Telescoping rounds have always theoretically been a very useful concept: Put the round and ammunition completely within the case, with the space not taken up by the round being filled with propellant:

The idea is that when the round is fired, the ignition charge kicks the round into the barrel.

The round is more compact shorter, and the full cylindrical shape is packs more efficiently.

The devil has always been in the detail, and people have been working on this for about 30 years, and the folks at BAE Systems have apparently worked out the kinks on a 40mm autocannon:

It feeds through the trunnion, the pivot point for elevation, and then rotates 90 degrees to be inline with the barrel. This means that the feed point never moves relative to the turret, you don’t have to deal with flexible feeds.

In any case, the telescoping round allows more propellant and more explosive charge in the round, so the performance for BAE’s MTIP 2 (Manned Turret Integration Program 2) has a punch equivalent to a 50mm round, but the cannon is about the size of a 30mm cannon.

Pretty neat, and even neater is where they got their feed concepts:

The cylindrical rounds also pack more efficiently and are easier to handle, which is where the brewery comes in: BAE Systems designers visited a local brewery to gather ideas for automated storage systems and conveyors that would be used in the turret’s autoloader.

Beer, it does a cannon good.