To answer your original question, it is highly unlikely that your gun will shoot both bullets with the same POI, since that is such a drastic change in bullet weight.

I think it's good advice to find something in the middle that your gun will shoot well for both.

What is the issue? If one sights in the 55 for 300 yards, the gun is sighted in for the 100 at what, 200?

I have the feeling that I am missing something? You can't just shoot the 100 and figure out what distance the sighting is good for? It seems like one would want the 100 to be sighted in for a shorter distance regardless.

What is the maximum distance you will shoot with either bullet?

And I have heard a number of posters that have an affection for the .243 say that they use an 85 or 87 grain bullet for everything.
Maybe that would be the ticket for you.

Unfortunately it doesn't work that way.

When you are talking about high velocity rifles and various weighted bullets, the rate of twist in your barrel will determine how heavy of a bullet your gun will "stabilize". For instance, if you have a slow rate of twist in your barrell (i.e. 1 in 14 or slower), your gun will shoot lighter bullets much more accurately than it will heavier bullets. If you have a fast rate of twist (i.e. 1 in 8) it will stabilize heavier bullets much more effectively.

With the 2 bullets you discribedbeing so far apart in weight, more than likely your gun will not shoot both bullets well, much less have the same POI.

I've seen guns shoot heavier bullets with a POI that is higher than a lighter bullet. I don't know that I have an explanation for that but I have seen it.

johnny thats a great idea!!