Not trying to be rude here but have you ever seen an AR-50 rifle????
Its probably easily the strongest receiver out there for long heavy barrels because it has a HUGE V-shape under the receiver that is hidden stock that you can not see.
Here is a pic of the receiver compared to a Rem 700 Long Action. This should clear up the size comparison issues for you. I do not think you fully understand the size and strength of the AR-50 receiver.
There is no bedded either. The stock is machined form a solid billet of heat treated aluminum. It has a huge V-channel in it and the bottom receiver matches this V and is pulled down into the stock and there is also a locking wedge that pulls and holds the recoil lug solidly back against the support surface in the stock so that there can be no for and aft movement.
If you have ever seen an AR-50 receiver, you would have no question how it could handle this barrel. The receiver was designed for the 12.5 Russian chambering so its MUCH longer then most standard 50 BMG receivers. Hell, the bolt body itself is as large in diameter and length as a Rem 700 Long Action and the AR bolt is longer then a complete Rem 700 Long Action receiver.
As far as barrel vibrations, again, have you ever handled a 1.750" straight cylinder barrel? There will be no barrel vibration problems with a barrel this massive.
I also never said 200 fps over a mil spec chamber. I do not work with mil spec chambers, I use only match chambers, have little need for mil spec. When shooting any 50 cal based on the BMG case, the best way to figure pressure is to chamber a fired case right after firing, the fired case should chamber easily which tells you chamber pressures are within the 50-55,000 psi range.
I load the 510 AM so that you can still chamber a fired case but there is a bit more resistance simply because of the min taper on the case design. Chambers are likely in the 55-60,000 psi range.
The factory recoil lug is used which is generally very square right from the factory. In fact, I have found very few that had more then 0.0002" variation in thickness around the barrel support ring. It is made from high quality heat treated 4140 chrome moly steel and is over a full inch in thickness, again, its more then up to any stress that the rifle will impose on the recoil lug.
I have never heard the expression COW referring to any barrel. Only time I have heard that is when someone was referring to a case forming process using Cream Of Wheat to form cases. I have never heard any references to what you are talking about.
Again, I just do not think you fully understand how large of a rifle you are looking at is all. The pic of the two receivers should make it clear just how big the AR receiver is.