The Mongoose is a wildcat round based on the .223/5.56 case. It uses standard magazines and can run bullets from 55 grains up to 95; the 105 amax will work at 2.29" COAL, but the heavy "VLD" bullets won't fit. The shoulder is 40 degrees and the case taper is blown out for increased case capacity, so it does require fire forming to reach its ultimate potential.
Making 6mm Mongoose brass consists of running through the die which pushes the shoulder back and then trimming to 1.690-1.695" (the rounds will shrink a little on the first fire as they stretch to fit the chamber). That is it - load them up and shoot to fire-form the case.
For load data, you can start with 6x45mm data. A good starting point is 1 grain under max loads for the 6x45 and work up. Max loads for the Mongoose will be above that of the 6x45 due to the Mongoose's larger case capacity once fire-formed..
Here are some velocities that can be expected
18 inch barrel
80 gr Federal pulls 2900-2990 fps
68 gr Match Burner 3100-3200
20 inch barrel
87 gr V-Max 2750-2880
90 gr FMJ 2750-2800
105 gr A-Max 2500-2600
22 inch barrel
55 gr Nosler tipped Varmegeddon 3300-3500
90 gr Nosler AccuBond 2700-2800
Powders between 4198 and CFE are all suitable. Faster ones for lighter bullets and slower for heavier ones. One powder that works with all bullet weights would be H-335. If I could only have one, H-335 would be it. BLC2 gives the highest velocity for 80gr and up bullets. The stick powders are less temp sensitive and forgiving, Most of them flatten out velocity wise at the top of the ladder. H-322 for the light weights, Benchmark for the heavies. LT-32 for the middle weights and lighter, Its a grain or so faster than H-322.
85 gr TSX =3000 warm