I am confused. Are you saying that the gun is a 51 and has a 51 barrel with a 4" extension by IGF that is usually used to make G3k rifle barrels 16" therefor making the V51 in question a 51 with about a G3k length barrel? A 51 barrel is about 8.5" depending on who made it, and the G3k is 12.4", so a 51 with the IGF extension would be about 12.5", about the length of the G3k barrel.
So what I wondering is if you want a 51 that has a barrel length of the G3k? If you do, what is the reason? Just for looks? It is still a pistol or SBR if you put a stock on it and you just put a 4" flash hider on a 51 barrel. Are you wanting a 51 gun with a G3k barrel and a 4" IGF extension so you can use a stock without a stamp?
Have a photo of that if that is the setup you mean?