I am converting a 14.5 inch barrel to 16 inch legal.
I am going to use an A2 flash hider, The hole has been drilled for the roll pin.

The part I don't know is where do I measure from at the reciever end to get
16 inches to the end of the flash hider.

Do I start from the edge of the feed ramps or the bginning of the chamber????

So the real question is how thick does the spacer have to be????

Thanks

An A2 flashider won't get you over 16"
You need to permanently affix a Vortex or a Phantom or similar length flashider to get over 16"
You measure from the bolt face to the end of the barrel.

Correct.  An A2 f/s will NOT get you to the legal 16".  You must use either a Phantom or a Vortex.  Both are better anyway than the A2.

If you must have an A2, some companies make an A2 f/s that is longer than the origional.  ONLY the longer A2 will get you legal.

TheRicker is right about how to measure bbl length.

A GemTech Bi-Lock will also get you out to 16",  and makes it easy to use a nice QD sound suppressor down the road.  Once you've got a Bi-Lock on there, it will remind you every day to get into the NFA game and buy the matching suppressor soon (at least, that was my theory ).