Just to clarify something that I think got missed, someone said there was a law against owning a rifle of larger caliber then 50 cal. That is not true, you simply have to submit a writen request with the ATF showing that the firearm is for sporting purposes only and you have to get a written approval from the ATF for building such a weapon. No problem at all, just time to submit the paperwork.
No different then building a 577 T-Rex, 585 Nyati(sp?), 577 Nitro, 600 Nitro, 700 Nitro. All need to have written approval by the ATF to build.
The FatMac was designed by Mr. McMillan and was an attempt to improve on standard 50 BMG ballistic performance. THe idea was to significantly reduce TOF for 1000 yard BR shooting, it did not work because again, teh match bullets are not able to handle the increased velocity. The reason he went with the shortened 20mm case was because of the lack of appropriate powders slower then what we are using in the BMG currently.
In the end, it was a serious attempt to improve on the 50 BMG ballistics but even McMillan admitted it ended up being little more then an experiment that did not prove useful.
Hell, my 510 AM is boarderline as far as being practical with the powders we have to use today and it only adds around 30 grains of useful powder capacity and its near the limits of what is practical.......