AR Upper .50BMG Conversions?

[Taurus44]

I've got the hankerin' for a .50BMG lately, so I've been doing a TON of Googling to find some options. I'm pretty intrigued by the bolt-action AR uppers in both single-shot and mag-fed flavors. I like that they basically use all standard AR lower parts (trigger groups, springs, etc), and I especially like the prices... It's as cheap as you can get to enter the 50 world.

So. Does anyone have any experience with any of the uppers in general? Anyone ever shot the Advanced Long-range Systems upper in particular? Does anyone besides DPMS make a single-shot (no magwell) AR lower?