Today’s understanding of the term “smallbore” is largely attributed to such calibers as the .22 inch rimfire family of cartridges. Did you know that in days past, “smallbore” referred to any caliber significantly less than .577 cal? What now constitutes smallbore would once have been described as “miniature caliber.”
Going back barely two or three generations, a .451 caliber Whitworth target rifle fell firmly into the smallbore category. At that time, almost all modern military small arms, now considered to be “fullbore” would have joined it.
Luckily, the NRA provides a definitive answer for modern shooters in Section 3 of the smallbore rules:
3.1 The Rifle―The rifle authorized for use in smallbore rifle matches is the .22 caliber rimfire chambered for cartridges commercially catalogued as the .22 Short, .22 Long, or .22 Long Rifle cartridges. There are no restrictions on the barrel length or overall weight of the rifle and accessories. No portion of the rifle or any attachment to the rifle shall extend more than 3 inches beyond the rear of the shooter’s shoulder. The trigger pull must be capable of lifting 3 pounds. The same rifle must be used throughout all stages of any one match (except aggregate) except in the case of a malfunction or disabled rifle, when the competitor may change rifles with permission of the Chief Range Officer.