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There’s nothing glaringly wrong with your bullet design. However, the 45-70 is not inherently precise cartridge no matter how precisely you load it. I was quite sure that I could make it shoot straighter than ever if I cracked my knuckles and gave it some MBT love (which I did for 3+ years and over 6000 rounds).
I made two custom 45-70 bullets myself with all care given to the lessons learned in the XCB project. I also fired over 6000 shots, testing ten shot groups side by side. 12 different powders, 12 different bullets, 7 different rifles, cast bullets only and smokeless powder only.
I found out what many already knew: The 45-70 and it’s brothers are straight walled cartridges that have no ability to create back pressure to make the powder burn to it’s most consistent potential. It’s a big pistol cartridge. Every single shot lets the pressure off too quickly and consistent burn is impossible. Couple that with the fact that cast bullets of this size are too weak to support their own bulk at these acceleration rates and you have some basic pillars of precision that are compromised by the nature of the design itself.
Now before you go screaming at me that this is all total bunk, let me say a few things right up front:
First, my testing was limited to cast bullets with smokeless powder exclusively. No jacketed bullets and no black powder (both of which can have a very positive effect on this more or less).
Second, I expected three ten shot groups in a row to deliver similar results before I would accept it as valid. The 45-70 in the scenario I mentioned has shown a remarkable propensity for giving almost satisfactory results with one group, and total disappointment with the next. Since I actually intend to hit what I’m shooting at every time rather than posting an impressive one time group, I set up my requirements accordingly.
Thirdly, I don’t call fliers unless I knew they were going wild when the trigger broke (which is a rare occurrence). There is a lot of excellent data that was thrown out by the shooter because it came in the form of a group the shooter was not happy with, which is a crying shame in my opinion and explains why there is so much misinformation floating out there.
My testing of this cartridge indicated that group sizes of less than 2MOA are rare and unrepeatable, however, I also discovered that nearly ANY 45-70 caliber rifle will shoot very similar groups to any other rifle of the same caliber regardless of barrel length or quality. (the Marlin 1895’s shot the exact same average group as the 1885 Highwall, and the 1886 Browning, but at no time was a rifle tested that could deliver less than 2MOA in three successive 10 shot groups.
Now, many people will site the Sandy Hook trials of the Springfield 1873 trapdoor rifle in 1879 in which Mr. R.T. Hare used three different rifles with milspec ammunition to hit a bullseye at 2500 yards (impressive to say the very least). However, few will quantify those results. If you consider the bullseye was 72″ across, and do the math, that’s well within the precision of black powder cartridges (6′ at 2500 yards is equal to 2.75MOA). Of course, considering wind and the extreme distance itself, these shots are no less impressive but that’s totally on the SHOOTER and not the equipment. The equipment was functioning well within it’s parameters.
There are few surprises with this cartridge. It’s greatest detriment is also it’s greatest strength, and after working with it for several years, I’ve concluded that its an interesting cartridge to play with from a historical standpoint, but also that it has been marginalized in almost every way by wave after wave of advancements in cartridge and propellant design.
The biggest tragedy of the cartridge is that I do believe that a propellant could be made that would sustain it’s own burn consistency without help from a shoulder (this has been done very effectively on 22LR propellants) but since it’s a cartridge that time forgot, and no one really expects it to perform sub MOA precision in the first place (especially not with cast bullets), no such propellant is likely to be forthcoming. Most people figure it’s a short range brush gun, which is sad, but that sentiment is what keeps it on the bottom of the barrel picking up the crumbs dropped from modern high performance propellants.
Regardless, it’s a fascinating pursuit that will teach you a lot if you keep your eyes open and let the data be what it will be.