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Just How Accurate Is Your Load?
Everyone likes to shoot small groups. But just how accurate is your load? Do you shoot three shot groups? Do you shoot ten shot groups?
Just how accurate a rifle shoots is most likely the area where most shooters tend to exaggerate to some extent. Everyone remembers the smallest groups but most are quick to forget the larger ones.
As groups are simply a series of bullet holes in a target, you would think that it is quite easy to determine exactly how accurate a particular load is compared to another.
This is not quite the case. Random selection and statistical variation tend to make the process complicated.
It is generally accepted that the more shots you fire, the larger that the group will be.
Now the most accurate group, will be the one consisting of only one shot! We as long range shooters rely on this one shot group to make first round hits on small targets at long range. But from a load development point of view the one shot group is of limited value.
Where it is of value is in determining the point of impact of the first shot from a totally cold bore. Or from a cold clean bore which is a completely different situation that generally involves a shift in the point of impact.
In the case where the shooter is trying to determine the point of impact of a cold bore shot, from a fouled barrel, the shooter will fire one shot and then put the rifle aside to totally cool before firing a second and subsequent shots. These shots will form a group.
In the case where the shooter is checking the point of impact of a cold bore shot from a clean barrel*, the rifle will be both cooled and cleaned between shots. These shots will form a group that may be in a different location to the above group*.
Knowing all this information is important in making first round hits at long range, as this information can be used as your known zero when calculating the dial up correction required.
Now lets look at sighting in, group size and load development!
As bullets tend to land in a random manner on the target. The centre of this randomness is called the centre of the group. The idea is to place the centre of the group at a particular location relative to the aiming mark. Once this is done, the rifle is "sighted in" and this is location relative to your aiming mark is called your zero.
It is most common for rifles to be "sighted in" so that the centre of the group is either "dead on" at a particular distance or slightly higher than the aiming mark at a particular distance. This is really a personal preference and can be compensated for when calculation your shot correction.
Fire ten shots at a your target at 100 yards and then go and have a look at it. The bullets will have landed in some form of a random pattern. Hopefully in will be a very small random paten!
Now pick any three bullets holes. These three holes, could have formed a random three shot group, had they been fired as such. So you could pick the holes further apart or three that were touching each other. This proves that single, three shot groups are useless for determining the accuracy. Its all up to chance which ones land in what order.
You could also have chosen the three holes on the left or the three from the right. So just where is your zero? Left or Right? Same can be said for picking the highest and lowest three holes. There is just not enough statistical information, provided by single, three shot groups to be of much value.
So if a person that tells you he has a 1/4 MOA shooting rifle and shows you a three shot group to prove it. Just smile and say well done, knowing that it is most likely just a statistical aberration.
Now having looked at statistical reality of shooting groups. Just how many shots should you fire to determine the most accurate load?
This will depend on the individual rifle and the type of shooting that you intend to do with it. I am not going to give you an exact answer to this question. It is up to the individual to determine this. Just make sure that enough shots are fire to make the sample statistically relevant.
If the rifle has a heavy barrel and is to be shot in competition, where 12 shots needs to be fired within a 15 minute time limit, then the number of shots and the time taken between shots to allow barrel cooling should reflect this.
If the rifle is to be used for hunting or field shooting where the barrel will be normally be fire cold. Then the group shooting should reflect this also.
The important thing to remember, is that a good load, is a load that is consistently accurate. Not one that produces a screamer of a group every now and then.
Taking this into account. If you fire a three shot group with a particular load, and the bullets are landing all over the place. It is safe to assume that the load is not going to be very reliable and therefore can be rejected. Of the two loads tested in the photo above left, the top load would be rejected and the bottom one could be used as a guide for further development.
Three shot groups are good for finding the bad loads, but are not reliable for finding the best loads. If you do shoot a tight three shot group, you must follow it up with more shots, to prove that it just wasn't a random fluke. The group shot in the photo above, right, contained 5 shots and this gives a far better indication that the load will provide consistent accuracy.
The more shots you fire with a particular load, the higher your confidence level will be that the load will be truly accurate.
Now you don't want to burn out a barrel before you find an accurate load. So use three shot groups to cull the bad loads. Then move onto 5 shot groups to sort out the best two loads. I then shoot these two loads at long range and compare the vertical component of the groups.
A load that has a high velocity spread will have a larger vertical component to the group size than a load that has a very low E.S.
This is something that the rifle manufacturers like to use as a marketing tool to sell rifles. "If your new rifle does not shoot 1 MOA at 100 yards we will replace it!" Yeah sure! Just try and claim on that guarantee!
distributor will ask for the rifle and then at your cost shoot the rifle at a
range. If he shoots a three shot group 1 MOA or less, you get the rifle back
with a bill for his ammo and time. He may claim that you are the problem, not
the rifle. Now most rifles will shoot Ok from new, and if the bloke fires enough
shots, sooner or later, three of them will land near enough together to form a 1
MOA group. So be wary of the 1 MOA claim.
The light weight sporter.
firing three shot groups with a typical light weight sporter. The groups quite
often have two holes close together and the third hole that is wide of the other
two. Firing more than three shots with a hot, thin barrel is pretty much a
waste of ammo and time. This is quite often due to the thin barrel heating up
and changing the point of impact of the shots. Letting the barrel totally cool
between each shot is the best way to test a light weight sporter for accuracy.
* A clean bore may result in a lower muzzle velocity. So you need to consider this if you expect to make first round hits at long range.
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