The Westerm Field 550 looks like a Mossberg 500, so I would start there looking for a plug. A good gun shop might have one, a gunsmith could make you one, or gunpartscorp.com may have one on the shelf.
To remove the plug from a Winchester 240W Western Field 12-gauge shotgun, first ensure the firearm is unloaded and pointed in a safe direction. Open the action and locate the plug, which is typically found in the magazine tube. Use a suitable tool, such as a screwdriver, to unscrew or push on the retaining cap or plug, then carefully pull the plug out of the magazine tube. Always refer to the owner's manual for specific instructions related to your shotgun model.
It is not difficult at all. It needs 14 gauge wire, freely available any hardware store.
Plain Plug Gauge
A gauge line can leak due to a simple break. It may also be caused from a line tearing away from the plug. The gauge itself may be too large or small for the plug as well.
With a spark plug gap gauge.
A thread plug gauge and a thread ring gauge, although designed for the same nominal size, serve different purposes and have different tolerances. The plug gauge is used to check internal threads, while the ring gauge checks external threads. Differences in manufacturing tolerances, lead angles, and thread profiles can cause a plug gauge to fit into a ring gauge even if they are both rated for the same size, leading to discrepancies in fit. Additionally, wear and deformation over time can affect how well the gauges interact.
"Gauges" is a slang term for large gauge plug jewelry. Technically "gauge" is the measure of the thickness of any type of body jewelry, including standard earring posts.
A feeler gauge is actually a set of very fine strips of steel used to measure small gaps such as plug gap of an auto plug, or gaps in bearing surfaces.
The spark plug gap is .020 on your gapper gauge!
Use a .5 feeler gauge for the TR Spark Plug Gap.
Depending on which intake manifold you have there may be a plug in it near the thermostat housing. Also, each head will have a plug on the side. One is usually fitted with the sensor for the original gauge. This is the usual spot for wiring in an aftermarket gauge.
anywhere between 16 and 18 on a spark plug gauge should work fine.