Contrary to Andrew Martin's assertion, it is very easy to modify replica firearms and is a serious problem here in the UK.
The issue has been somewhat obscured in the media due to its implications with legal firearms in the States: a number of mishandlings in the states (particularily the story about the ATF seizing 'toy guns') was purposely mangled by gun lobbyists to try and claim that it was impossible to convert replicas into the real thing. It isn't: at all.
I have been to an exhibition in the Leeds Armory that was dedicated to exploring exactly this problem: the amount of modified weapons recovered or submitted during weapons amnesties in Yorkshire alone is staggering.
This isn't the sort of thing that gets reported on the news, because the details are considered mundane, but there is even a number of competing commercial companies that specialize in the controlled destruction of seized modified firearms. Its a secret cottage industry.
Infact, Police powers were increased in the early 2000s to deal with this, and the legislation of the 1968 firearms act was amended to introduce "Possession of an Imitation firearm" as a chargeable offence; meaning since 2004, its actually illegal to carry a toy gun in public.
Because guns are illegal in the UK (unless owned under incredibly stringent licences), they are incredibly rare and obviously hard to acquire. The purpose of the Vinny/Sol- Bullet-tooth scene is a glorified dick measuring contest.
Vinny and Sol are amateurs, wannabe gangsters and Tony intimidates them into submission simply by pointing out the disparity of their equipment: Vinny and Sol have unreliable, amateur and largely ineffective 'toy-guns', whereas he has a Desert Eagle.
For a plan based entirely on intimidation, Vince and Sol are utterly defeated by Bullet-Tooth's lack of reaction. They realize they aren't going to win this, as he is totally unfazed.