i am going to install a new floating hand guard so i need a low profile gas block and i need to know if i can use the factory gas tube?

thanks

yes

Hack your front sight post.  It's gooder.

as cheap as gas blocks are im not wasting my time cutting up a fsb, plus i might want it for a future project?

For the most part the gas tubes are the same if the gas system is the same length.  There are some rare exceptions (a few gas blocks come with special gas tubes).

However, gas tubes and gas tube roll pins are pretty cheap, and the frustration of removing and re installing the gas tube roll pin is worth starting with new parts, and a spare gas tube isn't a  bad thing to have.

I say if it's pinned cut it yourself. It already a set screw/clamp on buy a new one. Not that either is really better than the other for general use, but pinned is always best if the option its already there.

The FSB and barrel have to be drilled together, so it will be of no use for a future project.  Shave it down and use it since it is pinned...

Shave it, you already know that it works and a pinned GB is the most secure.

This is NOT true at all.  It may be easier to install a "virgin" frt sight, but it isn't required.   I have installed several "used" frt sight bases and know a few others that have installed a bunch of used fronts. It's pretty common to do in the retro rifle building hobby, as there are very few unused 1960's A1 front sights to be had.

The is completely wrong

It is not completely wrong, or untrue... you can not simply swap a front sight to another barrel and expect everything to line up.  They are drilled together.

I guess you could possibly drill another barrel to match, but if you are off, it isn't going to line up.  It isn't simply a swap it over to another barrel.

Wow

No, it isn't.

Some right, some wrong.

A FSB that's been previously drilled can be used on a new non-drilled barrel.

A previously drilled barrel can use a new non-drilled FSB.

The problem arises when trying to match both a previously drilled FSB and barrel.

The FSB location is a given so as to line up over the gas port but if the holes on this block do not line up exactly with the present holes on the barrel another attempt at drilling will surely elongate the barrels
existing holes. Which will most likely not make for a secure connection.

There still is the chance that if both previously drilled parts are just slightly off that you may be able to enlarge the FSB's gas port hole. This small amount of adjustment should have no impact on the gas tubes function.

As others have stated, the best option is a shaved down block.