I checked in with the forum and noticed that there was some confusion about who offers small rifle primers in their 45-70 ammo. A while back we did, for a short time, offer all of our 45-70 loads with small primers. This was in response to concerns that were generated by three magazine ignitions that occurred with a competitors ammo. However, we had never experience such an event, and the guys at Marlin (Harold Waterman) told me that they had only seen three such occurrences in Marlin 45-70s, and they all occurred with the same competitor's ammo. So I found myself a bit "on the fence" regarding this issue, especially since I have chosen to offer extra wide meplats on my Hammerheads (which helps protect against the dreaded event). Subsequent to that, I did discover one cold morning (not all that cold, just 32 degrees) that when my Hammerheads were ignited by small primers that I would experience very well defined vertical stringing. I had never seen such a perfect example of it either, as my groups were one inch from left to right, but were six inches from top to bottom! This is the classic symptom of underignition. I then tried the same load, except for the use of a large primer, and the group size immediately shrunk to its normal 1-1.5" group. I then repeated this exercise several times, and each time I observed the same difference between the small and large primer performance.
After that I was convinced that small rifle primers have no place in the 45-70, since the conditions were not all that cold, certainly not as cold as one would expect in the far north. Given that, I had to conclude that if a customer were to fire on game in really cold weather, that the underignition of the small primer could cause real trouble, perhaps even a misfire. At that point I switched to the CCI34 Military Spec large rifle primer, which is a magnum primer. With it, I have not been able to produce anything like the vertical stringing I observed with the small rifle primer (CCI43).