..... I have been attempting to reload M-H using CBC brass (reformed 24g shotshells - very thin brass), but cannot get LESS than 100 grains FG, filled to the bottom of the neck. .....
Dave:
That, unfortunately, is one of the most significant problems with these admittedly economical CBC-based cases .... the other difficulty, in my view, is their potential weakness and/or likely short lifespan ..... being both thin-walled and semi-balloonhead as they are.
For those reasons, coupled with the wish to avoid the considerable extra hassle of forming the necessary bottleneck, I have no .577-450 cases formed from CBC brass. CBC brass is much better suited for forming straight-walled .577 Snider-Enfield cases, and I have a fair number of such cases made from CBC brass.
Frankly, I think you'd be better off getting a sufficient number of Jamieson case for the Martini, and hold the CBC cases in reserve .... or even put a Snider on your "wish list" and blow the CBC cases back out for use in that!
However, this wonderfully sage advice is of absolutely no help in resolving your immediate problem, is it?
To be honest, I often use a filler extending below the shoulder in various bottleneck cases .... including the Martin-Henry .... but for several years now have used only "50/50 active filler" on top of a reduced black powder charge, as discussed on the British Militaria Forums. There has been some suggestion of late that this mix can separate into its constituent parts, and "DoubleD" recently indicated there that some loads made with 50/50 filler which were pulled apart showed such separation. However, I am of the view that this problem will not occur provided the filler is mixed using a
suitable inactive component and that the powder charge and filler are compressed by the bullet during loading (just as a straight black powder charge should be compressed) .... and that is what I always do. I too have pulled apart some of my cartridges loaded in this fashion, following a considerable passage of time, and have noted no such separation.
Rather than cream of wheat, I have almost always used a brand of quite coarse-ground corn meal readily available here .... the particles of which are angular, and similar in size to 2F powder. I avoid the use of finer-ground stuff which is also available (closer to flour in appearance) and I believe that makes the difference .... especially since, in his recent post, "DD" specifically commented on the differential in grain size making it understandable that the two components would separate. He, or someone else, also mentioned how difficult it was to keep their loose 50/50 filler evenly mixed in its container before putting it into the cartridge cases .... but that has never been a problem with my mixture, either.)
The theory behind the use of a 50/50 mix as a filler (without empirical proof, so far as I know) is that the black powder mixed throughout the filler burns when the main powder charge goes off and thus prevents the danger posed by the filler possibly "slugging up" in the neck of the case.
On the issue of calculating the powder charge and quantity of 50/50 mix to use, the "conventional wisdom" on British Militaria Forums has been that the full quantity of black powder contained in the 50/50 mix should be factored into the "total powder charge". However, I personally don't think that it should count on a straight 1:1 basis, since its "contribution" to the effect of the powder charge must be compromised somewhat by being mixed with an equal volume of inert material.
Assuming for illustrative purposes, that the volumetric equivalent of 90 grains black powder constitutes a "full charge" which will be suitably compressed by whatever bullet/wad/grease cookie combination is being used, one wants to build a column of straight black powder, topped by the 50/50 mix, which will fill exactly the same volume in the case. (For practical purposes, of course, one measures both the straight black powder and the 50/50 mix using the same "grains by volume" type of measure.)
Although I honestly can't say where it comes from, another "gut feeling" I have is that it seems advisable to
avoid using a volume of the 50/50 filler equal to (or greater than) the volume of the powder charge. Intuition somehow tells me that the volume of the black powder charge should always be somewhat greater than the volume of even an "active" filler going on top of it.
Working with my suggested example based on "90 grains by volume", the conventional "simple" view would hold that a charge of 50 grains (volume measure) of black powder, topped by 40 grains (volume measure) of 50/50 mix should be viewed as the equivalent of a 70 grain charge of straight black powder - i.e. adding the 50 grain powder charge and the 20 grains of powder in that volume of 50/50 filler. In my view, however, the 'effective contribution' of the powder in the filler mix will undoubtedly be somewhat reduced.
In
practical terms, however, a bit of experimentation (such as always seems necessary in developing optimal black powder loads anyway) should soon give you a combination that works for you. (For what it is worth, my reduced Martini-Henry load for GAF shooting, using soft-cast 405 grain unsized .457"/.458'" bullets intended for the .45-70, has been precisely what I mention above - i.e. 50 grains (volume) powder topped by 40 grains (volume) of 50/50 mix. With the more voluminous CBC cases, in order to get the top of the filler right up into the neck, you might have to go with something like 55 grains powder topped by 45 grains 50/50 mix .... or perhaps even a bit more of each.
Now that I have you
totally confused ..... what the heck are you doing trespassing on my "British Empire" turf, anyway?