Daryl-

Your 58 is barreled somewhat heavy, as are most of my double rifles.
I've seen similar stability with mine, with the exception of my very light and long-barreled 8x60 BLE which was a bit finicky in many respects.
The stout barrels of the BPE rifles (similar to your 58) in my experience seem relatively stable.

I should get my WR 16bore out and check the 200yd leaf some time soon...

Cheers
Tinker