22 Winchester Automatic is indeed different than 22LR. Dimensionally, the case is .050" larger in diameter (it used a jacketed bullet instead of the lead heeled bullet of the 22LR) and .100" longer. Additionally, the rim is almost .100" larger in diameter. If you try to feed 22LR through your Model 1903, it will probably jam in the feeding tube or at the shell stop. Since there is so much difference in rim diameter, you would probably have trouble getting a good firing pin strike on the rim. If you were to single feed one into the chamber and actually fire it, I have absolutely no doubt the case would split. All in all, I would suggest you either have it modified to feed 22LR or buy some of the 22 Win Auto ammo available at $30/box. I myself just picked one up a few weeks ago, and am planning on rebarreling and modifying the feed mechanism to fire 22 LR.