Shooting over a chronograph is not going to tell you anything about what your group size will be down range-- ONLY that your load and loading procedures are consistent in generating velocity. Shooting Groups at 25 yards is mostly a waste of time, because almost all guns, and shooters can put bullets into " one hole " at 25 yards, using any kind of rest. ( Even me! :rotf: )
If you are trying to work up accurate loads, you first check WHERE the ball is hitting ( POI) at 25 yards, just to see if you have a large Windage, or Elevation problem to fix to get on the paper at longer distances!
Then move back to 50 yards. At that distance, using open sights, Good vs bad groups are going to begin to show up. When you isolate the good, Move the Target BACK to 100 yards, and shoot there. I have seen many good 50 yard shooters who could not hit the paper regularly, from a rest at 100 yards, largely because of the inconsistent way they loaded their rifles, and even MORE INCONSISTENT way they cleaned( didn't clean) their rifles between shots.
Now, using a chronograph set 15 feet from your muzzle is NOT going to tell you any of this other information. The Suggestion that a chronograph will do this for you is a bit of a Red Herring, IMHO. With an appropriate deflector plate, or shield, using a chronograph at distance, ( 50 or 100 yds) to determine exact velocity of each shot is the BEST and fastest way to determine how YOUR inconsistency in loading and cleaning your gun causes a change in the POI. Sometimes you can't tell. Its true that the way you hold the gun, the position you take with your body, and all the things needed to fire a gun accurately, from your eye and corrective lens, head position, hand positions, breath control, trigger squeeze and release, follow through, etc. all affect where that ball or bullet will go. Sometimes a shooter simply cannot see what he is doing wrong. Asking another experienced shooter to coach you as to shooting form will often improve your groups.
I have found one inconsistency that I Know most shooters do not address, and that is the nasty habit of the brain wanting to look at the target at the last split second, throwing the shot up and away from the main group. This happens with all kind of sights, so changing to a peep, or scope is not going to change this. Finding a way to focus on that front sight through your follow-through makes the difference between being a great shooter, and being that outstanding shooter who always seems to be in the winner's circle.
I have seen shooters who just STOP bouncing their ramrods on their PRBs see their groups at 50 and 100 yards shink, when the practice had no effect on their groups at 25 yards. I have seen shooters change their cleaning practices to clean their barrels thoroughly between shots, instead of NOT cleaning, see their 100 yd. groups shrink to 1/4 the size they were getting. NOne of this is diagnosed by using a chronograph.
You can spend a lot or a little on a chronograph. The Chrony chronograph still costs less than $100 from Sportsman's Guide. Competition Electronics, makes a good chronograph for about $125 or so. If you are talking about the OEHLER chronograph, it is really intended as a laboratory/ indoor shooting range testing machine, and its very expensive. I just checked, and the unit costs $1,800! Pact has a basic model for $169. RST has a basic unit at $199.
You need to do some comparative shopping on FEATURES provided by these companies to figure out what you need, and which machine is the best value for those needs.
Groups size as a measure of accurate loading and cleaning and shooting procedures has been done for hundreds of years. I worked up my best loads doing this for years.
Since I have a chronograph, I am able to find out what exact velocity is the " Sweet spot " for my given gun, and when changing components to work up a different load-- say, changing from FFFg to FFg powder, just to see if I can get better groups--- The chronograph allows me to quickly adjust the load so that I am shooting the ball at the same velocity to get into that Sweetspot. Only then, will I learn if the change in a component is an improvement, or a disaster, AS A RESULT OF THE CHANGE.
For instance, I tried some Walter's Fiber Wads as an OP wad in my .50, and my groups at 50 yards immediately began hitting an inch higher than when I was NOT USING the OP wad. I need to run that particular combination of components over the chronograph, and then shoot my normal load, to see if, and what change there is in the velocity. If I am getting more velocity, I will drop the powder charge amount down until I find out what load to use with the OP wad to get the same POI, since I don't want to change my fixed sights. Then I will decide, by shooting groups with both combinations, which gives the best accuracy for me, and decide if I am going to change my loading procedure and components to include an OP wad, and a reduced powder charge for future shooting.
Finally, I will do a comparison penetration test, to see if there is any difference in penetration performance of my PRB( There should not be.)
Chrnographs are not magic. They do what they do very well. The information they give the shooter is very useful, WHEN THE SHOOTER UNDERSTANDS FULLY HOW TO USE THE INFORMATION. Determining effective group sizes at any given range takes more, and a different kind of load work-up.