Barrel Extension/Upper Receiver Fit
It sounds like you have a barrel extension the small side and an upper on the big end. If you are unable to select alternative parts, I'd use green locktite to improve the "cyclinder" fit. You will get some degree of "compression" when you torque the conventional barrel nut, but not enough to take up the loose fit you describe.
I have even epoxied undersized barrels to uppers on match rifles with heavy barrels because I never expect to wear those barrels out. This gives me the confidence I need when shooting that I am the only element in the equation that can cause a shot to print outside an otherwise tight group.
(CB is David A. Lutz, Lt. Col. USMC (Ret'd)
@ Knight's Armament Company (KAC)
"...if you can read this, thank a Teacher,
if you are reading this in English, thank a Veteran..."