A Sage +3 ends app. 19-3/4" from the external face of the receiver. That is about 1/4" longer than the 20" barrel I have it on and assuming the Sage is slightly shorter than Remington /direct copy extensions that have the added step for bayonet mounting.
http://www.sageinternationalltd.com/SIL/MET.html
Been using one in 3 gun since 2014 with zero issues. Unless they have switched back to phosphate since then, the current ones are nitrided.