Complex sentence: Rifled guns have been kept in use by some nations (for instance Britain) because they are able to fire other ammunition such as HESH rounds with greater accuracy, however, the rifling wears down under regular APFSDS use and requires more maintenance.
Split into simpler sentences: Rifled guns have been kept in use by some nations (the UK, for example) because they are able to fire other ammunition such as HESH rounds with greater accuracy. However, the rifling wears down under regular APFSDS use and requires more maintenance.