I've already got a 10.5" pistol and am looking at a 5.5" barrel for a build with the intent of putting a can on it in the future (Please pass HPA!!!)  Which handguards?  Seeing how this will likely be suppressed 100%, do I just add barrel length + handguard length?  Do I need to add specific flash hider/length?  My 3 pistols all sport the same length barrel, 10" ALG EMR V1's and KAK flashcans right now.

I currently do not own any suppressors, but hope to get into them soon.

Thanks!

I've got a 6" barrel in my build and went with a 10.75" URX 4.  With an Octane 45 (direct thread) I have a little over 3 inches of the can outside the handguard.  I like the look and there's enough for me to grab to twist it on or off.  
 

I went with a 5" TROS 3 lug barrel, Odin Works 10" hand guard, and an Octane with a 3 lug mount. Lotsa fun and a lot of friends have "copied" the design.

Odin Works and Aeroprecision here..