bore measures when you are considering buying a secondhand gun.
If we take a modern 12 gauge that has stamped on the barrel 18.5 mm, the barrel can were up to but not beyond a maximum diameter at 9 inches from the breech face of 19.5 mm. At this point the gun must be resubmitted to one of the Proof Houses (London or Birmingham) for retesting. However, it is also important to remember that although a gun may pass test, the wall thickness of the barrel is reduced by the increase of the internal diameter being larger than originally made. I have had guns brought into me that have recently being sold as, “ Refurbished” and the barrel walls only measure .020 thousands of an inch in thickness. That is too thin and unsafe, as it does not allow for any wear and the tubes can be very easily damaged. When buying a gun you need to ask how much wear is left in the barrels and what is the wall thickness.
The next thing you should do is check the condition of the bores. Make sure the bores are clean and shiny as in a dull bore it is difficult to see faults. Also a clean greasy one can hide small marks, so you want it clean, grease and oil free for inspection. The bores should been clean and bright with no black marks showing or any dents or bulges. Once you are satisfied with the condition of the barrels you can move on to the next stage.
Remove the for-end from the gun and rest the barrels on a table. so the trigger guard is facing upwards. There should be no movement between the breech face and the barrels. The reason you remove the for-end is that on some occasions the for-end can take up some of the slack. If you feel movement, make sure where it is. Look for the movement between the breech face and the barrels, in some cases if it is really bad you will see light between the surfaces, On some occasions you will feel movement and it could be for two reasons, one there is a crack in the stock or the actual stock may be a little loose on the action body. If this is the case it may just require a tighten up. However, be sure where the movement is.
If the gun has ejectors make sure they both work. Use a pair of snap caps of the appropriate gauge to test that they work properly. Place the snap caps in the chambers and fire the gun. Open the breach and see if the snap caps eject cleanly. In some cases, particularly with an over and under, if the timing is not correct, the bottom snap cap will not eject properly, it will hit the back of the breech face and drop back into the chamber. This is not a serious problem but it needs fixing.