Let me cut to the chase and explain in more graphic detail this scenario for others edifications.
I have trained people in defensive knife for over a dozen years professionally. That includes three years at S+W's training academy as their defensive edged weapons instructor to LE [ 3 day courses ] and private citizens [ one day course ].
At 8 feet, you can be cut and stabbed a dozen times by one who doesn't know what he is doing with a knife before you can get to the gun, if you ever do.
In that time, you will have suffered so much damage that either:
1. You fall down and shortly die from the injuries sustained before you ever get to the gun from the slashes and stabs all over your body
2. You take so many hits trying to get to the gun that you stop trying and you are only trying to stay alive with both arms and hands, and forget you have a gun.
3. You have good H2H defensive edged weapons training [ not the mcdojo type you get with belts over the years ], and can stop his attack, taking a few peripheral cuts by knowing how to defend initially and stay up and running long enough to take control of him through some very effective and immediately damaging techniques that take him out.
In all the years training the LEO's and citizens, not one would ever get to the gun and would have suffered fatal injuries in trying to do just that with the trainers.
Now, take a jail bird who has experience training with a blade behind the walls who knows where to take you out properly and with no mercy. You'll go down faster than you can blink an eye. He'll be targeting the neck, stomach, armpits with stabbing blows, and he'll be on you like a gorilla, keeping you pinned to yourself so any effort to get to a weapon yourself is only going to give him more time to keep stabbing you.
It's ugly, it's nasty business, and to think someone who is presenting a knife at 8 feet and moving on you can be taken out in time to survive by your just defending with one hand/arm while going for the gun is going to get you killed.
You MAY end up pumping a few into him, but he has killed you nonetheless. You'd better have knowledge and training to protect the bodies core, get out of his kill zone [ and I don't mean run off or move laterally as he'll just stay with you and keep giving killing blows while being all over you ] but actually understanding how to use a startle response to protect and get those blows to stop.
You'll have to know how to tie him up with the blade arm quickly, make it a struggle and no longer a fight immediately or it's going to end in the above happening.
Thats the way it is, realisitic training has proven there is no other way to think you'll survive that encounter from that distance any other way.
Here's a link to think about this very subject:
http://www.combatcarry.com/vbulletin/showthread.php?t=9915
Brownie