Whiterabbit -
Howdy !
I read a post @ one of the shooting websites, from a guy that did ballistics lab work. He talked about efforts he made determining what charge level for each of several powders would result in " ringing the chamber ".
H4198 was one that was tested. In the end, his recommendation was to not use less than 70 % of the charge it took to fill case to base of bullet.
He also mentioned that any charge wt of 2400 could be used, and yet the chamber was not damaged. As regards this latter info, we note that 2400 is close in burn rate to WW296/H110; and both are used in .357Mag reloads by many. This would seem to imply that any initial loads worked-up w/ a sensible charge wt of 2400, would give you a good & safe starting point
I'll point out that Hodgdon lists H4198 as an "extreme rifle propellant ".
H4227 is not described individually on their website, the way most of their other powders are.
Perhaps they were being selectively non-commital, with an eye that 4227 could see use in both rifles, carbines; and handguns ?
For a top-break " test mule ", might I suggest you take a look at an H & R SB2 ULtra rifle; in .357Max ?
I say this because I had one, and I could not get even minimally seated .358" cal 200gr bullets to touch the rifling. This would seem to imply that (perhaps ) your longer/heavier chosen bullet might be able to reach the rifling in one of these rifles; while the bullet not encroach the powder spaced too severely ? Hmm....
IMHO -
.357MAx, .357 B & D; and .357AutoMag all can run charges like 24gr WW296 under a .357" cal 158gr boolit.
Rather than start w/ a case capacity of the sizes just listed above and deeply seat a boolit into it, ideally.... you would start w/ a larger-capacity case; and let the deeply-seated bullet itself limit the available powder space.
I've been doing a lot of reduced load work w/ .35 Rem in my Marlin M-336 XLR; and bullet/boolit weights of 140 -180gr ( not the 350gr you mention, though ).
The rifle has a 24" 1-16 12-groove barrel. My charges of H4198 were so low that I could not reliably stabilize Hornady 180 SSPs.
I have been able to shoot w/ pretty good results both Rem .358" cal 150PSPs, and the turned brass .358" cal polymer-capped 150gr Extended Range " Raptor " bullets from Cutting Edge.
My point:
With the monster boolit wt you describe and the desired low vel, I'm wondering whether any of the common factory-made .35 calibre single shot rifles have the twist rate needed to stabilize your chosen boolit ?
With regards,
357Mag