ok.. Me being a modeller i couldnt help but laugh at you for several minutes on end. but with that said and done, this is how u do it. First off lets give u the basics.
You have a w_MODEL which is a world model, the weapon you see on the ground this of course cannnot have a left or right preference.
You have the p_MODEL which is a player model, and funny thing is no matter what your setting is left or right all the 3d person perspectives of a player are always right handed.
Lastly you have a v_MODEL this is a visual model, the one with the major detail you view first person. Most modellers make a right and a left of these and signify it with a _r on the end of its name if its right handed, if infact there is a _r at the end, extract this and the w_, p_ models to the 'weapons' folder and rename the V_ model to that of the weapon. (I.E. v_p90_r.mdl would just be v_p90.mdl)
Some people are quite mean and do not signify with a _r, so its a matter of backing up the original and going for the gusto.
Hopefully this clarified anything you were wondering about, or atleast let you understand V_ P_ and W_... with that said and done I shall now go play many hours of videogames until i fall asleep in my chair. Good Day
~Sniper