Thanks Curly. So I made a quick call to Underwood ammo today. Spoke with a very helpful person who told me that they used a p64 in the testing of their loadings and stated the 95gr+p will be just fine out this gun. No issues at all. He said the p64 was a tough gun and was made to handle the hot Russian ammo that was loaded similarly.
He did recommend getting a fresh spring but it didn't have to necessarily be the extra power spring. Their loads could be beat up a gun with a weak, beat up spring.
He also confirmed the 115gr HC is a standard pressure load. But I won't be carrying this in my area. In their testing the HC load was getting 37" of penetration in ballistic gel! Just a bit of penetration there... Guess this would be a good woods load if you carry your 9x18 caliber gun for that sort of thing.
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I like the idea behind the 115gr flat nose for in the field supporting side arm, as they say it hits hard and has good penetration. I also carry Hornady Critical Defense for everyday carry. Would not use the Underwood ammo as a steady diet for my P-64.