[FlyFish]

I've spent my career in environmental research, which necessarily involves experimental design and statistical analysis. I just wrote a few paragraphs on both of those things that would have been well over the head of the average grad student, let alone a high school freshman, and would have also made me sound like a pompous ass, which I try not to be. So, I've erased all of that and will offer a few more appropriate (I hope) recommendations.

First, don't worry about your original question - you can measure your variable of interest in MOA or, more easily, simple linear dispersion on the target in inches, millimeters, or whatever. A basic rule of science is that results are independent of the units of measurement.

Second, you're attempting to determine the spread (deviation) of groups of shots using different bullet weights while keeping all other variables constant, or at least assuming you can and therefore throwing any of that additional variability into your "error" term. What you're measuring is precision, not accuracy. Precision is a measure of repeatability, accuracy is a measure of comparability to a known correct value (in your case, the aiming point). If one bullet weight shoots smaller groups than another it's more precise, not more accurate. You improve accuracy by moving the sights. Yes, just about everyone uses the terms incorrectly. (Edited to Add: But not Buzzcook, who posted a good description of the difference while I was typing)

Third, don't use group spread as the measure of precision. It has a number of flaws, notably that it only uses "information" from two shots, is highly dependent on low probability events ("flyers"), varies with group size, and can never become smaller as additional shots are added to the group. The pros use average deviation (it has other names as well), which is the average distance that each shot in the group falls from the group center. When I test loads, I measure each impact as the x,y distance from the aiming point and then it's a simple matter to calculate the center (average x, average y) and then, using the Pythagorean Theorem, the distance of each shot from that center. You can quickly and easily set up a spreadsheet do the calculations.

Finally, you obviously will see a difference between bullet weights. The question, however, is whether that difference is real or whether it's simply due to chance. That's what statistics can tell you. Test your results using some simple statistics, like the t-test (if you only have two bullet weights involved) or the somewhat more complicated Analysis of Variance (for three or more), or their even simpler analogs, (for example) the Mann-Whitney or Kruskal-Wallis tests. You can find them in any introductory statistics book, or on-line, or perhaps your teacher can help you with them (hope so!).

Good luck - please come back and share your results with us.