The present invention relates to barrel extensions for firearms that have been converted to fire different, most likely lower, caliber ammunition than the firearms were originally designed to accommodate. The present invention in several embodiments is a barrel extension designed to supplement or replace the barrels and/or barrel extensions of firearms that have a sub-caliber conversion installed. Several embodiments of the present invention are configured to be installed on AR15 family rifles with a .22LR ammunition conversion to allow for accurate shooting of the .22LR ammunition through the barrel extension.