Airsoft Atlanta have a range of SHS tightbore barrels in stock:
Brand new SHS 6.03mm tightbore barrels are in stock starting at $18.99-$46.99 each. These are stainless steel, custom, super smooth inner bore barrels for AEGs, closer to a spec of 6.01mm. These are not the older brass barrels, but a newly updated version for use in AEGs. 247mm – 650mm length barrels available. Check them out:
http://www.airsoftatlanta.com/SHS-6-03mm-Tightbore-Inner-Barrel-p/33146.htm
