British spelling vs .38 calibre

British spelling is a noun but .38 calibre is not a noun.

Word	Noun	Adjective	Verb	Adverb
British spelling	Yes	No	No	No
.38 calibre	No	Yes	No	No

British spelling (noun)	.38 calibre (noun)
English orthography used in the UK, Ireland and most of the Commonwealth (characterized by 'analyse', 'centre' and 'useable')

British spelling (adjective)	.38 calibre (adjective)
	of or relating to the bore of a gun (or its ammunition) that measures thirty-eight hundredths of an inch in diameter