Gil;
Interesting. The 10 gauge has about 37% more bore capacity than the 16. This would put the powder charges almost in direct proportion, but a proportionate shot charge would be about 1 7/8 oz for the 10 to be equivalent to the 1 3/8 oz in the 16. 4227 is, of course, a slow burning powder as shotshells go, It has been listed for some heavy loads in the 3" .410 as well as the 3" 20. I seem to recall the 220 Max-Fire Alcan primer was considered a quite Hot primer, which was likely necessary for good ignition of this load. As Zutz was using card & filler wadding he also put that 100 lbs of pressure on the column. At the time he wrote this article the Alcan primers were gone. He recommended to anyone who might try to duplicate it to use a Federal 209M (Magnum) primer.
Something I have never seen in print but have often wondered about is; as the 52.5-grain powder charge in the 16 is only about 75% of the 70.5-grain charge in the 10 then it would seem reasonable the same primer would ignite the smaller charge a bit more reliable than the larger one.
Do you know if the 10,400 pressure you listed was actual PSI or was it LUP?
What DuPont gave Zutz on his was LUP. All things considered as the difference in gauges as well as wadding etc the two loads are very comparable, These differences could easily explain the slight differences in velocity & pressure.
Miller
