I find the Berry 45-70 bullets troublesome to chamber as well.
From what I'm seeing, I think the reason is the wider meplat on the Berry. It's quite a bit bigger when compared to any of my lead cast bullets. All my lead cast bullets work just fine when at the same COAL as a Berry, which is usually between 2.26 and 2.30, depending on where I'm crimping - or not. It appears the meplat is catching at the lip of the chamber - there isn't any feed ramp on my Uberti 45-60. Not sure about any other 76'.
I found if I tip my rifle to the left when cocking, the Berry will roll in just fine. However, if I keep it vertical, they fail every time.
Bench shooting a Berry isn't much of a problem as I find it easy to tip the rifle to the side when dodging that big empty case anyway. I prefer not to use them though. Have gotten very happy with the the powder coated cast bullets I'm making.
See if that wider meplat makes some sense in your case too.