Yeah, here's an interesting article on modern day knife-wielders vs. armed police officers.
"The 21-Foot Rule was formulated by timing subjects beginning their headlong run from a dead stop on a flat surface offering good traction and officers standing stationary on the same plane, sidearm holstered and snapped in. The FSRC has extensively measured action and reaction times under these same conditions. Among other things, the Center has documented the time it takes officers to make 20 different actions that are common in deadly force encounters. Here are some of the relevant findings that the FSRC applied in reevaluating the 21-Foot Rule:
Once he perceives a signal to do so, the AVERAGE officer requires 1.5 seconds to draw from a snapped Level II holster and fire one unsighted round at center mass. Add 1/4 of a second for firing a second round, and another 1/10 of a second for obtaining a flash sight picture for the average officer.
The fastest officer tested required 1.31 seconds to draw from a Level II holster and get off his first unsighted round.The slowest officer tested required 2.25 seconds.
For the average officer to draw and fire an unsighted round from a snapped Level III holster, which is becoming increasingly popular in LE because of its extra security features, takes 1.7 seconds.
Meanwhile, the AVERAGE suspect with an edged weapon raised in the traditional "ice-pick" position can go from a dead stop to level, unobstructed surface offering good traction in 1.5-1.7 seconds.
The "fastest, most skillful, most powerful" subject FSRC tested "easily" covered that distance in 1.27 seconds. Intense rage, high agitation and/or the influence of stimulants may even shorten that time, Lewinski observes.
Even the slowest subject "lumbered" through this distance in just 2.5 seconds.
Bottom line: Within a 21-foot perimeter, most officers dealing with most edged-weapon suspects are at a decided - perhaps fatal - disadvantage if the suspect launches a sudden charge intent on harming them. "Certainly it is not safe to have your gun in your holster at this distance," Lewinski says, and firing in hopes of stopping an activated attack within this range may well be justified."
got it from: http://www.policeone.com/writers/columnists/ForceScience/articles/102828/
if you want to read the rest of the article. I think it's interesting, and I'm sure it takes longer for even a well-trained crossbow user to reload than it does for a well-trainer office to unsnap his holster, take out his gun and fire two shots.