madsenshooter wrote on Jan 18^th, 2018 at 9:37pm:

. . .What I'm thinking is fill up that space around the boat tail with powder. It'll be there, under pressure, even when the neck is expanded, and as the bullet travels through the neck and starts into the throat.  That should further minimize any chance of yaw. . . I wish I had quickload to play with, it'd be interesting to see what those ball powders would do with a 174gr bullet out of a 30" barrel. . .

I think that's pretty insane. Granular powder, especially spherical, has about the structural stability of dry sand - if sand could burn away.

In my limited experience (only been handloading since 1958), using super-slow powders in low-pressure, relatively small cartridges is a waste. They never get hot enough to burn much before the bullet exits the muzzle.

Here's QuickLOAD estimates for H870 (supposedly very close to WC860) in .30-40 with the 175 Sierra BTHP. Note the penultimate column showing the % propellant burnt.

Cartridge          : .30-40 Krag
Bullet             : .308, 175, Sierra HPBT MatchK 2275
Cartridge O.A.L. L6: 3.089 inch or 78.46 mm
Barrel Length      : 30.0 inch or 762.0 mm
Powder             : Hodgdon H870

Predicted data by increasing and decreasing the given charge,
incremented in steps of 1.0% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
%       %    Grains   fps   ft.lbs    psi    psi      %        ms

-10.0   90    42.30   1679    1095   14434   3309     52.9    2.469
-09.0   91    42.77   1698    1121   14809   3393     53.6    2.441
-08.0   92    43.24   1718    1146   15198   3477     54.4    2.413
-07.0   93    43.71   1737    1172   15596   3562     55.1    2.385
-06.0   94    44.18   1756    1199   16012   3648     55.9    2.358
-05.0   95    44.65   1776    1226   16439   3736     56.7    2.330
-04.0   96    45.12   1796    1254   16880   3824     57.4    2.303
-03.0   97    45.59   1817    1283   17339   3914     58.2    2.276
-02.0   98    46.06   1838    1313   17810   4004     59.0    2.249
-01.0   99    46.53   1860    1344   18300   4096     59.8    2.223
+00.0  100    47.00   1881    1375   18808   4189     60.6    2.196