The only reason the Gibbs was loaded to such a low pressure was because it got did what was needed. If this were my 'cat I'd use the same procedures as always to keep the brass safe, but there's soft brass in any caliber. The Nyati has a significantly bigger casehead, so bolt thrust is obviously going to be better for the Gibbs case at same pressures.
On a CZ or M1999 I'd plan on staying within 52k psi or so. Be interesting to see what Quick Load came up with.

With CZ coming out with a CZ action, and Montana Rifleman trying to get a big action out, there could be a way to make a nice long-range 50-cal.
Looks to me like a blown-out Gibbs case, necked to .510" will hold about 195g water. I think this will push a 750g V-Max (COL 4.8" or so?) to 2500 fps out of a 30" bbl. This would land 1000yd away at about 1750 fps, just about 200 fps behind what the BMG would give starting at 2750 fps (fair estimate?)
Someone with Quickload could figure it out better, but I see that the Nyati pusges a 750g out at 2420 with 140g powder; this Gibbs case might hold 136g powder (?), subtract for 13% smaller bore and add for 20% longer barrel, and I think 2500 fps is doable. And even if it's 50 fps less, you have 89-91% downrange speed with 40% less powder. Recoil-wise, that means a 19-20# gun instead of a 30# gun giving the same 100 ft lb of recoil! (Or 65-8 ft lb instead of 100 for the 30# gun.)
Pretty interesting to me.
Not sure about the capacity difference from the Chey-Tac, but if that survives and there's ever a 3.5" basic case for it....

Sure would be a good fun toy. At least it will fit in a CZ action and use normal primers which will be easier to get. I would like to try something like this later on when money is a bit more readily available.

the nyati runs at 23% higher pressure than the original Gibbs, so might run about 11% less velocity, so more likely 2200fps @ MAP.

rgds,

steve

The 505 Gibbs shows a 181gn H2O capacity. With 4.8col 30" bbl 750 A-max

quickload spit out

IMR 4350 125gns 2340fps 61,000psi 100% load density

35" bbl 2400fps



I looked at the Speer 647gn bullet also

30" bbl IMR 4350 137gns 2546fps 60,100psi 97% load density

35" 2616fps