fffg may be cleaner then ffg, but you should keep in mind the smaller granules of fffg will burn hotter and faster. I would not use fffg if you plan to use magnum charges.
and
One rule of thumb I read said to use FF in .50 cal or better and FFF in .45 or less.
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The thread-starter here states he uses 777 and none of-the-above applies to it.
There's very little difference in bore pressures between 777 FF and FFF.... sometimes none dependent on bore diameter/bullet grain/bullet diamerter. A reduction of only 5-10 grains for 'magnum" powder charges .... ie.. 115-120... is all that's necessary (on average).
Many shooters nowadays use 777 FFF in big-bores like 58-calibers. In my own 58, I found 75 gr FFF shot the same as 80 gr FF with no spikes in pressures.