[MS Hitman]

Somebody else jump in here also.  

Changeling, the only use I have heard of for gain twist barrels is shooting round balls in blackpowder guns.  You would have to have a short bearing surface or the differing rate of twist would cause the rear of the bullet to strip off material because it would be spinning slower than the fron of the bullet.  Page 209 of Elmer Keith's "Sixguns" gives a description of the use of this type of rifling.

 

[MS Hitman]

If you are spinning the front of the bullet faster than the rear, the actual "rate of twist" is inconsequential.  It's all about relative rates of twist.  I do not know what your definition of "fast" is, but on a gain twist barrel, you start at zero and get progressively faster as you go.  

Why don't you consullt with some barrel makers and see if they will build a gain twist barrel for you?  That seems like the logical thing to do.  I'm sure most of them have some expertise in this area also.