Its .223/5.56, and the ammo is .223. One of the magazines is a new 10 rounder which does have a very stiff spring. The other magazine is a used/pre-ban 30 round magazine that has a fairly loose spring.
I've come to the realization that in addition to downloading the magazines, I can circumvent the issue by loading the magazine into the rifle while the bolt is locked back, then pushing the charging handle forward (no backward motion), locking the charging handle into place. This way I can just hit the bolt release (terminology correct?) to chamber a round.
In extra magazines, I guess I shouldn't really have this problem because the bolt should be locked back before I care about loading it anyways.