You broke down the unfired ones, had 8 squibs and all the others fired OK?? There is of course no way to tell on what was in those 8.

The ones you broke down, did they all have exactly 3.5 grains??

You are using the LEE disc system? Yes? I do not know what "cartridge" type is?

3.5 gr is the Alliant load so I would guesstimate that your answer lies in the powder measure, it either needs to be cleaned, the charge hole did not fully come forward over the drop hole, short stroke or something. I use the LEE disc measure.

I doubt it is brass related.

If you size after cleaning then the sizing pin will clear the flash hole of any media.

The testing of other powder (BE) and 357 Brass just complicates the original issue.

__________________
Still Running Against the Wind