In post #36 the OP enlightened that the discussion is .357 Mag ammo fired from M686 . Specifically the moderate S&B 158 fmj vs various full power defense/ duty rounds .
I am somewhat outspoken in more accurate better than not accurate , aim small , miss small , etc , but there's also lots of context involved .
If the OP is consistently hitting 3 inch targets @ 30 yds , my hats off to OP for his marksmanship , and that load is certainly accurate in his gun .
Just a wild guess , but is that at a slow to moderate pace ? Are you shooting strictly reactive targets , or 3 inch aiming points on a larger piece of paper ? Trying to get a feel for the accuracy * in your hands * for the other full power loads .
Making hypothetical assumption that the 3 inch targets @ 30yds is at a relaxed cadence .
For more relevance to defensive situiations , also look to control at rapid firing @ ( arbitrary , pick own similar range) of 15-ish yards . If you can consistently hit 8 inch plate , rapid fire @ 15 yds , you can have a high degree of confidence in your ammo and shooting .
All that said , if you get significantly less rapid control with full power .357 , there's no shame in slightly toning down the velocity . Medium velocity
357 is still plenty potent in any absolute sense .
Ammo availability is still spotty , but until the current panic , W-W White Box 110gr JHP .357 was readily available , and inexpensive . Certainly old school , it is representative of that class of ammo , and performed reasonably well back in the day ( notwithstanding the underpenetrion debate ) .
First part of OP's inquiry , out of sequence :
In .357 Mag , any reasonably HP is Significantly more effective than FMJ . I won't get lost in the weeds quantifying in this thread , but by any measure or yardstick , the difference is significant .
* For Myself * , I would use the .38 +P HP of my choosing before .357 FMJ .
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I don't buy into the idea of " eh , overpenetration doesn't matter , because you're going to miss anyway " . Hitting is your goal and intention , and it is important to have at least one safe backstop ( the felon) to open fire at all in crowded environments .