Shipbucket

Why did British battleships have brass or bronze muzzles
on their main armament? I have seen photos and videos
of RN 14in., 15in., and 16in. guns with yellowish metal
ends on the barrels. I do not mean the caps used to
cover the muzzle openings, but the actual barrel ends
themselves. I have never seen this on large caliber guns
used by Germany or the US for their WWII ships. Does
anyone know what this is and why the RN used it?

British guns of that era used a wire-wound construction technique, whereas USN and German tubes were hoop-based. I assume that this is reflected in the brass/bronze caps on the ends of the gun tubes.

That sounds about right,it may have also had something to do with muzzle erosion.

Well, you'd want to seal the wire-wound portion, as it would be pretty prone to corrosion. Bronze/brass makes sense as a somewhat softer cover material that could be formed into shape. It's not idea from a galvanic corrosion standpoint, though. It might be a cosmetic color rather than actual brass.