From their website:
" - We do not sell additional barrels outright. All barrels must be factory fitted.
- We do not sell barrel blanks or do custom calibers.
- In order to obtain an additional barrel(s), the stock, forend and frame (receiver) must be sent to us so they may be properly hand-fitted to the barrel, to be sure they are correctly head-spaced and proof-tested. "
As Manitobahunter says, if an owner of Handi-Rifle has a spare barrel he will sell you, he can. The manufacturer's refusal to sell a barrel and their insistance on fitting the accessory barrels themselves is a precaution against potential liability if someone else were to do a bad job, not because it is too difficult for a competent gunsmith - it isn't.
There is that statement of policy right there that they won't do what mbogo3 is asking for, but that doesn't mean they won't ever change their minds. I hope. I'd buy a .303British barrel for my Handi-Rifle if I could.