Ok so first off this wouldn't be considered thaumiel, its a gun that somehow destroys energy. Ill go into why it shouldn't be thaumiel in a bit. This would probably be Euclid.
must be 2 x 3 x 2 feet
a Mosin is a hell of a lot longer than 3 feet, it wouldn't fit in this box. Also the foundation uses Standard Measurements, not US measurments.
Failure to do so will result in confiscation
confiscation by who? the Foundation already has it in its possession, unless this means like if someone tries to steal the firearm, in which case they would be detained and prosecuted.
magazines of this firearm must hold an amount of bullets in intervals of 5
Ok a Mosin Nagant doesn't use magazines, it has a single non-detachable magazine that's fed by stripper clips. They are different. Also specifying that the clips MUST be loaded in intervals of 5 is redundant since that's the maximum capacity of a Mosin rifle. (that is unless its anomalous property only works when the gun is fully loaded, in which case it should be worded differently and be in the description, not the containment procedures.) In either case this sentence is a mess, rewrite it.
SCP-2853 appears as a 1945 standard issue, Russian, Mosin Nagant bolt-action rifle
Is it actually a rifle or does it only APPEAR to be a rifle? This falls under breaking clinical tone guidelines.
SCP-2853's ammunition, SCP- 2853-2, will immediately reach the Supersonic level of speed
The muzzle velocity of a Mosin Nagant is well over the speed of sound, this sentence is redundant and written poorly.
SCP-2853's ammunition, SCP- 2853-2
Will any ammunition work? or does it have special ammo it needs to use? any other details on this ammo? like what kind it is/where it came from if it's special?
Ok back to the thaumiel stuff: this would most likely classify as an OTT (over the top) weapon. In your addendum you detail some situation where somehow a lieutenant got his hands on the weapon and shot a d-class for what seems like absolutely no reason what so ever. In doing so it destroyed every one of his mitochondria. This would effectively mean it would be able to kill every single biological entity in the facility, probably including 682. These items are usually looked down upon by the SCP community and rarely get any kind of support.
