I bet I can identify all or most of the guns used in the makings of most of these weapons.
X700: XM8 barrel and foregrip, TAR 21 stock and trigger, AK47 clip, Benelli 12 gauge shotgun underslung.
X40: AK47 body with a Dragunov SVD stock.
X55: AK47 clip on a Mk 12 SPR body.
X25: F2000, with an AK47 barrel, and the foregrip is probably an AK47 RIS.
The X50 and X45 are both M14's with G36 clips, the difference between the 50 and 45 being the 50 has a scope.
S30: M16 body, MP5 handle, G36 foregrip, Universal stock, but I am gonna say it's an M16.
BR40: AK47 clip and barrel, SCAR H body and stock, and an M4 handle with a SCAR trigger.
AR635A2: AK47 clip, G36 foregrip, SCAR L body, G36 handle and trigger, AK74 barrel with an M4 flash hider, and an M203 grenade launcher.
H40: Scar mostly, with an AR15 handle and trigger, G36 clip, AK47 barrel, SCAR flash hider, and an M203 grenade launcher.
MP340: AK47 body, VSS Vintorez stock, ACR clip, XM8 foregrip, XM8 Sharpshooter variant barrel.
MP340A2: M16 trigger grip and clip port, G36K carrying handle and ACOG, SCAR main body, G36K foregrip, XM8 barrel.
That was fun! Make some more, I'll try to identify them!