There could be many things causing a flyer and narrowing it down can be exhaustive.
Assuming the rifles are adequately bedded, all stock and scope screws are properly torqued, quality components and procedures in the reloads, I'd examine the easiest element first, my shooting technique and consistency.
The reason I point to this element is that you're getting 4 shots in 1/2 moa and there is one flyer in the 5 shot strings that's expanding the group. If you could get an experienced bench shooter to observe you shoot, you might be able to identify the problem. Do you call each shot? If you do, how's that going, are you fairly accurate in calling the shot? I can almost always tell when I pull a shot. 100 yards is a fairly modest distance so if it's shooter error, an experience bench shooter might be able to identify a technique issue since it would have to be fairly noticeable to show up at 100 yards.
Don't discount the mental aspect affecting your performance. Most of us experience this from time to time if something goes bad. We tend to lose confidence. A frequent approach to this for new shooters is to get an experience shooter to shoot a 5 shot group with your equipment. If his groups are tight, then you've identified the source of the problem. This should give you some ease knowing that the equipment is capable, you just have to work on your shooting.
Shooting, even off a bench, is ultimately an athletic event requiring concentration, breath control, muscle control, mental control, and most importantly training and practice in order to make repetitive good shots.
Many shooters I encounter at the range have little to no training in the fundamental of shooting. Not saying you're one of those, but there is no substitute for leaning the fundamentals and practicing. Equipment can only do so much; the shooter has to do their part. If it just was equipment, all of us would be world class shooters. But those top guys have mastered the fundamental of precise shooting.
