I went and measured my other .50 Disc Extreme MHC and it's serial # is only 15 away (earlier) from my other one. These are 16XX range. I will repost all of my measurements, and add the new ones.
My measurements are:
#1 .50 cal Extreme MHC = 4.272"
.45 cal Super DISC = 4.298"
.45 cal Disc Elite = 4.298"
#2 .50 cal Extreme MHC = 4.290" that is .018" longer than the other one.
I remeasured the #1, and came up with the exact same measurements as before, 4.272".
All my bolts came in at 2.560" as supplied, from bolt face to rear of bolt lug. Kinda shocking, but I knew that must be a fluke, and substantiated by jsteurrys measurements.