510mm is m24 length, m16 is 509mm, psg1 590mm
barrel cap it the thing on end of the barrel, two, i know stock hop up for ca m24 will, but it is an upgrade that works for aps2 as well
Stock barrel length for a CA M24 is 515mm. The closest you'll find is 509mm (M16 length) without having one custom cut for you.
A APS II PDI hop up will not fit in a CA M24, it isn't wide enough for the barrel. PDI does make a M24 hop up, which is damn near impossible to find, without ordering direct from the source. It will support an AEG barrel, as does the stock CA hop up. However, it is well worth the hunt to procure a PDI chamber for your M24.
Also, you will not find a barrel cap specifically for an M24, unless you can find it somewhere second hand. An APS barrel cap will fit, and you should be able to find it at some overseas retailers, but they're not quite the same size, so it's going to look out of place.
-Dutton