We claim the ornamental design for a magazine cap, as shown and described.
FIG. 1 is a perspective view of a magazine cap showing our new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a right-side elevation view thereof, the left-side, front, and back being identical;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof; and,
FIG. 6 is a perspective view of the magazine cap attached to the firearm.
The broken lines showing portions of the firearm are included for the purpose of illustrating environmental structure and form no part of the claimed design.