Fingerprint approaches just are not going to work, because the environment is insufficiently controlled.
Why not either design the assault rifle to use a small implantable RFID key device, that is coded to you and works every time? If it's implantable, it's always there..
That strikes me as a simple and elegant solution. You're always going to need a battery, but the power level might be low enough to measure the lifetime in years.
*shrug*
Another approach would be to code the ammunition not the rifle, and electrically detonate it. That way you could have a fresh "battery" every time. Likely cost prohibitive, however.
There's a few hundred million weapons in the US now anyway, millions more sold every year. I think the horse left the barn some time ago.. making this kind of moot.
If I ran the kingdom in light of the above, I'd have mandatory practical firearms training for every high school student. That'd make too much sense, though..