I have always been a shotgun "patterner". For decades I went the paper route, but finally built my own grease plate for it, and take pics of the patterns if I want to save them. A big purpose in patterning is to determine if a doublegun is regulated to shoot both barrels to the same point of impact, that being my point of aim. Some do not. All that don't can't be corrected to do so. Some can by the judicious switching of loads. The length of time the payload stays in the barrels is a big determining factor in regulation.
I may have related this story here before but hopefully it's worth repeating. It is a perfect example of the value of patterning with the loads you intend to use in a double. I traded for a 20 Ga. Philly Sterlingworth Ejector to make into my dedicated quail and woodcock gun. It has 28" barrels and was choked tighter than I wanted. I patterned it with 7/8 oz. loads at about 1250 fps and it crossfired terribly, to the tune of 8" laterally with each barrel. That is absolutely not satisfactory to me so I called Dean Harris. We discussed it and he agreed to open the chokes to .006"R and .016"L, lengthen the chambers to 2 3/4", and try to regulate the barrels by his method of honing the inside of the choke area to move the pattern.
He called me some time later with the news that all she wanted was to be fed 1 oz. loads at about 1150 fps. The longer time the payload was in the barrel and the slightly greater recoil with these loads, as compared to the 7/8 oz. loads at 100 fps faster velocity, gave the gun time to move more, laterally, under recoil before the payload exited the barrel. Result was a perfectly regulated gun with one pattern dead atop the other, and both 50/50 on POA.
A shot of my plate after shooting some .410 patterns. I keep a paint roller in a plastic 5 gal. bucket with a few inches of a mixture of white housepaint and motor oil in the bottom of it. After each pattern shot I just roll it, rendering a fresh coat to represent the pattern for the next shot.
Edited for clarity