I'm not a precision shooter or anything nor is my rifle anything special (stock MP10). Mainly, I handload and I'm obviously looking for the best load for a given rifle - in this case the MP10t. To that end I've been playing with a free program I found called Taran. It gives some nice stats.

I've been shooting about 20 rounds of each load (sometimes 5 shot groups sometimes 10 shot groups) and then using the software to overlay groups and calculate the data. CEP seems to be a pretty common measure of precision (called R50 in this software). Is that what I should be looking at or is something else better? In other words, if one load gives me an R50 of .40 and one .50 should I assume the .40 is the better load? All things being equal of course. Some (much) of the error could be me or weather (I try to test at an indoor range when I can).

While more shots is better, is 20 enough to give me a good idea of a load's accuracy? Once I dial into the better loads I can retest but I'd obviously like to weed out the poorer loads pretty quick.

Here is a sample of what the software shows.

The question of how many shots per group is going to bring ALOT of opinions.  some say 3, some say 5, some 10.  For me, i do 5 shot groups when I do work-ups.

As far as what is "precision", Now I think about it, im sure that could really depend on the shooters answer and what they are looking for.  For me, its best (smallest) groupings.  I will then take that load and start tweaking it (maybe go up/down a grain or 2 and see if velocity and/or SD changes for the better.  Mostly I am looking for the best, CONSISTENT, round for my rifle.

Differently than I do accuracy

Well shots per group wouldn't matter much if overlaying in software. You could do 20 single shot "groups" and have more or less the same overlay. Might introduce some measurement error with point of aim or something though.

30 shots would be better than 20, but is 20 enough to make a decision? And what measure of precision would you use to make that decision?

Do this: find the minimum and maximum load for your powder/bullet combo.  I wouldn't necessarily start at the minimum, maybe 2 grains into it (i.e., if min is 40grs and max is 45, I'd start at 42 or so).  Load one round at 42.2, then 42.4, 42.6, basically, adding .2 until you get through to your max.  Strap a chrono on the end of your rifle and go shoot the rounds.  Wherever you find a small variance in velocity, exploit that charge weight.

Example:
42.0 - 2700
42.2 - 2710
42.4 - 2718
42.6 - 2722
42.8 - 2723
43.0 - 2742

The change in velocity between 42.6 and 42.8 only added 1fps.  With that info, you'd probably want to load 10-20 rounds at 42.7 and see how they shoot.  Adjust seating depths after that, because if you have a capable rifle, I would almost guarantee that load will perform very well.

Scott Satterlee developed this method and it has worked amazingly well for me.  It is so damn easy and has proven very effective.

Another note is the simple fact that you may find multiple nodes in your load string.  With my last rifle, I had three different nodes, so I loaded up 10 rounds of each (seating just .020" off the lands) and each load shot sub-half minute.  Kind of cool because it gives you some options, I suppose, but then again, one of those loads put a 5 shot group at .18" and I am not the best "group" shooter, either.

Scott, if you're reading this, thanks again.  His method has made finding a load super easy.

Interesting. Thanks for sharing.

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Precision is applied to the shooter, rifle and ammo. I'm asking primarily about ammo.

http://ballistipedia.com/index.php?title=What_is_Precision%3F

Attached File

When comparing two or more loads average to center is better than extreme spread.
I like to shoot at least 6 or 7 shot groups multiple times, multiple trips at a size to be assured that it is capable of that size. I like to shoot 15 to 25 shot groups too to get an idea of worst case scenario. There comes a point where one can no longer concentrate properly and start screwing things up so....

As to precision required, that is up to you and your shooting needs.
If you are shooting 1k yards trying to hit a 3moa plate then 1 1/4 moa will be fine.
If you are trying to hit a 1/2 moa plate at 400 yards then you need near half moa or better precision.

This is the method I use as well.  It might not be the absolute smallest group at 100 yards, but for me I need the load to be precise down range.  It saves a ton of time headache and barrel life.  Working up loads for 6mms gets old quick.

I just load 5 of each load for test then shoot 5 let the rifle cool etc.

Example Max load for said rifle with bullet I want is 70 grains of powder. I'll load 69,68,67 and 66, 5 each. Then go shoot. If necessary I'll go to half, then 1/4 grain to fine tune. Looking for tightest group, don't give a crap about velocity.

I'll change bullets and seating depth to get the accuracy or performance I want. Don't over complicate things.

If my bullet causes a precise kill on its intended target....preferrably on the first shot.