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IMHO you don't have over pressure loads. My 30-30 brass all have a little bit of a ring around them similar to what you are experiencing. From the pictures I don't see signs of pressure on your primer. They appear very rounded. If the pressure was high the primer would be flat and you would have cratering around the firing pin strike. I think you have a low pressure load that did not seal the primer. It looks like gas blow by around the primer. That would indicate the primer didn't seal and may have backed out a smidgen allowing gas to get past causing that black mark.

According to my Hodgdon manual 20grs of either H4198 or IMR4198 is 6grs below the 'starting load'. Two things you can do....first you can load more rounds adding 1gr at a time until you reach 26grs. You won't need more than two of each just so you can do a primer check. Shoot those and watch the primers as you go. Option number two is to switch to a faster burning powder if you want to stay with the low velocity loads. Unique and 2400 will reach higher pressures faster in reduced loads. This will help you get good expansion on your brass and primers while still being able to shoot 1400-1600fps loads.

I use 16grs of 2400 with a 150gr GC cast bullet. This data is available in the old 44th Edition Lyman manual. Best of luck.

 

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I can't be 100% sure on this, but I will hazard a guess. The bottom 3/16" on your case is the thickest and strongest part. It does not expand much if any compared to the upper 90%. When you fire the rifle the upper portion expands and touches the chamber walls...the very bottom portion does not. At this junction you get a ring. The ring becomes even more pronounced when you run the case through a sizing die during reloading.

I have a similar ring on my 30-30 brass and I don't load any of them above SAAMI pressure. As a matter of fact, most of my 30-30 ammo is reduced loads using cast bullets. Gas checked cast bullets at 1600fps are fun and inexpensive to shoot.

I would like to add it was a good job on your part to inspect the brass. As a reloader it is always smart to watch your brass for signs of pressure. A fired case can tell you a lot.