Well if we are also considering crew-served weapons, may I nominate the USN's 5"-38 flat trajectory rifle, which has a barrel 190" or 15.83 feet long??
26" Mauser guild 1930's? case hardened and engraved k98 action, octagon to round ribbed barrel, 3 express sights, mannlicher stock, Hensoldt Jaeger mount scope .dual set trigger- IN 30-06
also has a ERMA .22 conversion unit although only a 24" barrel
Two 26" barrels I believe. One in 6.5 grendel and the other a 338-06. The 338 may be a 28ish barrel I don't remember what it came out to be after re-crowning and threading.
Damn, I have no clue how long the barrels are most of my long guns. I am guessing I have at least one shotgun with a 28" barrel but may have one or two with longer barrels.