In UK if you use a paintball gun in that manner it becomes an offensive weapon. The same with an air rifle or pistol.
A screwdriver is an offensive weapon if it is used as a stabbing knife. So is a table fork.
The law favours the intruder in UK, not the house occupiers.
Intruders have successfully sued for damages after being hit by their victims, turning a case of theft or illegal entry into one of ABH or GBH, which is a greater offence.
The thief gets two weeks for theft and the 'victim' gets two months for GBH and pays damages to the thief.
Who wins?
http://www.telegraph.co.uk/news/uknews/1398439/10000-damages-for-thief-injured-during-robbery.html . . . .
http://www.independent.co.uk/news/farmer-must-pay-thief-damages-for-injuries-1260415.html . . . .
http://www.dailymail.co.uk/news/article-1296768/Crook-frogmarched-police-station-THIEF-sign-neck-sues-employer-90-000-humiliated.html . . . . . .
http://www.dailymail.co.uk/news/article-1075809/No-barbed-wire--hurt-thieves-allotment-holders-told.html . . . . . .
The nice legal way is simply to say in a quiet and non-threatening voice.
"Hallo old bean. Err, take what you like of course. Could I have an address so the Police can come round and pick it all up again? I'd be so pleased."
You are allowed to use reasonable force in self defence in UK. Trouble is, whatever you do is then interpreted by the thief's lawyer as 'unreasonable force'.
How long is a piece of string? There is no definition of what is reasonable force in UK law that actually is free of being interpreted or mis-interpreted any convenient way by the lawyers....or by the Police and the courts.
Even in Canada....home of good manners and "Thank you Sir. You're very kind Sir. Much appreciated Sir" for a simple answer of "That way" and a point of the finger for a guy asking for street directions.
http://overlawyered.com/2007/04/canada-crook-wins-damages-for-injury-during-theft/ . . . .
Good old Britain eh? We are not alone.