To install a barrel in the Daisy 880 family of air rifles the barrel must be a tight fit in the valve body. The factory barrels are 0.300" in diameter. To install the barrel push it into the breach until it seats against the step in the breach just prior to the transfer port. You can verify the barrel is seated by looking through the loading port and making sure the end of the barrel is flush with the step.
After inserting the barrel use Muggy Weld Super Alloy 1 to braze the barrel to the valve body. Don't use other types of brazing rod or the valve body may warp or melt before the brazing rod melts.
Super Alloy 1 melts at 350 degrees f.