Weatherby's newest version of its Mark V bolt rifle is of a genre that has lately come to be known as the Walking Varmint class. It incorporates the features of a heavy varmint rifle -- fast, flat-shooting calibers, accurate barrel and a quick trigger -- all put together in a relatively lightweight, easier-to-carry rig. Our test sample in .22/250 proved finicky about ammo. It delivered groups as small as .387 inches with Winchester Supreme 40-grain Ballistic Silvertips, which it liked, and as large as 1.835 inches with Winchester's economy-priced 45-grain USA brand loads, which it apparently detested. During one phase of testing, the bolt became increasingly difficult to open and finally locked and couldn't be opened. Normal function was restored after disassembly, but the cause of the stoppage remained a mystery.