I have had this happen twice before on my 700MLS. Both times I was running it dry(no grease, teflon, etc in the brech/nipple threads) and shooting a lot at the range. What I did is used a small diameter hex wrench on the breech, then the nipple/breech tool on the nipple. Place the nipple/breech tool in a bench vise and unscrew the breech plug with the heck wrench. (has to be a thin diameter wrench not the regular shop style)
I now use grease or pink teflon tape in all shooting apps and remove the breech and nipple while still warm after a long practice session. If used in the field I do this as soon as I return home...not suggested to fool around with it in the field pretty hard to find stuff in the grass<img src=icon_smile_wink.gif border=0 align=middle>!