I have two versions of the sniper "Sole Survivor", version A has a 7 round magazine and version B has a 5 round magazine and has the perk Field Prep. I don't think that magazine size affects total reserve amount, but I'll note it regardless. Version A has a base reserve amount of 20 rounds, and B has 21 rounds. With 1 Sniper Rifle Reserves perk on my armor, version A and B both go up to 23 rounds. With 2 Sniper Rifle Reserves perks on my armor, version A goes up to 26 while B goes up to 24 rounds. It looks as if there's a weird interaction with reserve perks and field prep on some weapons.