Just started hand loading this year for the 7x57 (CZ550 American). Absolutely love the rifle and caliber. Have had good accuracy with everything I've run through it, 139 SST, 150 Partitions and the bullet I'll whitetail hunt with this year the 156 gr Norma Oryx. My PH supplied 180 gr 30-06 Oryx ammo on my first Safari and the results were fantastic. You can hand load these for around a dollar a round as opposed to 50 bucks a box for factory if you can even find them.
MOA with the 139 gr Hornady Superformance factory SST and my whitetail dropped in his tracks last year. My reloads with the 139 gr are grouping tighter than the factory. The factory SST's shot great as well in the wife's 7mm-08 but in my 257 Roberts they pattern like a shotgun. The only factory round worth a flip in the Roberts is 110 gr Nosler Accubond. So I'm loading for it as well.Sand Rat: How did the 139 gr SST bullets do? If factory ammo, I am assuming they would be the Hornady Superformance. I was first introduced to that ammo by a PH in the Eastern Cape and the rental Sako 7mm mag I used was very effective. My daughter and I had 8 PG with one shot kills. I decided to try the Superformance in my 270 and found the 130 gr SST to be quite accurate. I took a Dall sheep in the Yukon with one shot at ~230 yards. The SST has a high BC and I believe the plastic tip helps any rifle with feeding issues. As much as I like it though, I will be using the Swift A-Frame in my 338 wm this fall when I go elking hunting in Montana.