So 20x102 has under half the recoil of 30x113 and 7 times less recoil than 25x137.

That explains a lot of choices.

Do you happen to have the values for 50BMG and bigger guns (30x173, 35x228...)?

EmericD said:

Well, I also found another old topic dealing with that matter... http://forums.delphiforums.com/autogun/messages/7368/22 Just for the fun of it, the maximum recoil force of NEXTER 20M621 (20x102 mm) is ~250 daN, the 30M781 (30x113 mm) is ~550 daN and the 25M811 (20x137 mm) is 1750 daN

Wonderful info as always. 

I found this brochure for GD's earlier prototype thats very similar to the proposed XM915:

https://www.gd-ots.com/armaments/aircraft-guns-gun-systems/xm301/

The specs list recoil at 3.5kN. That would be 350 daN right? 

EmericD said:

Just for the fun of it, the maximum recoil force of NEXTER 20M621 (20x102 mm) is ~250 daN, the 30M781 (30x113 mm) is ~550 daN and the 25M811 (20x137 mm) is 1750 daN.

Typo - 25M811 uses 25x137.