In a gatling, The distance between the bolt and the shell. If the bolt is too close, the bolt will hit the shell. Too light and you get into a few problems, the firing pin will be too far away and if for some reason it did fire, the shell would be wobbling around in there loose.
Thats my take, others that know more will probably chime in.
It is easily adjustable on a Gat gun if your bolts are the same length....
This is why its so important to have each bolt the SAME length.... And the carrier block holes, the same depth...