Smith Enterprise offers 3 x G6-A2 Vortex threaded (1/2" x 28tpi) for the AR15 barrel thread:
1001V (AR15/M16)
1001AV (M240)
1001BV (M249)
Only the 1001V states on the web site that it is for the AR15/M16. What are the other two for? What are the differences?
Thanks

M240/M249 most likely for SAW...hence the designation.

Here are the current SEI part numbers for Vortex flash eliminator/suppressor interface units currently in production:

MODEL                  P/N         Application
ARSR-700              1000V         0.30 cal AR-10/SR25/Bolt Guns
M240                     1001AV       M240B/MAG-58 GPMG
MK48                     1041V        MK48 LW GPMG
M60                       1004V*       M60 GPMG
M249                      1001BV      M249 SAW
MK46/Para              1040AV      MK46 LW SAW & M249-Para
G6A2                      1001V        M4/M16/C-8 Family
G6A2D                    ------         Diemaco 10.5" bbl C-8 variant
M14DC                    2000V       M14SE/MK14 SEI (inc. modified M14/M1A rifles)
M14 OEM                 2002V*     M14
.458 SOCOM             TBD*       .458 SOCOM
FAL                          TBD         Metric FAL
AK                           TBD         AK47/AKM

*Denotes not suppressor capable
 
The purpose is pretty clear -- they're for the weapons indicated.  

Thanks - I got confused because the M240 is in 308.... OOPs - I should have noticed.