The ornamental design for an adapter for firearms, as shown and described.
FIG. 1 is a front perspective view of an adapter for firearms showing my new design.
FIG. 2 is a bottom plan view thereof.
FIG. 3 is a rear elevation view thereof.
FIG. 4 is a top plan view thereof.
FIG. 5 is a right side elevation view thereof.
FIG. 6 is a front elevation view thereof; and,
FIG. 7 is a left side elevation view thereof.
The broken lines in FIGS. 2 and 6 are included for the purpose of illustrating a portion of the adapter for firearms that forms no part of the claimed design.