There is a madatory 5 years for this offence unless the offence is 'exceptional'
The bar for it being exceptional is high!
One man took a gun to commit suicide and changed his mind and stored it in a 'reasonably secure place' - not exceptional - 5 years
a 24 year old mother of good character failed to notify the police on finding a firearm - not exceptional - 5 years
Gail Cochrane, aged 53, pleaded guilty to possessing an 80-year old Browning self-loading pistol. She had kept the gun at her home for 29 years following the death of her father (who had been in the Royal Navy). She thought it was just his war trophy. There was no ammunition for it. - not exceptional - 5 years
http://thejusticegap.com/2012/08/minimum-sentences-minimum-justice/
I cannot see that this case is any more deserving than any of the above but I suspect that because he is a soldier he'll get a more lenient sentence.
It really is a case of one law for us and one law for the military!