I'm looking to swap my A2 upper for a flattop on my Model1/BM carbine.
I've read here that all the headspacing is in the barrel/barrel ext/bolt relationship.
The only reason I ask about any concerns is that the USMC manual I have states that swithing B/BC between weapons can kill or injure personel.
Am I ok to swap upper if the bolt/barrel are the same?

Any sources on inexpensive but milspec uppers?

Any input appreciated.

The bolt carrier has nothing, repeat, nothing to do with headspace.  The bolt itself does.  Unless the bolt is a very old worn out piece, you will not likely have a problem.  If swapping  bolts between uppers was dangerous, I would have been killed long ago.  I have done it in the past, will continue to do it in the future.  
What swapping bolts will do is cause accellerated wear.  On any bolt and barrel, there are little high spots.  The lugs wear in until everything is a good fit, and the wear drops way down.  When you put a bolt with another barrel, it has to seat to that one, and will wear differently until it is settled in.

If switching parts (which is part of what makes a military weapon suitable) were dangerous, there would be a lot of dead GI's.

Check this out... when a firearm is tested for adoption, they get a number of them.  They don't test just one.  They take them apart, and mix the parts, reassembling at random.  This is so that they can be assured the parts will function , are all within tolerance, and that the firearms are not specially hand fitted.

Check out J&T for a nice upper.

[url]www.jtdistributing.con[/url]

Correction on linky [wave]

[url]http://www.jtdistributing.com[/url]