By Steven Dodd Hughes

A few months back on an Internet forum a fellow asked, “What are the screws in the side of the fences for?” A simple and straightforward question and the short sweet answer would be, “Those screws hold the striker disks in place.” The longer answer deals with strikers, disks and the tiny hole you probably never noticed in the above-mentioned retaining screw.
Britte action Striker screw
(The arrow indicates the retaining screw on a freshly case colored Hughes/Britte Shotgun.)

Basically there are two different types of double gun strikers, or firing pins: those common to boxlock guns that are integral with the hammer, and those common to sidelock guns that are a separate part, enclosed in the action body and struck by the hammer. The sidelock strikers are often held in the action by bushings or disks threaded into the breech face and are known as “disk-set strikers”. (As always with double guns there are no absolutes so there are boxlock guns with either disk-set or solid strikers and although I’ve not seen such, I’ll bet there are sidelocks with solid hammer/strikers.)
Figure One, Striker-
(Figure 1)
Referring to Figure One, let’s define the various parts of a disk set striker assembly: on the extreme right is the three diameter striker or firing pin, below is the striker retaining screw and to the left is the striker disk with the firing pin retractor spring in place. For convenience we will call them firing pin, retaining screw, disk and spring.
Looking at the action body in Figure One it is easy to see one striker disk installed in the breech face and the threaded hole where one was removed to the right. In the lower fence the hole for the retaining screw is visible. Looking a bit closer at the removed disk you can see the hole in the threaded portion where the unthreaded end of the retaining screw locks it in place. Also note that the disk, spring and firing pin holes in the action body are drilled at the upward angle of the firing pin. The firing pin angle is most easily recognized by the angled face of the disk.
Figure Two shows the parts up close with three variations of disk removal tools. On the right are two screwdrivers ground to fit the pin holes at the correct spacing. Upper left shows an adjustable disk removal tool (available from Brownells Inc.) with an eccentric cam to vary the distance between the pins to fit different disks. Unfortunately the tool’s pins are too large to fit this particular disk. Any ‘smith versed in double gun repair will have many specially ground disk removal tools and be ready to make another when your odd-ball gun comes in the shop.
Now is a good time to caution against home removal of disk-set strikers. I have found them to often be very tightly fit if not battered into place by repeated cartridge percussion. The pin holes are as easy to bugger as hair-thin screw slots on a “best” Italian guns. The disk face, not being square to the breech face adds to the complication of removal. And if you forget to first remove the retaining screw from the side of the action, which commonly have very thin slots, you will remember when the disks refuse to come out and the breech face is well scarred. (Or the gunsmith will recognize exactly what you tried to do when it winds up in his shop!) Also the retaining screw is often engraved with the continuation of a scroll that is easily damaged and must be perfectly realigned on reassembly. So attempted removal is at your own peril!
Figure Two, Striker & Tools-
(Figure 2)
Also in Figure Two you can see a hole in the end of the firing pin retaining screw. The screw is actually bore all the way through with a very tiny hole (.040”) that is so small the screw slot on the outside covers it up. This hole is for venting any gas that might leak from a blown primer of ruptured cartridge case. In the English gunmaker vernacular this screw is know as the vent pin because pin is British for machine screw and it vents the breech. Another method of venting the breech was accomplishes with a small groove machined into the breech face from the firing pin hole outward to the edge of the fence that would allow escaping gasses to bleed-off away from the shooter. These are easy to see when viewing the breech face. More on venting in a moment.
The mechanical operation is simple: the hammer, or tumbler in the lock strikes the firing pin that transfers energy to fire the cartridge primer. The spring retracts the firing pin to keep it from sticking in the primer and locking the gun shut. Boxlocks have solid pins because the hammer or tumbler can easily be aligned with a centered hole in the breech face and the cocking lever retracts the firing pin when the gun is opened. Because sidelocks can be easily removed from the gun it is more convenient and compact to separate the firing pins and install them in the action body. These disk-set strikers also make it relatively easy to remove and replace the firing pins.
Firing pins do break and look to be easy enough to make and replace. They are simply a three-diameter pin with a rounded tip on the ignition end, a shoulder for the spring to stop against and a larger diameter rear end that the tumbler strikes. When fabricating a new pin it is critical that the length of the front end be precisely measured to the correct firing pin protrusion; the length of the pin that protrudes beyond the breech face. Firing pins should be made from steel that can be hardened and selectively tempered because the front end can’t be brittle or it will break and the back end must be hard enough to resist mushrooming from repeated blows of the hammer. If the back end mushrooms because the steel is too soft they are extremely difficult to remove from the action.

Figure Three, Platinum Plug- copy

The notion of venting the breech goes back to the pre-flintlock era and as with much in the history of firearms development there is no factual proof, I surmise that there is a direct connection between flintlock vents and the tiny vent hole found in a modern striker disk retaining screw, or vent pin.
In order to fire a flintlock gun the vent or touchhole was a tiny hole in the breech next to the lock’s flashpan allowing sparks to flash through to the powder charge when the priming pan ignited. In later percussion guns it is not unusual to see platinum “blow-out plugs (Figure Three) in the side of the gun’s breech that would theoretically “blow-out” in the case of excessive breech pressure. Many of these platinum plugs had a tiny hole through them and when fired a squirt of smoke escapes from the hole. So putting a screw into the side of the breech of a gun has much historic president and the advance to adding a hole through the vent pin seems a logical step in double gun evolution.
So, that is the long answer to the question, "What are those small screws in the side of the fences?”