What Is a 10mm AR Upper and Who Should Consider Using It Spend time around people who build custom rifles, and you start to notice that not every AR platform is chambered in the usual calibers. Somewhere between standard 5.56 builds and larger .308 platforms sit in a category that has been quietly gaining attention: pistol caliber AR builds. Among them, the 10mm has carved out a very specific niche. Click here for more info:- https://telescope.ac/moriarti-armaments---a-leading-firearms-manufacturer/wlh5ida6uq2sdk24ss8lxb