So I have a 9mm SBR lower and I'm trying to figure out a barrel/forearm combo. I know that some of a 9mm barrel's "length" is actually in the receiver so my question is...

If I get say a 5.5" 9mm barrel, how much of the barrel actually extends out past the front of the receiver? Thanks

About 4.5".

Yep.

5.5" barrel, 6.5" can with about 1" overlap, 7" rail gives me about ½ of my can exposed beyond the rail.

Subtract 0.5" from the advertised barrel length, and that will be the distance from the forward end of the upper receiver rail to the muzzle of the barrel.
So for instance on my build, I have an 8" barrel and a true 7.5" free float SLR rail, and both the muzzle of the barrel and the far end of the rail are exactly flush with one another.




If you want your barrel's muzzle threads to not be under the rail, then subtract another 0.5" to get your max allowable rail length.
 

So 2 people say an inch and one person says 1/2 inch? Clear as mud

It's about a half inch if you want to go to the end of the barrel. If you want to go to the shoulder it should be about a inch depending on what manufacturer you go with. Our handguards are designed to come up just short of the shoulder to allow easy changing of flash hider/ suppressor after the handguard has been installed. Check out these drawings I made up for reference.


Shows the length from the front of the receiver to the shoulder on our barrels.

http://www.quartercircle10.com/wp-content/uploads/2015/02/Barrel-Length-Chart.pdf

Shows the rail length next compared to our barrels mounted with the handguards.

http://www.quartercircle10.com/wp-content/uploads/2015/08/HANDGUARD-LENGTH-GUIDE-e1455306270315.jpg

 
Technically both are right, depending what exactly you're trying to measure.  Here's a generic image that may help:






To get "A", Subtract one inch from advertised length.  This would tell you the longest rail you could have that ends at the shoulder on the barrel's muzzle and leaves the threads exposed.




To get "B", Subtract half an inch from advertised length.  This would tell you the longest rail that would completely cover the barrel, including threads.




Distance "C" is one inch, and more literally answers your initial question of how much of the barrel is "in the receiver". but this includes the threaded area of the receiver that extends out past the end of the receiver's rail, and thus the "half inch discrepancy" depends on what you call the "front of the receiver", is it the end of the threads or the end of the upper rail?








 

I just bought this barrel last week for mine.

http://micromoa.com/nitrided-6-9mm-ar-barrel-threaded-1-2-28/

It has a tape measure nect to the barrel to give you an idea. Should be pretty clear

Thank you all for the info especially D_Man (visuals are always a big help!)