1- As IG said, DPMS chambers are closer to SAAMI .223 chambers than 5.56 NATO chambers. Functionally this meant that there is less leeway in the throat. With that in mind...
2- Tula/Wolf/etc steel cased ammo tends to leave more carbon/crud in the chamber/throat.
3- High pressure 5.56 will cause the case mouth the seal more tighty throat/leade than standard pressure SAAMI rounds.
Combine the three, and hard extraction is a common issue.


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