A scraper for such work may be made of a small three-square file, say about four inches long; grind the sides so as to remove the teeth, and sharpen to a point. As a precaution against starting the rib from the barrel by heating, put pieces of binding wire tightly around, both above and below, twisting them tightly. With this security there will be no harm done even if the solder be melted between rib and barrel, in proximity to the thimble. _Height of Ribs._—The only way to ascertain the proper height of a gun rib, when put on the barrels, is to take the length of the barrels, and from this length make the calculation, as different lengths require some difference in height of rib. It is to be supposed that forty yards is the distance from the shooter to the object fired at, and at this distance a heavy charge of shot will drop about twelve inches. As the sights on shot guns are fixed, it is necessary that the rib be so elevated as to compensate for this dropping of twelve inches. Of course a lighter charge of shot will not drop so much as the heavy one, but the calculation may be based on the heavy charge. To ascertain the elevation at the breech, take the thickness of the breech and muzzle and multiply the difference by as many times as the length of the barrels is contained in the forty yards. This gives the elevation of the barrels without the rib, and the difference must be made out by elevating the rib. Many shooters complain that when their barrels have been shortened the shooting is not the same, and they lay this fault to the barrels being cut shorter, when it is often due to the difference of elevation from that they were accustomed to use. Suppose a pair of barrels were to be shortened, say, four inches, at the same elevation there will be more lengths in the forty yards, hence a difference in the sighting and consequently in the shooting. The calculation for the elevation of ribs is the same as that given for fitting barrels together, and the same principles there given will apply to this subject. CHAPTER XX. ON THIMBLES. _How to Make Thimbles._—What is called a thimble by gunsmiths is the short tube, soldered, or otherwise attached to the gun barrel, which retains the ramrod in place when not in use. To make these thimbles, form them on a piece of steel about a foot long, turned tapering; the large end being about nine-sixteenths, and the small end about five-sixteenths of an inch in diameter. This will answer for about all sizes of wooden rods as they are purchased. These rods are generally, the largest of them, about five-eighths of an inch diameter at the large end and about three-eighths at the small end.