One of the things that make the 45-70 somewhat touchier to chamber is that there is some variation in rim thickness between various maker of brass and even in some of the same lot at times. I just took a randum sampling from my range box and came up with an over all difference in rim thinkness of .005 between several rounds. For final sight ins and for hunting, I use nothing but newer Remington or Federal cases and they both have the thicker rims. When you do the headspacing, you have to take into account the thickest rims you will encounter. I don't know how closely your brass is manufactured, but it might pay you to measure a few rims. Have you tried trimming them back to original length and then re-firing and re-measuring a few of them to see if the streching continues? Brass work hardens as it is fired and resized, (that may help some, I'm not sure of this though),if it doesn't, you may have to set the barrels back .006 or so. Bob H.