Caliber selection really depends on the individual/use...
But I think a first focal plane 7.62x39 bdc optic would be pretty stellar for helping to use this caliber at range (and it does hold more energy at range than 5.56, it just drops a bunch more too...):
![](http://oi61.tinypic.com/2ibzuc6.jpg)
The problem with the VZ58 and optics though is that due to ejection setup, you're stuck w/ pretty much just a fixed zoom optic w/o a monolithic top rail which currently isn't available (you can diy your own deflector, but that's more work than most are willing to do...).
So that's the Bren's primary advantage as I see it -- the monolithic top rail. And the other is the burst function which is also absent on the VZ58. Otherwise, they're pretty equivalent.
But bear in mind that the reason the Bren was designed as it was, was to be multi-caliber capable. (It isn't as of yet as they couldn't make it reliable...) So recognizing the compromises made to support multiple calibers and then not being multi-caliber as of present, there were undoubtedly compromises made to allow that feature that weren't ideal in other respects... Moreover, modern militaries fight with optics. When you're removing and replacing the barrel, you either need to remove and replace your optic and reinstall at the same rail location or rezero your current optic for the new barrel/caliber. Vs what special forces do alread with a CQB upper and a longer accurized mid-length or rifle upper for general and longer range use... The optics and the barrels remain mounted so it's just a matter of pulling two pins and swapping complete uppers. (The vz58 obviously lacks this AR capability, but it could have been modernized/redesigned to allow for either a quick change or AR type barrel install (the latter requiring headspacing). For either, a wrench is much easier to move around than a shop press...
http://www.tactical-life.com/gear/colts-m4a1-556mm-carbine/Here's an example of a sopmod kit:
The two kits are painted with red numbers on all their components to identify their owners. Kit #4 is well worn with not much of its desert-colored spray paint remaining. Contrast that beat-up weapon to the pristine newness of the kit belonging to SEAL #16 and you?d wonder why there?s such a difference. Well, kit #16 belongs to a SEAL who is a machinegunner, and consequently all his pre-deployment work-up has been with his belt-fed and not his M4. SEAL #4, on the other hand, is an assaulter, so all his work has been with his M4 as his primary.
![](http://files.harrispublications.com/wp-content/uploads/sites/8/2009/02/seal.gif)