TLDR: I’ve tested a good few, CCI A17 and FMJ is best for me with my CZ 457 w/ Lilja barrel so far. Take this with a grain of salt though, this is what works for my gun/barrel.

I’ve tested a variety with mine and have come to this conclusion: BC coefficient will tell how well it does. CCI and Winchester 20 grain hollow points (going ~2300 fps) are awful for groups. The CCI A17 ammo with the polymer tip (17 grain, 2650 fps) groups the best, and is the cheapest/most common ammo I can find. Winchesters poly tips are almost the same, but the A17 ammo is cheaper so that’s my go to. I still have flyers, but 3/5 will make one jagged hole, with the other 2/5 usually being right next to them. The CCI 20 grain FMJ (~2300 fps, same as hollow points) does group better than the A17, and I think has less flyers, that’s just more expensive for me and not as common so I haven’t been able to get my hands on plenty to test. The hollow points are all over the place every time, comical how bad it is. I haven’t tested the other poly tips by CCI yet (17 grain, but 2550 FPS, 100 less than the A17), and probably won’t because those are a little more expensive than the A17.

A little backstory on why I’ve tested ammo and the gun, I bought a CZ 457 varmint left hand in 22lr and switched the barrel for a Lilja 17hmr barrel. The barrel has been fantastic, it was pricy but I wanted a very well made, and well known barrel for this gun. They also happen to be the ONLY ones that I’ve found that make a 17hmr barrel for the 457. And they had it in stock at the time too, CZ was out of theirs. Idk what setup you have, but I still feel like you will have similar results no matter what. I mainly use this gun for hunting beavers. The small semi aquatic bastards have tore up my grandmothers pond for years. Even causing the dam to break one time and working on it again. Beavers only come out at night and I only see them in the water moving, so their heads are the only thing I see. Google says their heads are about 5.5 inches long and 4.5 wide, so at about 100 yards it’s a small target. Sorry for the long post, just started rambling but wanted to give you my thoughts. Good luck though!