• Site updates are now live!

    If you see any issues, click on the VIEW THREAD button below to report them.

    VIEW THREAD

  • In order to be able to send private messages to other users, you must have a minimum of 10 posts on the site.

Does a flatter trajectory translate to a more accurate round?

Michael

Administrator
Staff member
Mar 25, 2015
3,893
222
Dripping Springs, Texas, USA
    We all know that different pellets like different speeds out of different barrels. But let's assume that most of the variables are the same.

    Use this basic example:
    Gun "A" is most accurate shooting pellets at 800 fps
    Gun "B" is most accurate shooting pellets at 900 fps

    Both guns shooting the same distance with the same weight pellets.

    Is gun "B" more accurate because the pellet has a more direct path & drops less in flight?
     
    Nov 15, 2015
    139
    12
    Idaho
      "AZBOBCAT"If you had zero wind I would say it doesn't matter what the fps is. Now if we are talking real world with wind the slower round would have more time to target to be acted upon by the wind,.
      
Not quite how that works. The wind effect minimizes at about 800 to 850 ft/s. When your pellet or bullet goes faster than that there is more drift due to wind, until it is pushed at a bit above 2000 ft/s. The wind drift maximizes at about 1500 to 1700 ft/s and is more dependent on shape than at lower velocities. 

      At extreme ranges (greater than 100 yds for a pellet), then you want a subsonic pellet or bullet to have the average speed between that 800 to 850 ft/s for the minimum wind drift. 

      The reason for this effect is that the drag on the projectile goes way up as the speed approaches the speed of sound. Below Mach 0.8, air can be treated as a non-compressible fluid. Between that and about Mach 1.2, air is semi-compressible, while above Mach 1.2, it becomes a compressible fluid.
       
      • Like
      Reactions: fe7565

      WCT_Editor

      Member
      Jul 10, 2015
      116
      16
      Ohio
        My answer would be maybe, but maybe not. There are too many factors that cannot be accounted for even if we say same pellet function (head size, weight, skirt expansion, etc.), identical shooting conditions, identical distance (meaning that the actual distance from the barrel to the POI is exactly the same), identical function of the mechanical components, etc. In order to make the determination, you'd need a large data group to compare.

        Just because one pellet has a larger arc than the other, won't always make it more or less accurate. After all, this should already be accounted for with the zeroing of the scope. I see this a lot in bow hunting articles. Everyone seems to focus on the FPS, and while it is true that an arrow shot at 325 FPS will arrive quicker to the target than one traveling at 275 FPS, there is still so much more to accuracy then just speed. 

        Ultimately, I don't feel you can say that FPS is the determining factor with accuracy, rather it is just another component to the overall picture.
         
        Jul 21, 2015
        151
        1
        Colorado
          I think there might be another factor at work here that would tend to make B more accurate. In traditional firearms we call it "lock time"; the time it takes from the moment the trigger is pulled until the firing pin hits the primer and sends the bullet on its way. It has been demonstrated repeatedly that faster lock time equates to better accuracy because it reduces the time the shooter's movements have to influence accuracy. Similarly slower pellet velocities provide more time for such factors as heart beat, small muscle movements, etc. to negatively effect the ability of the shooter to hit his/her mark. So unless the gun is locked into a gun vise and is being fired remotely then B would likely be more accurate than A.
           
          Nov 15, 2015
          139
          12
          Idaho
            Usually when I read the word "accuracy" I believe that the writer generally implies precision. That is, instead of hitting the target, the writer wants to know about group size. This is how most folks interpret the term. But, accuracy really means being able to hit the target at will, while precision is the less ambiguous term for group sizes.

            However, if we are looking at being able to hit a target at various ranges, then a faster projectile will have a flatter trajectory, and thus it's point blank range will be greater. That is, the distance between where the trajectory intersects the bottom of the "kill zone", does not rise above it, and where the trajectory again drops to the bottom of the "kill zone".

            The effect of wind, as I stated above, still holds, in that if you do not hold off or adjust sights for wind, then hitting the target "kill zone" will be easier when the velocity minimizes the wind drift.

            Most folks put the crosshairs at the center of the kill zone and base the point blank range on that. The projectile crosses the line of sight at two points, while the extension of the point blank range is where the trajectory intersects the bottom of the "kill zone". This is what programs like Chairgun show. 

            However, one can use an old iron sight technique to extend the point blank range: Hold the crosshairs at the bottom of the "kill zone". Now your point blank range is the near and far zero, but the apex of the trajectory is now the full height of the "kill zone" not half of it. To further increase the likelihood of hitting the target is knowing what the distance at the apex, and both the near and far zeros look like in the field. When your target is close to the apex range, hold a little bit lower. If it is close to the near zero, then hold a bit higher. If it is around the far zero, consider getting closer if you can. Otherwise, also hold a bit higher. 

