WB
One note about cast bullets and short neck ctgs.
Thursday, January 25, 2018, 16:00

The .401 GNR is a good example and the .429 GNR too. A short neck and struggle to retain as much neck tension as possible. Increasing bullet weight is also a recipe for bullet creep from the case with increased and duration of recoil. I'd not go past 300 gr. in the .429 GNR and have not tested this 250/10mm as of yet.

Look at the sides of that cast bullet comparing to the available internal case neck sidewall. What will touch the inside case neck when the bullet is seated to a particular depth? In my first earlier picture seated in the crimp groove, only the first band under, that narrow band is contacting the case neck to hold the bullet, and whatever crimp. By seating deeper I can shift the band lower, still in that neck area, and have that front band ahead of the crimp groove in the neck. That's two slim bands but an improvement in area and bullet grip with neck tension. The long necked antique .38-40 contacts all the bullet bands, those lube groves are just air to the neck tension but it has a bullet holding advantage.

Jacketed bullets have solid sides and are of no concern here. Lube groove laden cast slugs may need a little thinking of application or technique.

Look over the photo and added notes. Hopefully you can see what I mean. It's not the case's fault, maybe the bullet is not as good a fit as another design for this special application. Actually it was designed specifically as a high performance .38-40 bullet but with custom 10mm gas check bullets so limited it's worth a try for sure. I'll keep you posted.