How does the 125gr hp 30-30 compare to the 357 mag in leverguns?
This is a hard question to answer. It would help if you gave a clue. What is it you want to compare? Range? Accuracy? Killing Power? Cost of ammo? Ease of reloading?
I will start but may be off the mark.
The 125 Federal 30-30 round (or handloads using the Sierra 125 HP) are very good hunting rounds. The HP is not a frangible bullet. It is pretty tough and opens nicely. However, the ballistic coefficient of the Sierra 125 HP is not good. So the bullet looses velocity quickly. Also, at only 125 grains, it is not a great penetrator. But for deer size (or smaller) animals at range out to 150 yards, it is a very good round.
The 30-30 is easy to load and there is very good data available for the Sierra 125.
You don't specify a load for the 357 mag, so its hard to compare, but I would not recommend 125 gr bullets in a 357 levergun. Most 357 mag rifles shoot better with heavier/longer bullets. Hunting range with a 357 levergun is comparable to the 30-30 with the 125 Sierra. 30-30 range will be much better with other loads but loaded with the 125HP, it is very similar to the 357 loaded with 158 gr bullets.
Recoil is similar but the 357 is likely a bit lighter.
The 357 can get good performance with shorter barrels, giving it a potential advantage if you like the "handiness" of a short carbine.
The 357 is also easy to reload.
So, thats a start. Maybe others will fill in the gaps
Bob A