Snowflakes in Hell did a post looking at apparent changes in the BATFE rule book regarding pinned and welded extensions on barrels otherwise too short to avoid being labeled NFA weapons…
http://www.snowflakesinhell.com/2009/07/28/pin-weld-no-longer-viable/
The BATFE hand book can be found here…
http://www.atf.gov/firearms/nfa/nfa_handbook/index.htm
In summary the newest version of the NFA hand book appears to have taken out language that would allow pinned or welded accessories on a barrel being added into the length measurement of the barrel. Hence if you have a 15″ barrel with a 1″ pinned or welded flash suppressor it would now be considered a short barreled rifle.
This could turn many law abiding citizens into instant felons since this method has been used by many people to make their otherwise too short barrel long enough.