Thread: 45/70 questions
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Old January 26, 2019, 12:22 AM   #5
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Join Date: March 11, 2006
Location: Upper US
Posts: 22,773
So my question is how will this change in seating depth affect the pressures.
The answer is that deeper seating the bullet reduces the space available for powder and that increases pressures.

But, there is a second part of the answer, which begins with the question "Does that matter?" and the answer to that is, in some cartridges yes, and in others, "not so much".

The .45-70 is a large capacity case, and its a low pressure round a small change in the bullet seating depth doesn't have nearly the effect it has in a small capacity high pressure case like 9mm Luger or .40 S&W.

Plus, you are shooting from single shot rifles, strong ones, at that, so crimp is irrelevant. you don't need to crimp for single shots. If you were running a lever gun, or a bolt, crimp would matter. In a single shot, it doesn't.

What matters is the length of the throat of the barrel. And the profile of the bullet being used. Max overall loaded length (with bullet) for the .45-70 is 2.550". With the usual profile bullet, rounds loaded to that length ought to chamber in all firearms in that caliber. Ought to, isn't "will" some guns aren't able to take all bullets at that length, and need something a little shorter. SHORTER hurts NOTHING, in this case.

The max loaded length number is a number you should not try to exceed, not a number you need to meet. Load your bullets to a length that chambers in your gun(s) and don't worry if its a little bit below listed max length. If you are loading to levels at the max working pressure of the gun, you do need to be carefully working up your loads and a tiny change might move you from Very hot, to "too hot" but if you aren't, maxing things out, small differences in seating depth (resulting in shorter overall length) aren't going change the pressure enough to matter.

I've been loading .45-70 since '83 and my guns have been Ruger No.3, Marlin 1895, Siamese Mauser (rebarreled) and a T/C Contender. Its a great round, amazingly accurate in many guns but does drop fast, so long range shooting is an art not easily mastered, but it can be done, with enough "work".

the straight case, and (relatively) low pressure of the .45-70 means slow "magnum" powders simply don't work worth a damn. "Medium" burn rate rifle powders are the slowest burning thing you should use in the .45-70. "Fast" rifle powders like IMR 4198, 4227, and 3031 work pretty well.
Faster powders (considered slow pistol powders) can also be used, but take great care to ensure you get one, and only one charge of powder in the case. A double charge of something like Unique or 2400 WILL blow up your gun (even a Ruger).
All else being equal (and it almost never is) bigger bullets tend to work better.
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