Hey Canoeguy;
I have a trapdoor that has same problem with keyholing.
I used 36.0g. 3031 behing Magnus 405g. bullet.
at 25 and 50 and 75 yds. everything OK. At 100 yds, keyholing
I raised powder charge to 38.0g 3031 and no keyholing.
I used mag primers for all loads.
This keyholing probably due to lower powerder charge and longer twist. A more modern rifle, like a Marlin 95 or Ruger
has a faster twist.
Remember the trapdoors , at end of their career used 500 grain bullets, especially for long range shooting.
Hope this helps...
Smikem