You say that you are only loading 40-43 grains, yet the max load shown is 48.2. I seriously doubt that a load that is 5 grains shy of the maximum is making enough pressure to streach the case enough to reseat the primer. Your first inclination is most likely the road to follow.
If you dont have one, get a chronograph. This will tell you more about your loads, and the approximate pressure levels than any other peice of equipment a reloader can own. While many things in reloading are not absolute, there is a very linear, progressive relationship between pressure and velocity. Typically, the higher the velocity, the higher the pressure. A chronograph gives us a window into the pressure progression of our loads. A good many starting level loads for a cartridge like the 308 are only making in the neighborhood of 40,000 psi. That is only about 70% of the operating pressure of the cartridge. Are the case necks clean and bright or are they sooty? If they are sooty, it is a sure sign of too little pressure. Necks that are bright (an obvioulsy bright shiny as compared to the rest of the case) are indicitive of high pressures, the pressure acting against the case as it tries to move against the chamber wall creates this brightness.
If you have a sooty neck and protruding primers, you are at low pressure. Primers running at high pressure generally don't protrude, they get flatter and flatter till they are blown out. If the primers don't have sharp edges, but rather are round edged still, you are at low pressure. Again, generally primers don't protude at high pressure, they will show a raised ridge around the firing pin crater (indention, strike) This is almost always in conjuction with bright marks on the case head, usually from the extractor. The Handi won't show this kind of mark, it will be hard to open and there will probably a brass colored ring on the breech face, or even a brass colored drag mark. At these pressure levels the primer pockests will begin to loosen in short order. I don't mean a primer is easier to seat, I mean a primer won't stay in the pocket. At these pressure levels, you will also see a dark ring around the primer pocket, gas leakage.
There is a pretty strong disscussion around this..... that over time, cases that headspace on the shoulder, that are fired constantly at low pressure will develope excessive headspace as a result of the firing pin blow driving the case into the chamber, but with a lack of sufficient pressure to restreach it. I don't buy, especially when we are talking about a Handi. But incorrect sizing die dimensions could create this.... probably a red herring in this. My 30-30 exhibited very proud primers, even in factory loads. It was not untill I exceeded 2550 fps (app) that I even began to get primers flattening at the edges. Since I was operating well outside any available, reliable data, I simply quit experimenting.
The short side of this converstion is that without a chronograph you are guessing at what the load is doing. Guessing is ok, a great many of us do it, but study the manuals very carefully. Speer and Lyman both have good sections and drawing and photos of what normal and overloaded cases look like.
With primers, if it protrudes and shows no loss of round edges, pressure is low. The greater the loss of rounded edge, the higher the pressure. As the rounded edge begins to disappear, look more and more closely at the condition fo the brass, the opening qualities of the gun and the breech face for brass coloration, as well as for discoloration around the primer and for primer peircing as well.
