Loading for vintage doubles my normal routine is work up the load then shoot it in a modern gun for a while before going to a Parker. I don't own a modern 10 G though. Be good if we could work off published data using components that are available, but that’s not possible today. Be clear this works for me in my gun may not in yours. Use caution !!
All loads are for 2 7/8 inch chambers, folded hull is 2 7/8 inches. Roll crimped hull I cut the fold off ending up with a hull about 2 1/2 inches
My short 10 light spreader shell load is Charlie Cleveland's 19 G of Red dot 1 1/8 oz in RST hulls Remington SP 10 wad with a 12 G Polywad yellow spreader disk on top of the shot, roll crimped. Latest loads went up to 20 grs looking for a better burn, still need to shoot some more of the 20grs to see if it's better than 19 grs. Caution Short 10 PDF file has some very hot Red Dot loads, some way too hot. Lightest Recommended is 22.5 with Red Dot and 1 1/8. I think that's a least maximum and don't plan to go over 20 with mine. Far as I know no published pressure data on Red Dot loads
Green dot lightest recommended load off the PDF file is 28.5 using 1 1/4 oz. My loads are 27.5 Green Dot in the RST once fired hulls. Remington SP 10 wad with a cut down fiber filler wad under the shot, fold crimped. Short 10 PDF has Alliant Green Dot Data for 3 1/2 inch hulls using same components = 28.5 charge 1 1/4 oz shot about 8500 lbs
Unique is a back up powder for me, however it ought to substitute about same grain weight as Green Dot. It’s a little slower which gives some safety factor substituting. And both GD and Unique are slower than 7625. Don’t take these charts as gospel, indicators only. If you pull them off the internet they don't all say the same thing.
http://www.t2ammo.com/data/_uploaded...te%20Chart.pdf
Once I settle on both loads will send them off for pressure testing.
William