I've read as many of the relevant threads that I can find. The rifle is a model 12 with a PTG replacement bolt head, 223. Barrel was installed using this bolt head, go and no go gauges. Chamber is regularly cleaned. Larger detent ball is in place. New extractor (from Savage) is in place. Bolt face area in the area of the detent ball, spring slot, and extractor have been cleaned. About half of the fired cases drop off and are left on the magazine lips. These same cases are briskly extracted and ejected from another Mod 12. It does appear that specific empty cases are not extracted and ejected, while other cases will consistently extract and eject. What am I missing? Any thoughts as to what I should try next?