With a hot bbl, first the inside heats up then the rest of the bbl, when the first part starts, the bore actually gets smaller this is why we try not to shoot multiple rounds that heat a bbl, and the reason that cold shot will not be with the rest of the group. After the bbl fully heats up then the bore is back to normal size, albeit that expanded metal does have to go somewhere. All of the attached parts, dovetails holding sights, forearm, tube magazine all react to the equation. To lessen this we try to keep the bbl cool while load testing. Now take in the other things we do, we go out in the woods with a recently cleaned bbl, and expect that first shot to be right where we zeroed. Lot of things that can go wrong. Now back to basics, that 3.5 inch group is actually a good group for a levergun, in fact a bragging group. Figuring deer sized targets, the heart is larger than that. Just to put 3.5 inches in perspective lay three quarters and a dime down that is 3.5 inches. Now think about the size of a deers heart, it is larger than 3.5 X3.5 or a common file card folded in half. The most important thing you can do is to center this group and know it's size. Check the same load at 50 and 75yds this will be your average shot not 100yds. Getting back to your groups, Have you had a trigger job? If not, do so it will help all the way around. If you have a gunsmith do this have your rifles bbl recrowned, though it may look perfect check it anyway. Sometimes this is a problem overlooked. Another thing that needs to be checked is your self. Do this put an empty in your rifle, concentrate on an aimpoint at 25yds on sand bags leave both eyes open while squeezing the trigger concentrate on keeping the crosshairs on the bullseye, after the shot. While this seems strange to do, it is very important to total accuracy. Often times we will do little things at the last moment that throw us off, very minute but still the same. If you are left or right of the target, you may be flexing your pectoral muscles when firing. If you are up or down you may be releasing your grip or tightening it. This is why you should shoot through the target not only with a bullet but your aimpoint. By shoot through I mean aim through also, or keep you sights on the target after the shot. Without recoil these movements can be detected. So while you shot a 1/2 inch group and a 3.5 inch group with the same rifle all your concentration to get that 3.5 inch was not as relaxed as the 1/2 inch group you were not expecting. Tensing at the wrong time has done it to me a quite a few times, and I call this your compeditors best ally. This was a little thing I lived by for many years during my benchrest days. My best ally was my compeditors that beat me, because they will tell you anything if you but ask. Soon they ask the questions. You are doing the right thing by asking, and will get many answers all well intended and mayhaps you'll find an answer. I do hope I have helped. A gun that is capable of a 1/2 inch group will shoot more than one. Get your mechanics aligned and then work on the equations.
