.223 77 TMK deforming during bullet seating

[menhaden_man]

 

[menhaden_man]

Chamfer/debur your necks, wager that fixes it.

 

[Lawnboi]

You have one of three problems.

Too much neck tension, the interference fit
Too much friction from clean brass
Too much compression on the powder

The first two can easily be fixed. The third I would just avoid, when you get that compressed you start getting wild runout pretty quick.

A perfect fit stem might help but probably isn’t the answer.