To resist the pressure of the explosion against the breech end, a tightly-fitting breech-plug must be employed. The most modern and ingenious is the Welin plug, invented by a Swedish engineer. The ordinary interrupted screw breech-plug has three parts of its circumference plane and the other three parts “threaded,” or grooved, to screw into corresponding grooves in the breech; thus only half of the circumference is engaged by the screw. Mr. Welin has cut steps on the plug, three of which would be threaded to one plane segment, each locking with its counterpart in the breech. In this case there are three segments engaged to each one left plane, and the strength of the screw is almost irresistible. The plug, which is hinged at the side, has therefore been shortened by one-third, and is light enough to swing clear with one touch of the handwheel that first rotates and unlocks it.
The method of firing is this: The projectile lifted (by hydraulic power on a ship) into the loading tray is swung to the mouth of the breech and pushed into the bore. A driving-band attached near its base is so notched at the edges that it jams the shell closely and prevents it slipping back if loaded at a high angle of elevation. The powder charge being placed in the chamber the breech-plug is now swung-to and turned till it locks close. The vent-axial or inner part of this breech-plug (next to the charge), which is called from its shape the “mushroom-head,” encloses between its head and the screw-plug the de Bange obturator, a flat canvas pad of many layers soaked with mutton fat tightly packed between discs of tin. When the charge explodes, the mushroom-head—forced back upon the pad—compresses it till its edges bulge against the tube and prevent any escape of gas breechwards.
The electric spark which fires the charge is passed in from outside by means of a minute and ingenious apparatus fitted into a little vent or tube in the mushroom-head. As the electric circuit cannot be completed till the breech-plug is screwed quite home there is now no more fear of a premature explosion than of double loading. If the electric gear is disordered the gun can be fired equally well and safely by a percussion tube.
This description is of a typical large gun, and may be applied to all calibres and also to the larger quick-firers. The mechanism as the breech is swung open again withdraws the empty cartridge. So valuable has de Bange’s obturator proved, however, that guns up to the 6-inch calibre now have the powder charge thrown into the chamber in bags, thus saving the weight of the metal tubes hitherto necessary.
Of course several types of breech-loading guns are used in the Service, but the above are the most modern.
The favourite mode of construction at the present time is the wire-wound barrel, the building up of which is completed by covering the many layers of wire with an outer tube or jacket expanded by heat before it is slipped on in order that it may fit closely when cold. A previous make, without wire, is strengthened by rings or hoops also shrunk on hot.
The quick-firers proper are of many sizes, 8-inch, 7.5-inch, 6-inch, 4.7-inch, 4-inch, and 3-inch (12-pounders). The naval type is as a rule longer and lighter than those made for the rough usage of field campaigning and have a much greater range. There are also smaller quick-firers, 3-pounders and 6-pounders with bore something over 1-inch and 2-inch (Nordenfelt, Hotchkiss, Vickers-Maxim). Some of the high velocity 12-pounders being employed as garrison guns along with 6-inch and 4.7-inch, and the large calibre howitzers.
We still use howitzer batteries of 5-inch bore in the field and in the siege-train, all being short, rifled, breech-loading weapons, as they throw a heavy shell with smallish charges at a high angle of elevation, but cover a relatively short distance. A new pattern of 8-inch calibre is now under consideration.
It is interesting to contrast the potencies of some of these guns, all of which use cordite charges.
| Calibre. | Charge. | Weight of Shot. | Muzzle Velocity in Foot Seconds. | Number of Rounds per Minute. |
| 12 inch | 207 lbs. | 850 lbs. | 2750 | 1 |
| 8 inch | 52 lbs | 210 lbs | 2750 | 5 |
| 6 inch | 25 lbs | 100 lbs | 2775 | 8 |
| 4.7 inch | 9 lbs | 45 lbs | 2600 | 12 |
| 3 inch | 2 lbs. 9 oz. | 12.5 lbs | 2600 | 20 |