Owen wrote:Maybe inconsistent bullet weight? Or some other manufacturing sloppiness?
+1
Bullet weight, bullet seating, powder load, etc.
However, my personal experience is that most inexpensive .223/5.56 shows pretty similar performance out of my Stag Model 4 20" rifle at 50 yards. Going to 100 and then 200 yards, the differences between manufacturers and grades are amplified. Of course, those longer distances also require me to be a more consistent shooter to see those differences clearly.
These days, I tend to buy a little ammo here and there when I see it (either at the local gun shop or trying to avoid shipping costs or using promo codes for a discount for an online store). Always better to have it than need it!