The site Firebug linked to does a comparison of a 1 pound rock thrown at point blank range to a pistol. Now details are emerging that an AR15 rifle was used, and we know the location of the incident and can estimate distances based on where the legal midpoint of the river was and where the victim was when he was shot. So, here are my numbers (with open invitation to better them by my superiors in terms of physics)
With an AR15:
velocity =
3200 feet per second if they used standard M193 ball military grade ammo or 3000 feet per second if using SS109 (M855 ball) combat grade
the ss109 will penetrate 10 gauge steel plate up to 623 yards distance, the 193 ball will do the same up to 400 yards.
Typical accuracy of AR15 rifles with either types of ammo is about 1MOA, which means a competent shooter can put bullets within a 1-inch circle of aimpoint at 100 yards. at 100 yards, the kinetic energy of the ss109 is 1430 foot pounds (enough energy to move a 1430 pound object one foot)
If the CBP were on the US side of the river, as legally required, they are probably at least 105 feet from the spot where Guillermo Arévalo Pedroza was shot, at the water's edge on the mexican side. At that distance, we can safely rule out a 1 pound rock that Firebug's linked site used. We're talking at most a 5 oz baseball weight (sized smaller than a baseball) rock.
Let's suppose Guillermo Arévalo Pedroza were as accurate and strong in his arm as Roger Clemons at his steroid-pumped best. He's throwing 142 feet per second at 65 feet, which carries a kinetic energy of 0.35 foot pounds (if I'm doing this correctly), and both speed and kinetic energy will be dropping rapidly, probably by 30%-40% at 105 feet. Estimating MOA is 3 inches at 65 feet, 12 inches at 105 feet.
So, if the rock thrower were lucky enough to even hit the CBP at this distance, the KE is probably 0.20 foot pounds.
Baseball hitters are routinely hit by pitches at 97 mph and survive with permanent injuries being vary rare. The CBP agents were being pelted by rocks at 62 mph at most, at these distances, IF Roger Clemons were throwing. Most likely, the speed developed by these rummies on the mexican side was half that, say 31 mph.
Still say lethal force was justified, and rocks were comparable to what the agents used?