Why can't the two guys in Snatch use their replica pistols which they previously tested successfully?

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.

In light of @John Smith Optional's answer, I'll rewrite/clarify my own answer.

In the film, the guns used are blank-firing replicas, i.e. they fire blank bullets, not live ammunition.

Whilst some weapons can be converted to fire real bullets, it's implied that this didn't happen in the film (hence the annoyance of several characters at having only replicas. On a site not, it's worth noting that this conversion is a dangerous and often ill-advised procedure, as the weapons often can't handle the pressure of real ammunition. For more info, see here or here, or do a simple google search on the topic.

This wikipedia page details the guns that were used in the film. Effectively, the film used real guns adapted to blank fire as there were no commercially available replicas of the guns they used. However, they were intended to be replicas in the film.

What all this effectively means is that in the scene in the car when they did shoot the bullets, they shot blanks. To quote from the wiki on blanks:

Blanks are commonly used when the sound and flash of gunfire is needed, but a projectile would not be safe...

The appearance of a blank cartridge can give a false sense of safety. Although blank cartridges do not contain a bullet, precautions are still required because fatalities and severe injuries have resulted on occasions when blank cartridges have been fired at very close ranges.

Blank cartridges frequently contain a paper, wood or plastic plug called a wad which seals the powder in the case. This wad can cause severe penetrating wounds at close range and bruising at medium ranges.

So they could in theory have squared up to Tony with their blank-firing replicas, but his real Desert Eagle would have caused far more damage and been far safer to fire (in comparison to their replicas).

A little necromancy here...

In the scene where they test fire the replica guns inside the car the shot itself is seen only from outside the car as they drive.

There are a few potential interpretations for the firing, none of them particularly satisfying.

1. The blow out of the windows is supposed to imply that the pressure wave from firing the gun within the car with the windows up. In this scenario all the windows should blow out. At this level of pressure everyone's ear drums should also probably rupture being made of a much softer material than the glass. Notwithstanding cars are hardly air tight and the same measures designed to prevent the windows from being stressed when you slam the car door closed would dissipate the pressure wave of the gun.
2. If we are to believe that the gun was pointed at the rear window and that the near proximity of the discharge or because of any projectile then there is no reason for the other windows to shatter.

A little frame by frame shows the passenger rear windows blown out in the scene whilst the windshield and passenger front window are intact for a moment before we observe all of the windows being blown out.

Since our intrepid thieves are not permanently injured we can conclude that the pressure wave did not blow out the windows. Since all the windows shatter we can conclude that the blowout was not caused by a directional discharge/projectile.

In other words the test fire scene is a complete goof that defies logic.

Since it takes frame by frame to see the single window blow out and the multiple window blow out is clearly shown, I think it is safe to assume the intention was to suggest that the replica gun was loud and scary enough to intimidate anyone.

Taking the lack of damage to the crew, and that such a test firing wouldn't blow out the windows as revealing mistakes, the internal logic shown here suggests that the crew would have had no belief that the replica gun was capable of injuring anyone, even at close range and that they were meant purely for intimidation.

Thus when Bullet Tooth Tony is not intimidated and is clearly armed and dangerous and they are not, they wisely flee.