doketx-
Jamison brass does have a thick rim. IFn you use a 45-60 shell holder the Jamison brass will fit into it as you have found out the Jamison brass won't fit into a 45-70 shell holder.
I have not had the problem you state about Jamison brass not working in the Uberti '76. Jamison works well in both of my '76's, I have a Uberti and a Chaparral.
Some folks like the Jamison Brass w/the headstamp... my opinion- go to Starline and buy 45-70 brass and then size it for the 45-60. Then you won't have that problem with the "thick" rim. Jamison is too expensive for the product and I do not find that it is better than the 45-70 trimmed down, again my opinion. The headstamp does not make it shoot any better... IF it did, more folks would buy it, pricey or not. For me, I bought 200 cartridges of Jamison 3 years ago before Jamison sold out... I shot it and was not impressed, but to each his own... I prefer the 45-70 cutdown plus the Starline 45-70 I do believe has thicker walls which will give you more shots before the brass breaks down. But I may be off on that so do not quote me on the wall thickness.