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Based on the displayed operational characteristics of the rail-gun rifle in "Eraser" (the latest Arnie movie), an attempt is made to deduce the performance specifications of the rifle.
It was stated that the bullets travelled at "near light-speed". Since "near light speed" means a very high speed, the mass of the bullet must be correspondingly small due to the limited amount of energy that the rifle can generate. The mass of the bullet can be so small as to be a serious drawback. We will estimate the maximum possible bullet mass.
For a conservative (high) estimate as to the maximum bullet mass, we will make generous assumptions as to the power of the rifle (high) and the speed of the bullet (low).
The rifle is man-portable. Therefore, assuming a power dissipation of 10 kilowatts maximum for such a small unit (with a sustained firing rate of one round per second), and an overall system efficiency of 99%, 1% of the power generated is dissipated as 10 kilowatts of heat.
Therefore, the bullet has kinetic energy of 10 kilowatts X 99 = 990 kilowatts.
Since kinetic Energy, E = 1/2 X m X v X v,
m, or the mass of the projectile = 2 X E / ( v X v)
Assume a speed, v, of one tenth light-speed, 0.1 x 3 x 10**8 metres/second
The mass of the projectile can be calculated as
2 X 990,000 / (0.3 x 10**8 x 0.3 X 10**8) kilogrammes
= 0.0000000022 kilogrammes
= 0.0000022 milligrammes
An incredibly lightweight bullet - considering that 1 litre of Evian mineral water contains 78 milligrammes of Calcium!
This bullet weight, gives a recoil of m X v
= 0.0000000022 X 0.3 x 10**8
= 0.066 kilogramme metres/second
For comparison, a 100 gramme object moving at 1 metre per second (3.6
kilometres per hour), gives a recoil of 0.1 x 1 = 0.1 kilogramme
metres/second
The "Eraser" rifle has therefore very little recoil when fired, as can be seen in the movie. This would make it possible to fire the rifle with high accuracy.
Unfortunately, the bullet's path is marked by a visible smoke trail. This implies that the bullet is ablated by atmospheric friction during its high speed flight. Considering the density of the smoke trail, and the bullet's already light weight, it can be deduced that the bullet's mass is significantly reduced by ablation even after travelling only a few tens of metres.
In the movie, the rifle was not used beyond ranges of 200 metres. Together with the severe ablation deduced above, it is speculated that the rifle is not a long range weapon, but a short range gun limited to perhaps less than 200 meters.
For comparison, the 5.56 mm calibre Colt M16 is effective up to 300 metres (Colt claims 500 metres, and US Special Forces do use it up to that range). 7.62 mm calibre rifles are considered effective to above 700 metres and 0.5 inch calibre sniping rifles such as the Barret are designed to be used above 1000 metres.
The large smoke trail of the bullets also precludes it for use as a sniping rifle as the trail would betray the position of the sniper.
Therefore, despite its awesome short-range destructive capabilities, the "Eraser" rifle is of perhaps limited utility. It is closer to a shotgun in performance and should be more useful in situations currently well served by shotguns i.e. close quarters combat such as jungle and urban street fighting (as is the case in the movie).