What do I have here?

[tankbarrell]

It is a practice flat shot. Flat topped to limit range and ricochet.

Anti tank guns had a practice flat shot for range practice, usually with a reduced load.

These are often called a proof shot today but that is incorrect as a proof shot has no tracer and is fired with an overcharge to proof the gun.

The markings indicate 2 pounder Mk 1 (shot) with tracer (T)

For Mk IX and X guns which were the anti tank guns, rather than the pom pom type.

RL likely Royal Laboratories.

10/38 is the date.

BS indicates it is turned from bar stock