I must be antiquated. In the military, you can customize your AR?
In a fashion yes. Troopers in Iraq & Afghanistan made mods & used PMags before they were authorized.
Also, the "A5" configuration was un-officially authorized for use as a field modification to the M16A2 and A4's.
From:
The M16A5 configuration, though somewhat ambiguous, is some of the most fun you can have with the AR15 platform. You might know it as any combination of collapsible stock and 20" barrel, but there is actually a history here I wanted to get into. At the end, we'll talk about building your own...
www.everydaymarksman.co
The US Army AAL M-16 Hybrid Buttstock Kit
By 2009, many individual soldiers were unofficially experimenting with the collapsible buttstock configuration. United States Army Tank-automotive and Armaments Command (TACOM) started fielding requests for proposals to make it official. They created
NSN 1005-01-569-6938 in 2008 for the project, but never officially announced it as an approved accessory until 2011.
TACOM authorized the hybrid buttock for any M16A2 or M16A4 in service. The Army apparently ran it through a 200,000-round torture test before approving it.
Excerpt from PS 703, June 2011 declaring the new hybrid buttstock kit for M16 rifles as authorized for installation
The kit contained a standard M4 buffer tube, M4 stock, carbine spring, and specially-weighted “H6” buffer. Army units could purchase the kit for about $100 apiece for installation on their own rifles.
The hybrid buttstock kit still required an armorer to install it. Furthermore, it was considered a
temporary modification and any rifle with it had to be returned to its standard full-length configuration before returning to the armory at the end of a rotation.
The June 2011 edition of the Army’s PS Magazine, #703, announced the change.
The image in the announcement, found on page 24 and seen above, looks suspiciously like a Magpul UBR. Don’t be fooled, though, the kit came with the standard M4 stock.
Excerpt from Army PS 771 discussing interchangeability of hybrid buffer parts with standard carbine parts