The same thing happens to me some days. I have some theories about this.I attribute it to a couple things.

First. when you start shooting, the rifle barrel is nice and clean. Granted we swab between shots, but we really do not "clean" the barrel when we swab, what we do is remove part of the fowling to assist in loading and to make things more consistent. So I suspect it might be a build up of fowling. The more you shoot, hence the further into the range session the accuracy decreases.

To combat this, you can use a brass bore brush as your swabbing tool. Put that on the end of a separate ramrod if you can. Then wrap your damp patch around the bristles. As you swab down the bore, the bristles move into the rifling and scrape some of the fowling that is attempting to accumulate there. A patch or two like this every fifth shot, and you will see the difference. After you brush swab the barrel, then run with your cleaning jag with another damp patch and it will pull even more from the barrel. This allows some rifles to shoot longer on the range with better accuracy.

Another theory of mine is barrel heat. As the barrel heats up, it changes. As we know we swab and shoot but lots of times that barrel is really hot. Grab a hold of them sometimes. They will be very uncomfortable. Especially for some of you that live in areas of the country where it never cools down.

So I take a coupe rifles with me. I shoot one, swab it, walk down and check the target (need the exercise anyway) putter around marking holes, enter my notebook, walk back, run my dry patches down the swabbed barrel, then set it off to the side and shoot a different rifle. All of this gives the rifle time to return to normal or as close to it as we get. So there is a good reason to tell the wife or other half, why you need more guns... Some people even go so far as to take a cooler with ice and water in it, and towels. They dampen the towel with ice water and wrap the barrel to cool it down faster. I personally have never done this, but do understand their reasoning behind it. Also keep your sabots and bullets in the shade when ever possible. Set then sabots out on a hot shooting bench when the temperature is high sometime and see if they do not load or feel different when loading.

Still another is, the longer I am at the range, the more I find myself checking my position behind the firearm. I am actually shocked that I am suddenly giving the death grip on the rifle instead of a controlled grip. I think this is due to muscle fatigue, strain, and a desire to match the accuracy of the first shots. Perhaps I could call it over concentration.

That's when I get up, walk around, have a soda pop, throw the stick for the dog, but the big thing is I calm down. It is too easy to get into shooting. And you have to step back sometime to really examine what you are doing. Check yourself sometime. Have you moved forward on the seat? Are your elbows flatter on the rest table? Is your cheek in the same spot on the rifle or are you moving back and fourth tying to get that perfect scope picture by adjusting yourself on the stock. I could especially notice this at longer distance where, because of optics, you have to take care not to over adjust. Now with open sights, I do not find this as such a problem.

Another way to check your accuracy sometimes is start at the longer distances first. See what happens there. Then move in to the target. After all the shots and optics get better as we get closer, there is less margin for error. Just some of my thoughts.