I'm taking an upper off and replacing with a new one. Should I leave the bolt in if I use an upper vise? What about if I use a barrel vise instead?

Thanks in advance
M4-AK

Out... Out... Out...

[url]www.ar15.com/content/books/TM9-1005-319-23.pdf[/url]

I'm a little confused, are you replacing an upper or a barrel in an upper??  Either way, sounds like you could benefit from spending some time with a book or two to understand how these rifles fit together and operate.

Upper reciever vises come with an insert to replace the
bolt and carrier when in the vise.
Always take the carrier out and put this insert in
when changing barrels.

When tightening the barrel nut, I have the bolt carrier in the receiver.

I took an old gas tube and cut it down to 2" or 3" and stick that into the bolt carrier key.  While turning the barrel nut, I move the bolt carrier back and forth so the gas tube stub sticks out of the front of the receiver.  Very handy for keeping the notches in the circlip, weld spring, and delta ring at the top and aligning the gas tube notch in the barrel nut.

It's a very handy little alignment tool.  And it was free.  [:D]

very tricky, Homo_Erectus!  I could have used that the other night! I'll have to make one.  

This was shown to me by a retired US Army armorer in RVN.  (He was in RVN, not me. [;)])  One hand works the bolt carrier with the "alignment tube" and the other leans on the barrel nut wrench and everything lines up!!  It turns the dreaded barrel replacement into a ten minute job.

This same armorer told me to throw away the torque wrench and use a breaker bar instead.  His exact words were "watch the barrel nut, not the wrench!!"  Now I use one of those "combo" wrenches with a built in handle and mounting AR barrels is quick and painless.  [:D]

I completely agree. Was told essentially the same thing by a Colt assembly person over 35 years ago. He basically said; "Go to where you first start feeling resistance, and look at the position of the nut.  If it's less than half way aligned w/the hole, tighten one more cutout beyond. If it's halfway or more beyond the alignement hole, go to the second cutout."

Works every time[:D]