Test-firing your shotgun is simple but requires a few things to be done correctly. Follow these steps to get a good shot pattern for analysis.
What you need
Shotgun, shells, cardboard plate as target (recommended minimum 1×1 m), tape measure, and a marker.
Make sure you have your shotgun, the shells you want to test, a large cardboard plate (at least 1×1 m), a tape measure, and a marker pen.

Draw a horizontal line of 10 cm in the centre of the plate. Then mark a small aiming point 5 cm below the middle of that line.

Place the target securely and pace out 28 metres from it. This is the standard distance for pattern testing.

Aim at the aiming point and fire. Then photograph the shot pattern on the plate and upload the image for analysis.
