Brad gave a good description of the terminology used with the “big guns.” In my studies of the Civil War, gun breeches were typically closed since most of those fine cannon were muzzle loaders. A few, definitely the minority, were breechloaders, pointing to the future. Forty years later, at the start of the 20th century, all guns in the US Army and Navy, save a few ancient Civil War types retained in reserve, were breechloaders. The evolution of breechloading was as important to the overall gun system as rifling and recoil systems.
To help visualize the difference between interrupted screw and sliding breech-blocks, let me pull up some photos from the USS Olympia, with guns dating to the 1890s. Here’s a the breech block from a 5-inch gun:
Notice the breech block (to the right). It has two “steps” of threads. Notice also the locking lug in the center of the step closest to the camera. The tube running from under the gun to the 11 o’clock position is, I think, part of the exhaust venting system. If I’m correct that tube blasted compressed air into the breech after firing to force the fumes out the muzzle end.
Closer look at the block:
So there’s a flat step, an intermediate threaded step, and a top threaded step. There’s also an obturation cone at the front face of the block (left side in this view). That cone pressed against the gun where the chamber met the breech, sealing off the escape of gas when the gun was fired. Again notice the locking lug. The threads and lug matched into the breech end of the gun:
Again a flat, intermediate and top step, with the last two threaded. There is a “seat” for the obduration cone in the mouth of the breech. Also notice the trail for the locking lug at the upper left. A trace shows the path of the locking lug as the block entered and turned within the breech (sorry for the fuzzy photo).
So the top threaded step with the locking lug entered the breech against a “flat” step, then it turned about 45 degrees to lock against the intermediate step in the breech. Here’s a view of a similar 5-inch gun with the breech closed. Notice the entry slots for the steps (from 9 o’clock to about 11 o’clock). Also notice the open hole in the center for the primer.
There were of course several variations on the interrupted screw. This particular block uses the British Welin system, with the steps in the screw. The French de Bange system lacked the “steps.” But the principle was generally the same – a slotted breechblock inserted into the breech then turned and wedged in place using threads.
A few steps away is a sliding breech block on one of the Olympia’s 6-pdr guns.
On this example, the breech block hangs low, reading for loading. Missing on the side of the breech is a handle that gunners used to push the block up and against the breech face. Here’s a view from the back.
This gun used a “U” jaw. Other sliding block guns had open jaws (without the lower end of the “U”) or closed jaws (with only a hole in the breech to insert the round. The gunner worked a handle (missing here but the axis of the mechanism sticks out to the left) which in turn worked to draw the block upwards. The trace of action would press the block against the breech face. Hard to see, but the upward trace of the block is not true perpendicular to the line of the bore. It is instead slightly angled towards the muzzle, thus pressing the block against the breech tightly as the block is driven upwards.
In some weapons a two-action motion would also push a small obduration cone into the mouth of the breech. But in most systems, a brass case provided sufficient closure to seal the breech against gas leaks. Here’s a view of another 6-pdr with the block in the closed, or firing position.
Note the “U” shape of the top of the block. This cleared the breech block when in the loading position. But there is a small gap between the top of the breech mouth and the block.
For reference, the 6-pdr is 57mm caliber weapon. Generally, but certainly not absolutely, sliding blocks are found on guns using fixed ammunition. Interrupted screws worked better where bagged propellant charges are used.