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Last year I had an AR in that also didn't function properly with Aquila ammo. After inspection of the rounds, the rounds were out of spec in the neck area.

The neck dia of my Aquila is .253. The neck dia of my Federal ammo is .247.

I colored the neck area of both with a sharpie. The Federal round ejected easily with no marks. The Aquila round was stuck, i pulled hard... Faint rub marks on the neck area.

Its the ammo, but im curious now.... What is the nominal neck dia and tolerance of a 556?
 
I found a reference to tolerances on luckygunner....
Neck dia .253" +0/-.008"

Sampled 5rds and all measured .253 (Mitutoyo calipers). There is also concentricity and roundness I cant measure.

Is there any way to measure my chamber? Without taking the gun apart?





 

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