Mike: First of all, if your gun is an "OO" Grade it was made prior to 1913. Secondly, it was NEVER EVER designed, intended, or proofed for the 10-bore 3 1/2" mags that are even more powerful today than when originally conceived in the 1930s. Finally, I am sure there is some knucklehead out there in this big old world who would open those chambers to 3 1/2"; but I honestly could NEVER imagine there is a reputable double gunsmith anywhere in this entire country who would accept the kind of potential liability he would surely entertain for making such a dangerous alteration on your vintage Smith gun as the barrel walls will be very thin at the end of those lengthened chambers. Now, assuming the barrels did not rupture on the first shot, a serious shooter armed with the above knowledge regarding potential dangers associated with shooting such an altered piece will have a difficutl time concentrating on shot/s placement and shooting fundamentals knowing that he will be counting the fingers on his left hand every time he pulled the trigger (and no pun is intended here, as I have personally witnessed a barrel rupture that did indeed remove a significant portion of a man's hand!). If you are into heavy loads and magnum guns; you are best served with a gun designed for that kind of abuse. If the bores on your gun remain in good condition and your stock is not busted, your gun is likely to have enough value to offset the cost of a Remington or Browning 10-bore pump, or even a semi-auto if your Smith gun remains in even better condition. If you keep your Smith, and wish to use it; I suggest you call the Double Gun Journal and acquire Sherman Bell's articles on loading and shooting vintage large bore guns. Properly loaded, these guns are more than adequate to take any game that flys using the 2 7/8" loads for which they are intended.