Here's a thought if your going to use "brass bullets" you might try a coating.. kinda like the old 1970's KTW rounds years ago. They used Teflon coated bullets which if memory serves me were brass; this being a pistol round, however'.., similar things have been done on rifles, one that comes to mind is Barnes "VLC" bullets.
There are coatings that can holdup to near copper performance while sealing gas pressure.
Just my thoughts...
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