I'm not to sure what you are trying to say... but the rifles you mentioned are all very different and the similarity between all of them is that they are bullpup rifles. Rifles are all different in their design, their internal mechanisms are different though some parts are similar due to the fact that they are rifles or their designers copied.
The SAR-21 and Styer AUG may look alike but they are very different in their designs. It's not the laser that gets magnified, itÂ’s the optics.
I don't know of any assault rifle that can be fired underwater but firing an assault rifle underwater is not recommended as they tend to go Kaboom. Over-pressure. There were people who shot their AR15s underwater; the results were well... bad.

As for why Singapore designed a new rifle instead of buying one, may be due to the fact the Singapore government wanted a locally designed and built rifle. They didn't redesign any thing, they built it from scratch.
