"Why use .357 brass with a way loaded down .38 Special load?"
People sometimes have more brass in one caliber over another.
Or,, to avoid the dreaded carbon ring buildup,, using longer cases helps.
I used the poly fiberfill in my .38 Spl loads to assist in uniformity of my ammo. It must work,, because I'd get single digit Standard Deviations over the chrono.
When I'm loading,, I always try to find the most accurate ammo for my purposes. Sometimes it's light charges,, sometimes it's heavier charges. But most often,, it's a load that's from about mid-way to a bit above mid level loads that are the most accurate.
Just yesterday,, I was working on testing loads in an AR-15. I had (7) different charges to try out with that bullet & primer combo. I fired my fouling & settling rounds,, then after a cool down,, I fired my first group. I'd let it cool down,, & fire the next group. I got 6 of the 7 tested so far before it got too late. But right smack dab in the middle,, one load preformed notably better. That's most likely going to be my starting area to re-test things. Accuracy is king,, as all else is irrelevant if you can't hit the target. No amount of power, or velocity matters if you aren't accurate.
