M4A1

Copy the v_m4a1.mdl, p_m4a1.mdl and w_m4a1.mdl files into cstrike/models, replacing the originals. If your game uses a localized folder, put them in cstrike_russian/models instead - files there take priority. Back up the original files first. Then start the game and select M4A1 in a match.

Weapon models go to cstrike/models: v_m4a1.mdl is the first-person view, p_m4a1.mdl is the model in the player's hands, and w_m4a1.mdl is the one lying on the ground. Custom sounds go to cstrike/sound/weapons. If you use a localized version, the cstrike_russian folder overrides cstrike.

Check that the file names match exactly: v_m4a1.mdl, p_m4a1.mdl, w_m4a1.mdl in cstrike/models. Make sure you did not leave the files in a subfolder. If you use cstrike_russian, place them there as well. Restart the game after replacing files.

Delete the custom v_m4a1.mdl, p_m4a1.mdl and w_m4a1.mdl from cstrike/models and restore the backups you made before installing. If you did not back them up, verify the game files through Steam or reinstall the game. The default models will be restored automatically.

On most servers custom models are client-side and work, but some servers enforce consistency checks and may replace your files or disconnect you. If the server uses sv_consistency 1, your custom M4A1 model may be rejected. In that case use the default model while playing on that server.

Place the sound files in cstrike/sound/weapons, keeping the original file names the game expects for M4A1. If the model pack includes sounds, copy them to the same path. Do not rename them unless the pack's readme says otherwise.

Remove the custom v_m4a1.mdl, p_m4a1.mdl and w_m4a1.mdl from cstrike/models and restore the originals. A corrupted or incompatible model file is the usual cause. Also check that the model was made for CS 1.6 and not for another game.