NFB = "Nitro for black" and refers to the (reduced) loading of smokeless powder (hence "nitro") to aproximate the orginal black powder loading of some early cartridges.

As for the confusion over "sleeving" it seems you've sorted out the intent of the orginal replies. As you get deeper into this double gun stuff you'll soon learn there are many unique terms associated with it, often the terms are not what they seem. Double-gun jargon like any language dictates everyone use the same terms to avoid confusion. That said we all have our colloquialisms no matter what the language. What's really important is that you know what is meant regardless of what is said! Don't fret over the terminology but make sure you know what you're being told and how reliable that advise is. As for reliable advise in this arena and experience in actually doing it, you've come to the right place welcome aboard.

BTW - check the "suppliers" thread at the top of this section. 4D has 450NE reamers for rental.