Right. It's unlikely for IRIS to possess the capability to do so. Likewise, SERT itself probably won't though one of it's member nations might. Not sure about Murder Night. This is because nuclear isomers generally require a nuclear reaction to produce. Needless to say this sort of stuff would be heavily restricted.
I shall for now assume it is the use of hafnium-178m2 as there are actual figures on it and it can actually be stabilized by magic (most other isomers tend to decay in less than a second, even if you have magic, I doubt you'd be able to magically stabilize it on time), though with a half-life of 31 years, that is optional anyway.
Hafnium-178m2 would be capable of releasing approximately 1.330x109 joules of energy. Note that it releases this energy in the form of gamma radiation. However, it is rather impractical for any real application because as stated, it has a half-life of 31 years. Of course, if magic were to cause a sample of it to fully decay instantly, then it is fully usable as a power source.
Should such a mechanism be paired with a mechanism that focuses the gamma radiation, it would be possible to create a laser rifle that fires gamma radiation bursts of significant power with each energy cell being a lump of hafnium-178m2. I doubt it would be easy to produce hafnium-178m2 in enough quantities to mass produce such firearms without drawing unwanted attention. However, using a similar principle it could be possible to create a laser-based missile defense unit or weapon for elite units. Either way, a single shot would be rather potent seeing as it would essentially be a nuclear bomb-pumped laser.
Consider this instead: the Nuclear Non-Proliferation Treaty restricts the creation of nuclear weapon. A nuclear weapon by definition uses nuclear reactions such as fission or fusion. A Nuclear Isomer Bomb does NOT come under that definition. Hence it's deployment does not violate such treaties. See the implications?