Project X flexes = old Rifle flexes?
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We discussed this around Christmas.
The old Rifle 5.5 flex had an FCM = 5.5; but, the Project X 5.5 has FCM = 6.0, about a half a flex stiffer.
This can be confusing, since Royal Precision originally produced both Rifle and Project X shafts. (True Temper bought out financially stressed Royal P in 2006.)
I don't consider it shady, Project X are super stiff shafts designed for control while Rifle are more of a neutral profile. You shouldn't fit into both profiles so why should the flexes be called the same thing? I believe 5.5 is considered about equivalent to "stiff", though there is no industry standard. PX just play about half a flex strong so you would notice a bigger difference in performance, and so fitters can stock fewer shafts but still fit a lot of people.
5.0 or soft stepped 5.5 should be close to what you want, but I know PX at least is progressive weight, so it gets heavier in 5g increments as the flex gets higher where Dynamic Gold flexes are basically each 130g unless you go for one of their uncommon flexes like x300 that are slightly different weight. That might make the 5.5 play a bit softer than Dynamic gold unless you get the PX version. Rifles are also not a low launching profile like DG.
I'd suggest, however, that you consider KBS shafts. The KBS Tour S or R+ is higher launching than DG and as smooth as they come; it's also 10$ per shaft cheaper than project X, and it's designed by Kim Braly, the maker of the original project x. They are very high quality and their specifications are very good. The non tour issue dynamic golds are a bit cheaper though.
I've recently used the newer Project X pxi and Project X 95 flighted iron shafts, in both 5.5 and 5.0 flex.
I also own several Ping, Titleist and Callaway irons, all in Regular flex.
Compared to the above, my experience is that 5.5 plays stiff and 5.0 plays firm (reg +)
Hope this information is of help to you.