There is mod Better Attachments by Dana. It uses OnEntitySpawned hook to find and modify weapon attachments (aimcone, aimsway, recoil, etc).
Does this way still work? I did same thing in my code, but attachments changed only on server-side.

        void OnEntitySpawned(BaseNetworkable entity)
        {
            if (entity is ProjectileWeaponMod)
            {
                ProjectileWeaponMod attachment = entity as ProjectileWeaponMod;
                if (!attachment) return;

                if (attachment.ShortPrefabName == "silencer.entity")
                {
                    attachment.recoil.enabled = true;
                    attachment.recoil.scalar = 0.00f;
                    attachment.recoil.offset = 0.00f;

                    attachment.aimsway.enabled = true;
                    attachment.aimsway.scalar = 0.00f;
                    attachment.aimsway.offset = 0.00f;
                }
            }
        }