I've noticed that if I'm shooting at a target with a well defined "bull's eye", then I can keep a fairly tight group in the center (with maybe one or two flyers). But, when I am shooting silhouettes, which do not have a well defined center, my groups get wider.

Example: Last Sunday I had one of my recent acquisitions at the range and I was zeroing the Aimpoint that's on it. I was using a scope target (grid with cross hairs and a diamond in the center. When I kept the dot from my aimpoint on the center between the cross hairs I was able to put all of my rounds into the diamond (once I was zero'd). Then I switched to a silhouette (which had rings for bull's eye, 10, 9 ,8 and 7, but they could not be seen from 50 yards). After shooting a couple of mags at the silhouette, I found that my group had doubled in size. All of the shots were still COM, but my accuracy definately took a hit.

My hypothesis is that on the silhouette there isn't the same consistent point of aim that there is on targets with a well defined bullseye, so I tend to drift around a little more. Anyone else ever experience this?

Aim small, miss small.

Actually, I'm the opposite.  I find shooting at a silhouette focuses me, and, I get my
groups tighter.  

I shot drug and armor drills at the range a little bit back and, I was amazed at my
accuracy.  Shooting at a bullseye target my groups actually drifted.

I'm sure it's a vision discrimination things.  

Your way sounds more normal than mine though.

Size of target versus size of group is basic theory.

Quoted: I've noticed that if I'm shooting at a target with a well defined "bull's eye", then I can keep a fairly tight group in the center (with maybe one or two flyers). But, when I am shooting silhouettes, which do not have a well defined center, my groups get wider. Example: Last Sunday I had one of my recent acquisitions at the range and I was zeroing the Aimpoint that's on it. I was using a scope target (grid with cross hairs and a diamond in the center. When I kept the dot from my aimpoint on the center between the cross hairs I was able to put all of my rounds into the diamond (once I was zero'd). Then I switched to a silhouette (which had rings for bull's eye, 10, 9 ,8 and 7, but they could not be seen from 50 yards). After shooting a couple of mags at the silhouette, I found that my group had doubled in size. All of the shots were still COM, but my accuracy definately took a hit. My hypothesis is that on the silhouette there isn't the same consistent point of aim that there is on targets with a well defined bullseye, so I tend to drift around a little more. Anyone else ever experience this?

My experience to a "T."

That is why a practice for hunting season on lifelike target pictures. Hitting a well defined aiming point is fine for load development and zeroing, but nothing beats shooting at a realistic target to find out how well you can practically shoot.

Quoted: Aim small, miss small.

YES

Put a black circle @ center-mass on your man shaped target...?!  

Well,a bullseye is made for just that purpose. It makes you concentrate more on the center without thinking about it. A sillhouette doesn't. Now one thing I concentrated on for competition was hitting the X ring and that was it. There was nothing else I wanted to hit so I didn't look at anything but the X ring.

It's all about focus.  On a large ambiguous target, you have to focus on getting the "picture" surrounding the sights identical for each shot, or shoot at a discrete point such as a corner.  With iron sights, you must focus on the front sight for every shot - if you alternate your focus from the front sight to the target for each shot, the group will get larger.

I had an instructor tell me years ago that once you put a hole in the target, aim for the hole and not the target

 A smaller aiming point results in a smaller group. On a silohuette you have to pick your own aiming point.  Just like you shoot a spot on a deer, not just the deer. One of my old instructors used to say "Shoot em in the Kools" meaning aim for the cigarette pocket area on someone's shirt.

Quoted: Aim small, miss small.

According to an awful mel gibson movie