Often guns with these types of perks have no alterations when selected. An example would be my exotic Telesto with the "enhanced battery" perk, before selection the green arrow on Magazine shows but upon selection; no effect is applied.
I have seen this upon other weapons too. I'm unaware of how widespread this issue is with others but it would be nice to have a fix in a future patch if it is.
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There's a bug where the perk selection screen WHILST IN ORBIT does not reflect things correctly. Try messing with the perks in a combat zone, such as a planet.