If you used the gun smith I believe it's more productive to buy weapon parts to turn in instead of heavy synth, but you can do the math.
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weapon parts are 1250 for 50 rep (25000 glimmer = 1000 rep) heavy synths are 1900 for 50 rep (25000 glimmer = 650 rep) special synths are 2000 for 50 rep (25000 glimmer = 600 rep)
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I don't know if it's faster but it's 2400000 glimmer to get from 0 to 25. I don't know about you but that's way above my pay grade. (cheap ass vanguard)