Yes. Every part changes the overall weapon stats. So does the type of ammunition. Which is filled into mags bullet by bullet. There is no „magic refill“ of a mag just because you press the reload button. As a full auto lover, bring a couple of mags.
Want to know how many shots you got left? If the mag has peep holes, you can tell e.g. it‘s between one third and two thirds.
Oh-oh, the derailment alert just went off 
go here Escape from Tarkov - #58 by Poneybirds