Plating is done to quickly and easily learn where the gun shoots (Point Of Impact or POI). It is also used
for a quick look-see of choke and cartridge performance. To make a final determination of cartridge
and choke performance (note that choke is not a "dial" measurement, it is a specific combination choke and cartridge performance level...) the pattern frame comes into use. Here is a picture showing examples of both:
The plate is piece of sheet steel about 1/4" (6.35 mm) thick, 36" (0.914 m) square, covered with (white lithium) grease or a mixture of white paint pigment and oil, then fired at the desired distance. The pattern is then inspected, the plate re-greased and so on until the POI is adjusted to the desired value or a decision is reached as to which choke/cartridge is desirable. Most gun clubs will have one. Sometimes, a plate will have a large 3/4" (19 mm) bolt in the center as an aiming aid. Or simply a big hole.
A better plate arrangement is with a wider plate, allowing two barrels (or two choke tubes or two cartridges or two seperate guns) to be compared simultaneously. See the following pictures:
Here's the rear view to show the mounting method. Note that, for safety, the plate is angled slightly down to prevent pellets from bouncing directly back at the shooter.
While a plate can certainly be used to determine pattern percentages and uniformity, most find it an easier job to pattern on paper. Then the slog work of drawing the circles, counting pellet marks and evaluating uniformity can be done more easier at a later time in greater comfort. I had a 24" (0.61m) clear plastic disk made up to expedite finding the center of the pattern. I then use a compass to draw in the 10" (0.25 m) and 20" (0.51 m) circles.
An alternative is to incur the expense of a digital camera, specialized pattern calculation software and the required computer to run it on. If you have a lot of patterns to run, the cost is easily justifiable as it's a tremendous time saver. See www.shotgun-insight.com, for one software example.
The portable pattern frame shown above is a simple, but rugged, design to allow easy, no-strain transportation with quick set-up and breakdown. The latter is done with only two screws (see picture below; arrows point to the conduit couplings). Loosen the two bottom coupling screws, and the frame comes away for easy transport; each base is easily picked up in one hand. Frame is stable enough to use in any wind one would pattern in. I have supplied a Bill of Materials at the bottom of this page.
For a more permanent one, two posts in the ground, 4 feet (1.25 m ) apart, connected together by PVC or steel electrical conduit at 4' (1.25 m) apart works well. Paper is attached to the bars by duck tape.
I have found 36" (0.914 m) white butchers wrap paper, available from any paper supply store, works
quite well with 24 gram loads out to 35 yards (32 m). The cheaper brown Kraft paper is harder to
find pellet holes in; the slight extra cost for the white paper is well worth it to speed up the work. For only
a few patterns, supermarket shopping bags can be taped together. Blueprints also work, and so forth. With
heavier loads or longer distances, 48" (1.22 m) works better. It's harder to accurately aim a shotgun at the
longer distances and 36" is insufficient width to see the big picture with the heavier loads.