            To use Chairgun, or other programs to find out what this "extended" point blank range is, double the diameter of the kill zone. The extended point blank range will now be the near and far zeros. Ignore the extensions before and beyond these points. You will find that the effective point blank range is extended. It will require a retraining on your part to hold at the 6 o'clock of the "kill zone" to use this technique effectively.
             
            Nov 15, 2015
            139
            12
            Idaho
              Let's look at lock time a bit more closely.

              First, assume that the acceleration of the pellet in the barrel is constant. It isn't; it accelerates much quicker at the beginning and has little at the end, but we will disregard that for this discussion.

              Given a 24" barrel or two feet, the 800 ft/s pellet has an average velocity of 400 ft/s and takes .0050 seconds or 5.0ms to traverse the barrel. The 900 ft/s pellet traverses the barrel in .0044 seconds or 4.4 ms. This is a difference of 0.6 ms. 

              Now let us look at the hammer in a couple of hypothetical, but realistic PCP rifles. The first has a hammer that has a mass of 150 grams, while the second has a mass of 100 grams. The first one has a hammer stroke length of 2.0 cm, while the second has a stroke length of 1.6 cm. Let us assume that they both have the same hammer energy (for this is what knocks the valve open, not the momentum.) The hammer energy can be expressed by force x distance. 

              Let us now assume that the average force on the heavier and longer stroke hammer is 45 newtons. This works out to be 0.9 N-meters or joules. The other hammer has an average force of 56 N. to make them the about the same.

              Let us assume also that the acceleration of the two hammers is constant. The first one will have an average time from trigger release to valve strike of 1.3 ms , while the second hammer spring system will do it in 0.57ms. This is a difference of 0.73 ms.

              The hammer time needs to be accounted for as well as barrel time if you are worried about movement in half a mili-second.
               
              Nov 15, 2015
              139
              12
              Idaho
                "AZBOBCAT"I like irons that is really odd I can go out on a windy day and shoot at 3 different power levels and the slower speeds see more impact on groups Than the faster speed. Just telling you my results might not be scientific or maybe it is some other force other than the wind, but the target tells the tale.

                What are the velocities? Less than 800 ft/s to 850ft/s will give more drift as will above that. You can check for yourself with Chairgun. Look at the wind drift chart and tables. Compare different velocities for your pellet and see which minimizes the wind at your given target distance. There will be a bit of variability depending on pellet, and range. But, if you are not shooting at 10 yards or 100 yards, you should see where modest changes in velocity does not have a significant effect on the drift. It will be somewhere between 800 and 850 ft/s. 
                 
                I think your wording is throwing me off:

                "Gun “A” is most accurate shooting pellets at 800 fps
                Gun “B” is most accurate shooting pellets at 900 fps

                Is gun “B” more accurate?"

                My two cents says that accuracy, while often dependent on velocity, is only comparable in group size/precision as mentioned above. 

                another way to look at this is to say:
                car 'A' gets optimal mpg at 29mph 
                car 'B' gets optimal mpg at 32mph,

                which car is more fuel efficient?
                 
                Flatter trajectory translates into a more accurate round for someone that doesn't know yardage to target and how to compensate for bullet rise/drop and 'points and shoots'. I used to know many younger shooters that had a rifle zero'd at X yards and basically guessed at where the bullet would land after that. If you know distance to target, winds and accompanying turret adjustments (or reticle adjustments) the relative trajectory doesn't make much difference.
                 

                Kim

                Member
                May 9, 2015
                264
                9
                CA, United States
                  "Bstalder85"I think your wording is throwing me off:

                  "Gun “A” is most accurate shooting pellets at 800 fps
                  Gun “B” is most accurate shooting pellets at 900 fps

                  Is gun “B” more accurate?"

                  My two cents says that accuracy, while often dependent on velocity, is only comparable in group size/precision as mentioned above. 

                  another way to look at this is to say:
                  car 'A' gets optimal mpg at 29mph 
                  car 'B' gets optimal mpg at 32mph,

                  which car is more fuel efficient?
                  
Yes the question is mis-specified. There is nothing in the premise to indicate which is more accurate. A may be the more accurate at 900fps, or B may be at 800.
                   
                  Even a slow and heavy round can be "accurate", but as far as adjusting for the trajectory goes, the slower round is more difficult due to drop. The faster a projectile goes, the less the drop at distance, and thus seems to be more accurate due to the fact that you can put the crosshairs on the spot you want to hit, without much fuss.

                  If velocity did not matter, then ballisticians would not bother with making the more efficient pellet/bullet.
                   

                  Yrrah

                  Member
                  Apr 1, 2015
                  461
                  73
                  NSW, Australia
                    "Michael" let's assume that most of the variables are the same.

                    Both guns shooting the same distance with the same weight pellets.

                    Is gun "B" more accurate because the pellet has a more direct path & drops less in flight?


