According to wikipedia (you know...the bastion of all things true on the internet) the effective range of a Desert Eagle is 200m. But
Burt didn't shoot
The One With Markings with
Shirley. He had a rifle. At the 9:00 minute mark,
Michael says "One arm or not, I will get that rifle from you..."
The cut was on the bottom of
Pegs' foot. It might have been something
Gatekeeper missed, or it did not look enough like a bite or
scratch to him to warrant barring her entry.
It's also possible its something he noticed, and questioned her about but was left out for the dramatic effect of the cliffhanger of Michael not being allowed to enter. Especially since Pegs mentioned it in front of Gatekeeper. If it wasn't something he hadn't noticed before, there would have to be something that happened when Pegs mentioned her foot in Gatekeeper's presence.
Good observations :)