The maximum diameter of a P51 is around 1/2" (12.7mm), possibly as small as 12mm - had one with me yesterday but didn't think to measure it. I know I was surprised when I found the P51 was the same as a P61. The P61's shape makes it feel much more slender.
The J911 is 10.59mm (measured 2 minutes ago).
Regards,
Richard.