I voted for only when testing ammo, but the true answer is more complex. In my M&P in 40 S&W I shoot plated bullets, Berry's or Rainier, however in my 9 mm I routinely shoot 115 gr JHP's as I have been able to find decent prices on them in the past, but may switch to plated bullets in the future due to rising component prices. In my 1911's I shoot cast lead bullets almost exclusively.
I've rarely noticed a difference between FMJ and JHP of the same gr, so I normally don't shoot my JHP at the range what-so-ever. Honestly, I don't think I would be surprised at all if someone gave me a new round to use and I had to blindly fire it. However, I have shot some JHP in bulk before when I forget to check my mags (through my AR15), and didn't even know until I couldn't find the JHPs... I also try to shoot a couple of rounds out the gun just to check for surprises, and then tend to forget about it.
Of course, I only use ammo brands I trust with my life for the job of carry, so I won't just go and buy some random load and carry it w/o knowing the brand. If you noticed in some of my pics there was a box of American Eagle 45 FMJ that I was trying out, b/c I had previously not used to much Federal made ammo.
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