If you have a caliper just lay it across the face and use the tip going into the breach face to measure the depth. The bolt will either measure .136 +or- .001 or .125 +or- .001. The .136 is the standard 6.5 Grendel bolt face depth or commonly referred to as Type II. If the measurement is close to .125 then the bolt is commonly used in 7.62x39 and is Type I. On most 6.5 Grendel bolts the length from the tail to the face of the lugs is 2.810 inches plus or minus around .002.
The AR-15 5.56/223 bolts measure 2.800 inches but since AA added .01 to the depth of the bolt face and AA added .01 to the length of the bolt to use standard mil spec firing pins. A couple manufacturers have used 2.800 length bolts and use shorter firing pins. Odin Works and Brownells are reportedly using 2.800 length bolts.
The critical dimension is the firing pin protrusion. The firing pin protrusion standard is .028-.036 inches or a tolerance of .008 inches. If the firing pin is out of spec you can get pierced primers or slam fires. If the firing pin protrusion is to short you might experience failure to fire.
If you have several AR-15s I would be very careful to keep the same firing pin with the same bolt.
The AR-15 5.56/223 bolts measure 2.800 inches but since AA added .01 to the depth of the bolt face and AA added .01 to the length of the bolt to use standard mil spec firing pins. A couple manufacturers have used 2.800 length bolts and use shorter firing pins. Odin Works and Brownells are reportedly using 2.800 length bolts.
The critical dimension is the firing pin protrusion. The firing pin protrusion standard is .028-.036 inches or a tolerance of .008 inches. If the firing pin is out of spec you can get pierced primers or slam fires. If the firing pin protrusion is to short you might experience failure to fire.
If you have several AR-15s I would be very careful to keep the same firing pin with the same bolt.