I thought I saw an ad for these, but I can't find the name of the company. It's a side mag, bolt action, AR15 upper, .50BMG rifle. They also make a single shot bolt action AR upper.
Safety Harbor Firearms has what you are looking for.
Originally Posted By Duchess:
Safety Harbor Firearms has what you are looking for.
Yep. Walter has what you seek.
Brownells shows the T-2 as in stock if you're in a hurry!

The 18.5 barrel weighs a wooping 16.5 lbs. There is something ex·traor·di·nar·y about standing with a .50 rifle and emptying a 5 rd. mag. Plus love loading/unloading it seeing thoose big brass flying out bouncing on the ground. The look on peoples faces when they see 10-25 rds laying on the ground.
The only issue with these guns are if you forget the mags. no one at the range has one you can borrow.
On a serious note I have the PRS stock on it with no pad and after the third rd. wearing just a teeshirt, the steel butplate begins to dig into the shoulder.
Safety Harbor
This is the one I saw. Thanks for the links!