I'm trying to install a carbine length PRI free float tube.  When I try to place the front sight post back on, it hits the tube and will no go forward far enough.  

I'm wondering if I have to replace the front sight with a flip down or milled version.?

I've noticed there are about three threads showing after snugging the collar.  It stops when it reaches the allen holes on the tube.  (I've just looked at some pics of PRI tube in the equipment exchange with about three threads showing)

It looks like if I tighten the tube down the remaining threads, It might move enough to place the sight.  Problem is, it's not going to go without a lot of force.  

Any information or ideas are appreciated.  

I've had this happen with some otherfloat tubes.  If it almost but not quite fitting then just file the front sight a little bit so that it clears.  You can then use some cold blue solution or a touch-up pen to cover the exposed metal.