The std bbl length for an M14 service rifle is 22inches.
Here is a ref pic of the 3 types of bbl lengths that SAI produces...
22in at top ( std/loaded/NM/SM/m25 models)
18in in the middle. ( This one is a bush model, but the scouts are the same ecxept they utilize a muzzle brake instead of a flash hider, and have a scout mount, along with a handguard that has a hole cut in it to allow the mount to stick up thru it......where as the bush model uses the std hg.)
16in ( SocomII, and Socom 16 models.)
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Like mentioned b4...my fav is the 18ish types for all around use, and also use them for hunting.
M1A-A1 Bush rifle at the top, and the other is a custom built "tanker" using an M14S norinco receiver at it's heart, along with a Criterion 18.5in chrome lined std contour bbl/ SEI "front end".
My next rifle is an LRB ARMS M14SA 18.5inbbl Tanker. (Will do pics after I get a hold of it when i get home)