It has been a commonly held belief that because the annihilator beam uses both matter and anti-matter, it must be capable of producing energy comparable to that of the atomic bomb dropped on Hiroshima, Japan. This likely comes from the scan regarding Quadraxis, which tells us, “Its primary weapon system fires destructive matter-antimatter blasts.”  This was then applied to Samus to give people the impression that because she can survive Quadraxis’ matter-anti-matter attack, she must be able to survive 21 kilotons of TNT. While it’s true that half a gram of matter and half a gram of anti-matter would generate an energy equivalence, this doesn’t necessarily mean it’s true for the annihilator beam, since we never observe such destruction in the first place. The word “destructive” can apply to many things, including locusts eating crops.
If we look up the specific energy of anti-matter, we can find out that it is 1.8 * 10^17 MJ/kg. This is found by 2c^2. Unfortunately, I do not know what this would be by simply knowing the diameter of Samus’ arm cannon muzzle. The reason is because I don’t know what the energy density is, which would be found in J/m^3. While we could ignore the idea of anti-matter production rate in the Metroid universe to take as long as it would in reality, the production rate for 10 nanograms of anti-matter takes at least a year.  If we took this amount per shot for the annihilator beam, we would know the amount of energy. I am willing to work with this, although Yoshio Sakamoto’s answer to the destruction of the B.S.L. and planet SR388 gives me the impression that the Metroid universe has larger quantities of anti-matter. 
In order to find out the energy from 10 ng, we would need to use E = mc^2. Where we’d use the amount for anti-matter, we also need to add matter, since that is what the annihilator beam uses. This means we’ll be using 20 ng. which would be (2.0 * 10^-11 kg.)(299,792,458 m/s)^2. This would be equal to 1.798 MJ. This would be from a single shot alone, which doesn’t look bad, quite honestly. This is still speculation, since we don’t know the amount of anti-matter used, but I find this at least more reasonable based on what we know about the production rate of anti-matter, as well as the visual effects in spite of the Metroid universe probably containing a lot more anti-matter.