We’re gonna write about some naval guns soon. But before we do, we wanted to discuss some of the basic terminology used with big guns.
1. Caliber- When we talk about small arms, caliber is the diameter of the bore of a weapon, expressed as a decimal of an inch. Thus, .50cal has a diameter of half an inch. But when we talk about big guns, caliber has an entirely different meaning. It refers to the length of the barrel, expressed as a multiple of the diameter of the bore. For instance, one of the more common guns during WWII was the 5”/38. This was a gun with a bore diameter of 5 inches, and the barrel was 38 calibers, or (5” x 38) 190 inches long. Since there are several different guns of 5” bore diameter, it’s common to differentiate between them by listing the caliber, as shown above, thus, 5”/25, 5”/38, 5”/54, 3”/50, etc.…
2. Fixed, Semi-fixed, and separate loading ammunition- Most of us have a vision of what a round of ammo looks like, from .22 bullets or what not. Some large guns use the same style ammo, where the projectile and the powder case are fixed as a single piece. Not surprisingly, this is called “fixed” ammunition. Generally, naval guns up to about 3” diameter used fixed case ammunition. Larger guns, 4”, 5”, and occasionally 6” guns, used “semi-fixed” ammunition. The projectile and the powder casing were loaded separately. Still larger guns, most 6” guns up to the massive 18.1” guns of the Yamato class Japanese battleships used separate loading ammunition. The powder for the round was sewn in silk bags that would be loaded behind the shell. As a rule of thumb, naval guns always fired at full charge, regardless of whether using fixed, semi-fixed, or separate loading ammunition. Army artillery use the same basic types of ammunition, but the charge for semi-fixed and separate loading was varied as a means of adjusting the range of the shot.
3. Breechblocks- Just as the barrel of a gun is called a “tube” there has to be a plug at the end to use it for shooting. This end of the tube is called the breech (the other end is the muzzle) and the “plug” for the breech is known as the breechblock. Separate loading guns almost universally use an “interrupted screw” breechblock. A screw like plug went into grooves cut on the inside of the breech end of the tube. One quarter turn and the breechblock forms a gas tight seal. Fixed and semi-fixed guns usually used a “sliding wedge” breechblock. This was pretty much what the name described. Just a large block of steel that wedged close to the breech end of the tube. Because the powder was carried in a case, the gas tight seal was provided by the powder case, not the actual breechblock itself. Sliding wedge breechblocks were quicker and simpler to operate, and could be powered by the recoil of the weapon. Thus, when the gun recoiled, some recoil energy would be utilized to drop the breechblock. A mechanical trip meant that as soon as the next powder case is rammed home, the spring loaded block slides back into place and the piece is ready to fire again. If a high rate of fire is called for, the gun can be set to fire as soon as the breechblock slides home.
Some of this will make more sense as we discuss individual guns. Of course, if you have questions, fire away.