In training we tried cutting seat belts with good EMT shears as well. The shears did a far better job. They also where superior to knives. I doubt that this would change in real world scenarios.
Newbies tend to struggle when trying to cut a seat belt with the seat belt cutter without prior instruction on the technique. It is hard to do if you try to cut at a right angle.
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If it isnīt broken, it doesnīt have enough features yet.