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Years ago my brother and I played around with Ackleys and Gibbs improved cartridges. My 6.5 Gibbs didn't pan out at all; when velocity gains over, say your basic 270 Winchester, were realized, they were accompanied by serious over-pressure symptoms like blown primers and sticky bolt lift. I had the rifle to the gunsmith several times and tried everything I could think of to get even a percentage of the benefits touted by the wildcat faithful. No dice; after much expense and frustration the LGS I acquired it from bought it back from me.
My brother still has a couple he had made, a 257 Roberts improved and a 6.5 Swede improved. Ackley never did the Swede but he had a custom reamer made following Ackley's principles. He gets a very modest gain, but I'm suspicious that he's exceeding prudent pressures a little bit.
My experiences changed my thinking from, "velocity is king" to, "there is nothing more effective than putting a good bullet were its supposed to go" (even if it's 100 fps slower when it arrives).
The 7.62x39 is such a small case with a 30 cal bullet, I doubt if anything measurable would result from "improvement".
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