I would doubt that you could find a better powder, then RL-7, for the 45-70.
I've been shooting the 45-70 for 20 some odd years, when I first started, I used jacket bullets and as I remember I crimped into the groove closest to the lead up front. You know I've got several boxes of Remington and Speer 405s out in the loading room. Once I started casting there was no reason to use them. My 1886 has not had a jacketed bullet shot thru it.
I am new to the 45/70 as far as loading goes. I have only used RL#7 and have been so pleased at first time loading attempts that i doubt i'll try other powders. Like arkypete mentioned, the powder seems to be a "natural" for the 45/70 case. As far as the seating depth goes, check the manuals for o.a.l.(I think you will find the cannelures on the 400 gr. Speer S.P. match Remingtons)
I certainly can not disagree with your decision on RL-7 as your primary powder. It shot extremely well in my 1886 EL, with good velocities.
The Remington 405 grain bullet is a double diameter bullet, which will allow it to be seated out in the rifling, but the feed mechanism is the limiting factor in that rifle. I loaded a dummy round as long as I felt would feed. It failed to feed, so I shortened the OAL until it would feed easily. (The feed I am talking about is through the gate into the magazine, incidently. The far wall of the receiver must allow the round to slide into the magazine, and I have gone so far as to polish that wall in a .450 Alaskan on a Browning 71 so the rounds would feed with additional OAL.) On the .45-70 I have finally settled on 2.650 as my OAL, which crimps right at the front edge of the forward cannalure and shoot fairly well in my rifle. YMMV.
Tom: My 86 has a very short throat and it was necessary to use the forward crimp groove for Rem 405's or they would not chamber due to rifling contact. I've pretty much quit using them although they shot very well, and primarily use cast RCBS 300FNGC which cast at about 325 and RCBS400FNGC which cast at 415. Either bullet is very accurate. I use the lighter bullet a lot for practice as it uses less metal and like the heavier for hunting loads although I also seem to send a lot of them down the range too. About a 6" elevation adjustment needed between the two weights.
I have a Marlin 1895SS which probably requires a shorter LOA than the Winchester. But I love the Remington 405gr SP and Rel 7. If the bullet shoots well crimped to the first crimping groove, what is wrong with that.
My Marlin requires me to crimp ahead of the first crimping groove. The trick is to set the LOA uncrimped (for my Marlin 2.55 in), then crimp with the Lee Factory Crimp die for the 45-70. My marlin loves this load at a chronographed 1790 fps. You will be surprised how well the Lee die works, even though you are not in one of the two crimp grooves. It seems to make its own grove on the bullet. Seems too good to be true but it works, in my marlin. The die is about $9.00 from midway.
dodgestdshift: There was nothing wrong with it at all: as stated, very accurate . That's just how they had to be used. I understand that Ruger 1's can use the rear crimp groove by virtue of a longer throat. Suspect some .458's can too. I quit using them only because I wanted to use cast bullets. I get a kick out of making and using them. (No double entendre intended). cheers.
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