Andrew of GY6Vids reviews the FM-9 beltfed 9mm AR upper made by Freedom Ordnance. It is an interesting system. I am curious how well it would work with a Slide Fire setup. It looks like a lot of fun as long as it is well maintained. However, as Andrew mentioned in the video below, chasing links does not seem like a fun time.
The FM-9 is available at an introductory price of $1895. Regular MSRP is $2495. Check it out on their website.
Also additional links are available on their webstore. You can purchase 500 links for $35 or 1000 links for $60. That is not terrible at all. Definitely puts chasing links into perspective. You can order them here.