[Riktoven]

Is there any reason to avoid using +P ammo during a P99's break-in period? Namely after only 100 or so WWB have been shot?

 

[9 and 10 mm Fan]

I wouldn't worry about it. Normal European 9 mm is around +P pressures anyway. FWIW I hand load and for first 500 rounds use factory ammunition. The first 500 rounds through my P-99 were Speer 124 +P Gold Dots that I bought in bulk at a good price. After 4000+ rounds at max SAAMI or +P pressure loads the gun is not showing any wear.

Just be safe and have fun.

 

[kbc]

Yes, should be fine. I've fired many boxes of +P stuff.