You can still use the shorter barrels, as long as you pin and weld a muzzle device on them that takes them out to the required 16" length. I recently completed a retro XM-177 commemorative build for a friend (his father carried one in Vietnam while with The Big Red One). We used the original length 10.5" barrel then had my gunsmith pin/weld the 5.5" muzzle device used on the XM-177s which brought it to the legal 16" limit. It was personalized to his father with his name, rank, dates of service, Big Red One logos and the Big Red One motto. Turned out really well and brought tears to his father's eyes when he was presented with it. The scary part is it actually works and his 74 year-old father is still pretty damn good with it!!