GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on February 24, 2014, 01:54:51 AM
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I have had a request to explain my view of how the Extreme Spread (ES) of the velocities in a PCP shot string affects the accuracy.... and why I typically use a 4% ES as my baseline to determine how many shots to include in the string.... This is for the noob's primarily, to help them understand why this is important....
First of all, what is the ES?.... It's the difference between the Muzzle Velocity (MV) of the slowest shot in the string and the fastest.... expressed as pure fps, or as a percentage of the fastest shot in the string.... Let's assume the fastest shot is 1000 fps.... If the slowest shot is 960 fps, that is 96% of the fastest, and the difference of 40 fps is the ES, which in this example is 4%.... A 3% ES (30 fps) would have the slowest shot be 970 fps (97%), and a 2% ES (20 fps) would mean the slowest shot was 980 fps (98%).... If the slowest shot was 900 fps (90% of 1000) the ES would be 100 fps which would be 10%....
Now, how does that relate to what happens at the target?.... All other things being equal (and they never are), the faster a pellet leaves the muzzle the higher on the target it will impact.... The ideal would be for all pellets to have exactly the same muzzle velocity (zero ES) but even regulated guns are usually around a 1% ES, and sometimes more.... If the pellets are different in weight, just that will cause a slight difference in MV.... The same goes for a difference in how they fit the bore, how hard the hammer hits the valve, and a myriad of other factors, so IMO a 1% ES is about as good as it gets most of the time.... For unregulated PCPs, in an ideal tune, the MV starts out a bit slow, rises to a peak, and then drops again.... It is up to the shooter how far he will allow that drop to be, ie how large an ES he is prepared to accept.... Once he decides on a given ES that he will accept, that in turn governs how many shots he will include in the shot string.... and that in turn will determine his fill and refill pressures at that tune.... Those pressures are chosen to exclude shots outside his acceptable ES.... If the fill pressure isn't what he wants, then he will retune the gun accordingly....
Let's look at a pellet with a good Ballistics Coefficient (BC) and see what happens at 50 yards with slight changes in velocity.... We'll pick a pellet with a BC of 0.035 and a muzzle velocity of 950 fps as the peak of the shot string, and a 4% ES, which makes the slowest shot 96% of 950 = 912 fps.... We'll sight the rifle in so that it is dead on at 50 yards at 950 fps with that pellet, and then use ChairGun to figure out where the same pellet travelling at 912 fps will hit.... You can do that by calling up a column in the table called "Drop", which is how far the pellet is below the boreline.... The number opposite 50 yards is the drop, so the difference in the drop is the difference in Point of Impact (POI) due to the 38 fps (4%) difference in velocity.... In this case, the pellet travelling 912 fps drops 5.920" and the one travelling 950 fps drops 5.456", and the difference is 0.464", so just under 1/2".... I don't know about you, but I don't generally shoot 1/2" groups at 50 yards, so for me, that indicates that the effect of the velocity difference on the group size falls within what the group size is likely to be.... That is why I consider a 4% ES to be OK out to 50 yards.... Doing the same thing at 75 yards with a 3% ES gives a drop of 13.891" at 922 fps and 13.084" at 950 fps, a difference of 0.807".... and using a 2% ES at 100 yards gives a drop of 25.834" at 931 fps and 24.811" at 950 fps, a difference of 1.023".... If we check to see what an 8% ES does at 25 yards, we find the drop is 1.513" at 874 fps and 1.280" at 950 fps, a difference of 0.233"....
Those examples produce about a 1 Minute of Angle (MOA = 1" at 100 yards) change in the POI at their respective yardages.... To summarize, for a roughly 1 MOA vertical dispersion, you need to stay within an ES of:
8% at 25 yards
4% at 50 yards
3% at 75 yards
2% at 100 yards
If the velocity is higher, or the BC is greater, then the difference in the vertical stringing is less.... Conversely, if you are shooting wadcutters at 600 fps at 25 yards, you will want to stay within about a 4% ES or you will start to see vertical stringing in your groups.... Perhaps now you can see why I always use 4% as my maximum ES when I am counting up the number of shots in my strings, and those are the only shots that I include on my graphs.... That 4% works for pretty much any shooting on "normal" targets, for me, out to 50 yards, with the appropriate pellets.... If I'm concentrating on longer ranges, then I tighten up what I look at for my ES, and at 100 yards I insist on a 2% ES or less....
Bob
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Very well written and understandable Mr. Bob.
Richard
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Thanks
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Clear. Concise. Perfect.
Thank you for the great write-up and example.
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Excellent write up, it really helped me to better understand how to nail down an acceptable shot string. Thanks!
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awesome write-up! It takes some time and effort to put that much information into a post and to explain it so clearly. Nicely done.
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Thank you for such an understandable and informative post.
Brett
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If just going for group size, the news is even better.
A rifle with a reasonable shot count seldom has a large % difference within any set of 5 shots. OVERALL it might be a 4% change, but within any set of 5 shots, the change is usually a whole lot less. The center of the group may be a bit higher or lower, but the 5 shot group will still be about as good as the gun can do.