I just bought a super nice Bushmaster [barely] used. It has a Magpul grip combined with a trigger guard. The grip also has an end cap for storing five rounds. Five were in there and it freaked me out to see live ammo., it was not expected.

 My questions are;
It is attached by a shorter than commonly used grip screw with no lock washer. Is this normal?
Is this a desirable modification? I have not fired it yet but would just as soon have the familiar A-2 grip.

When I replaced my A2 grip with a magpul I used the original washer and screw.
As far as it being desirable that is up to you. I prefer the magpul grip, it fits my hand better. If its the full kit (which it sounds like) with the different front and back straps switch them out until its comfortable. Or send it to me and I'll send you an A2 grip with a lockwasher and stock screw. Thats a $50 grip you have there

Magpul includes a shorter grip screw that has thread locker on it. The instructions specfically state to use the Magpul provided screw and not the original A2 screw and washer.

It is a good grip. I have them on all of my ARs

Edit: PICS or it didn't happen

Quoted: I just bought a super nice Bushmaster [barely] used. It has a Magpul grip combined with a trigger guard. The grip also has an end cap for storing five rounds. Five were in there and it freaked me out to see live ammo., it was not expected.  My questions are; It is attached by a shorter than commonly used grip screw with no lock washer. Is this normal? Is this a desirable modification? I have not fired it yet but would just as soon have the familiar A-2 grip.

Yes it is. Did you happen to get the unused interchangable parts along with the rifle? If A2 is your style, you can reduce a MIAD down by changing panels to closely match an A2. IF not, A2's are cheap (<$5). Throw one in on your next order for AR stuff.

Oh yea you'll need the trigger guard + roll pin too if you switch back

I dont remember getting a new screw with my magpul grip
now I want to take it off to see