This is the text of the email I sent to the participants in the project. It is not confidential, and I consider the hive here resource/sounding board. No surprises except splitting cases on the heeled bullet loads, apparently do to my using an excessive crimp.
After months of COVID delay. I finally did a range day within the .38 Colt Konversions. All shooting was done free standing, using a Weaver stance. Range was 7 yards, typical SASS revolver target range.
Summary:
1) Black Hills factory .38 Colt. .357," 150 solid base RN: From both a 4" and a 7.5" .375 bore gave minute of dinner plate accuracy, with ALL bullets key-holing and tumbling through the targets at 7 yards. Useless for practical purposes in BP revolvers with .375" bore converted with centerfire cylinder, regardless of whether the cylinder has a .357" or .375" throat.
2) Black Hills factory .38 Special 148 HBWC. From the 7.5 " with the .357" throated cylinder did a 3.5" group, and from the same revolver with the .375" throated cylinder did a little better at 3".
3). Bear Creek 140 grain .375" heeled bullets over 3.7 grains Trail Boss, crimped with OWBM's heel bullet crimp in Black Hills .38 Colt cases. These cases split (no other cases in test split), but I attribute that to my using too severe a heel bullet crimp. The rupture started where the wrinkle in the crimp occurred. The same powder charge with a 148 grain HBW set forward 1/4" in the same .38 Colt case did not rupture. Groups were 1.75" to 3.0" from the test gun, with the 4" EMF 1851 giving the best group.
4) 148 grain HBW set forward 1/4" in the same .38 Colt case, with 3.7 grains Trail Boss gave 2" from the 7.5" barrel using the .357" throated cylinder, and 2.5" when using the .375" throated cylinder.
5) 148 grain HBW set forward 1/4" in the same .38 Colt case, with 3.3 grains of Unique gave 4" from the 7.5" barrel using the .357" throated cylinder, and 3.0" when using the .375" throated cylinder.
6). Several other powders were tried (WW231, Bullseye) and gave results essentially the same as Unique
A) In ALL cases, whether from .357" or .375" throat Kirst cylinder, Hollow Base Wadcutters seemed to obdurate into the .375" bore to give as good of accuracy (or bretter in some cases) than the 140 grain .375" heeled bullet. Groups were consistent and the holes cut crisp and round, with no evidence of tumbling or keyholing.
B) Barrel length (4.0" or 7.5") did not seem to have any effect on this obduration.
C) The hollow base loads, while not quite as accurate as the 140 grain .375" heeled loads, were certainly accurate enough for SASS use.
D) The tests show that hollow base bullets work well in both .357 and .375" bore revolvers.
E) (FOR JEFF) The expense of commercially loading .375" heeled bullets is prohibitive, and appeals to the somewhat limited market share of converted revolver shooters. OIn the other hand, the 150 grain, hollow base round nose bullet (of the original 1892 Army loading of the .38 Colt produced into the 1960s) would be attractive to both SASS shooters shooting .357" bore revolvers and SASS shooters shooting .375" bore conversion revolvers.
F). (FOR WALT) There was no consistent difference in accuracy from the `.357" throated cylinders to the .375" throated ones. It would be easier to simply produce all the cylinders with the .375", rather than in .357" with the .375" being special order. The production being in .357" eliminates traditional shooters )like myself) that may load the original style heeled bullets.