If both his shots hit the same place it is probaly not flinch (unless he can flinch the same every time) It is more likely his cheek well is different then yours. I have seen alot of guys able to shoot the same rifle with out any trouble then there will be someone come along and totally ruin the whole thing. It is almost always a cheek well issue. Check his trigger pull, it should be like clicking a ball point pen. I sometimes take one with me to show them how it works. Practice yourself if you need to it wont hurt anythingYour shells have nothing to do with it.