Does not increase the magazine size. It already has a base size of 5 and does not increase when the perk is selected.
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This has occured on other Linear fusions before. I am assuming rounding is what's happening here. Let's say the thing has a base magazine size of 4.5 (rounded up to 5) and enh battery is a 20% increase. That would then land on 5.4 and get round down to 5 again, therefore no increase. However, this is just guesswork and we have (on quite a few reports regarding this) yet to get an official answer on that