This section applies where—
a person is convicted of an offence listed in Schedule 20 (certain offences involving firearms that are prohibited weapons), and
the offender was aged 16 or over when the offence was committed.
The court must impose an appropriate custodial sentence for a term of at least the required minimum term unless the court is of the opinion that there are exceptional circumstances which—
relate to the offence or to the offender, and
justify not doing so.
In this section “appropriate custodial sentence” means—
in the case of a person who is aged under 18 when convicted, a sentence of detention under section 250 or, in a case to which section 252A applies, under that section;
in the case of a person who is aged 18 or over but under 21 when convicted, a sentence of detention in a young offender institution (and includes, if the offence is an offence for which a person aged 21 or over would be liable to imprisonment for life, a sentence of custody for life);
in the case of a person who is aged 21 or over when convicted, a sentence of imprisonment.
In the case of an offence within paragraph 5 of Schedule 20, “the required minimum term” means—
in the case of an offender who was aged under 18 when convicted, 3 years;
in the case of an offender who was aged 18 or over when convicted, 5 years.