To expand on what Fast14riod said, I'll add this: As he said, the most important single thing to improve precision is to know how your loads shoot. And by that I mean, really know and understand what's going on. I know some reloaders who have all the gear, all the skill, and use the best components. Yet they don't know how their "good stuff" actually performs because they measure their groups with a coin or a carpenters tape and scribble some stuff in a rumpled old notebook.
I recommend something similar to what I do. I make a load sheet for each batch of ammo using M.S. Word. The data includes just about every specification for the bullet, case, primer, seating depth, case volume, etc. I also add a comment section and when I return with the results I add the collected data along with more comments and usually a graph or two. Then I print the page and keep it much like many people keep a hand written notebook. I try to log each and every thing I might wonder about if I wanted to duplicate the recipe in the future. I number these recipes with a sequence number unique to each gun I shoot. The subsequent load sheet amounts to copying a similar sheet, renaming the file, changing the date and a couple of other pieces of data. That's a lot easier and/or more informative than a hand written range log.
I print 10 targets on letter sized card stock with an offset POI. I chronograph every round. After a range session I scan the pages into the On Target scoring software where I mark and measure each bullet hole along with comments about that particular group; i.e. recipe number and whatever I'm testing that day like charge weight or seating depth.
I pay the most attention to the Mean Radius of 5 shot groups at 100 yards in order to develop competition ammo for 600 yard F-Class matches. Calculating Mean Radius is not easy if you do it by hand, but On Target provides it automatically. Of course, I also study group size (converted to MOA), MV, and SD.
The On Target software lets me keep a file for each range session as an Excel worksheet where I can add MV data, powder type, etc. Then I can sort by MOA, or Mean Radius, or Group Height, or whatever and study the day's data in order to try and make sense of what happened. That way I can see, for example, which charge weight produced the lowest Mean Radius for every round fired from that barrel. Or, if I want, I can see how SD relates to Muzzle Velocity during the past month. Most times I ask Excel to produce a graph which is particularly useful when doing load work ups.
I copy that day's data and merge it with a master Excel spread sheet for each gun. I can easily tell you, for example, how the average MOA or Mean Radius of 80gr Berger VLD bullets using Varget compares to 90gr Berger VLDs using Vihtavuori N-140. In a few seconds I can tell you which powder produces the highest MV. Or how neck tension relates to SDs. In other words, I can study any parameter having to do with precision and get a good idea of what works and what doesn't and how one load compares with another.
If you don't know exactly how your best loads perform and, more importantly, how they compare with your other loads, then you can't fine tune and develop a winning recipe. When it comes to shooting matches, those guys who did a little casual testing and found a recipe which shoots "pretty good" are not usually the ones taking home the trophies.
