357 mag very inconsistent velocity

[243winxb]

I can easily see the bulge where the base of the bullet is.*

*I am using mixed brass,

Check to see if the bullet is being sized by the brass. Buy new brass of the same lot/ brand. Primer- mag or standard? Try the other.

 

[gspn]

I don't have a chronograph, so I'm left wondering...how does that 100 or 200 fps variation affect accuracy?

Is there a noticeable difference in plinking ammo?

 

[joneb]

how does that 100 or 200 fps variation affect accuracy?
Is there a noticeable difference in plinking ammo.

I depends on Your expectations and how far you want to plink and hit what you are shooting at.