I bought some  missouri coated 125 gr bullets to load in my s&w 66 revolver.I was loading these in 38 special cases, and wanted a light target plinking load
I planned on using  red  dot powder, since i have a bunch.I loaded a few, and went to test, for groups.The groups were horrible, i mean 4-6 " at 15 yards.I tried varrying the charge, up and down, no luck
Next, i tried u clays, shit, then unique, crap,  then hp38, poopy.This is getting out of hand.I tried a few groups with 357 mags, and 38+p' i had loaded before, groups were fine.
I was beginning to think the bullets were just crap.I had one last idea, and wondered what would happen if i found the hottest 38+p data i could find, and load a few.I ended up with  a max load of power pistol.powder.The results just blew me away.The 3 shot group was litterally  touching.
Just thought  you fellow reloaders may like this information.
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Do you have a micrometer to measure the diameter of the bullets?  ( to .001 )  It may help explain what has happened.

Cool story.
Never give up.

I agree. Plated bullets are soft. Your high end load is very likely causing the bullet to obsturate and fill the bore where the lighter loads were not.

You either have smalish bullets or a large bore. Probably smalish bullets.

Is it possible that you are using bullets that were intended for 9mm?

Motor1

I never mic-ed the bullets.I could check them with my caliper, but the riddle has been solved, they need velocity for accuracy.
I wont be  able to use them for a fun casual load, but at least i can use them.
The interesting thing, is when i orderd these, i also got some 300, and 405 gr bullets for my 45/70, and  250 gr for my 45 long colt.The first load tried with all 3 , were awesome.

I came here to post this.

The bullets don't "need velocity to be accurate" they need enough of a boot in the ass to make them expand enough to seal the bore, and take the rifling full depth.

Lead bullets for .38/.357 should mic at .357 minimum to reasonably assure the bullet will obturate in the bore. I'll bet those 125's you have will mic at .356 or a tad less, which means they're meant to use in 9mm.

Boosting the powder charge and velocity causes the ass-end of the bullet to soften from the increased chamber pressure and heat and allows it to fill the bore.

This might be an excuse to buy a 9mm if you don't own one already.

The base of a lead bullet is not affected by the heat, unless there is a poor seal to the bore and the bullet suffers gas cutting (like having an oxyacetylene torch blowing past the bullet.

I use .358" lead bullets in .38 Special/.357 Magnum.

I would suspect yu had some .355" dis bullets.  Loaded at normal velocities, they were undersized and accuracy sucked.  At full throttle loads, the bullets obturated, obtained a better seal, and shot better.

As was already stated, you had undersized bullets. With lead and plated bullets, it is important that you have bullets that fit your throat.

I have discovered that some plated or coated bullets I have tried ( that were sized properly for caliber ) were not going nearly the velocity I expected. I suspect they are so slick they start moving quickly and the pressure doesn't build as much and the powder doesn't burn as well.

If your pressure/velocity is lower than expected accuracy and uniformity can go south .

Got a chronograph you can use ?