                    
The assumption: " ...let's assume that most of the variables are the same."
                    The question: ..." Is gun "B" more accurate because the pellet has a more direct path & drops less in flight?"
                    The question implies that gun B IS the more accurate.
                    Therefore we are actually being asked if the reason is that B's pellets, which are the same for both guns, fly flatter.

                    There is much good and soundly based discussion above; and in particular Iron's explanations that, though he has not mentioned the terms - possibly to avoid longer and deeper explanations - relate to "lag" or "delay" time. This phenomenon explains why a slower projectile may deflect less in wind than a faster one. The velocity bracket he indicates takes into account both "lag" time and actual time of flight and can be a wider window than indicated; or can be shifted up/higher or down in velocity depending on drag coefficient. Some of my pellet/rifle combinations have the sweet spot for that window as around 875 fps and extending from approx 850 to 900 fps or a tad higher.
                    But Iron's window is a good rule of thumb for least wind deflection.

                    However because the question clearly states/implies B is the more accurate and assumes most variables to be the same, we have to look at things along with the flatness of trajectory as reasons. Folks above have covered some of those. Again Iron has given an excellent explanation as to why the shooter should consider the bracket between the near and far zero ranges in the deliberations; and has given sound advice as to strategies for capitalizing on the "point blank range" issue. I could add that sight height has some influence over the distance at which the "point blank range" will operate for any given pellet BC and velocity.

                    My simple answer would be: no we cannot assume the flatter trajectory is the reason for B presumably being the most accurate; so must give credence to other reasons some of which have been discussed above.

                    The topic heading is a somewhat different question: " Does a flatter trajectory translate to a more accurate round?"
                    As a statement I will write that it is a mistake to assume that a faster velocity and "therefore presumed flatter trajectory" will always give better accuracy or indeed better precision grouping; and this is especially so when we are shooting at long range where many pellets begin to spiral because of loss of dynamic stability.
                    Such loss of stability seems to me now to be much more prevalent as we get to shooting at above Mach 0.80 and certainly near and above Mach 0.85. This relates to increased static stability and the pellet fighting gravity force in trying to maintain its axis pointing in the original flight direction and not adjusting to the downward trajectory curve after reaching the apex of that trajectory. ... I have many SM videos now to support that statement.

                    I will stop now, I had a cataract operation yesterday and this is a real strain for me to type through blurred vision with many typing corrections. Thank you Michael for the invitation to respond. Best regards, Harry. 
                     
                    Nov 15, 2015
                    139
                    12
                    Idaho
                      Now, what Yrrah brought up about the gyroscopic stability trying to keep the pellet point on as it is falling is something that I've been giving a lot of thought about lately. 

                      A few anecdotes I have read about the FX smooth twist having greater drop, but with better down range speed than predicted indicates to me that most airguns are spinning the pellets faster than need to be---especially for longer range shooting. As the pellet slows in speed, it's rotational speed slows down much less proportionally. By the time the pellet is a substantial distance down range, its gyroscopic stability is now much greater compared to the aerodynamic forces trying to keep it point on. It does not fall as fast because it now has a bit of upward angle of attack and thus has a bit of lift. But, being pointed upward a bit, it now has more eccentric bite to the wind which will tend to cause it to precess and spiral. 

                      The smooth twist of the FX has a much slower effective twist as Yrrah has demonstrated in other posts and videos. This is one reason why, I think, that these barrels give remarkable longer range precision. What is needed, in my opinion, is an airgun barrel maker to make slow conventional rifling twist rates. For .177 for example, I believe something on the order of 1:48 would be quite effective. Of course the slow twist rates would be for diabolo type pellets and not for slugs.

                      However, this discussion probably deserves another thread.

                       

                      fe7565

                      Member
                      Oct 13, 2015
                      1,667
                      46
                      VA, United States
                        Please, keep it coming...there is a lot of excellent info in this thread that would be hard to compile from various sources. 

                        Is there a relation between pellet BC and twist-rate and the resulting pellet rpm? Is there a way to calculate what's the best speed and rpm for a given pellet over a certain range that would prevent spiralling? 
                         

                        Yrrah

                        Member
                        Apr 1, 2015
                        461
                        73
                        NSW, Australia
                          "I_Like_Irons" As the pellet slows in speed, it's rotational speed slows down much less proportionally. By the time the pellet is a substantial distance down range, its gyroscopic stability is now much greater compared to the aerodynamic forces trying to keep it point on.




                          
For those interested in this phenomenon: I did this study quite some time ago.
                          It is a field study on effective spin rates at ranges to 200 yards; Possibly the only such study done on air rifle pellets' range-comparative spin rates. Or for that matter, on actual measured spin rates to 200 yards. It was specific to the pellet under scrutiny.
                          Enjoy please.
                          Quote: Circa 2010.
                          Topic: "Pellet spin rate increase with range to 200 yards study and data ... long with graphics..
                          August 8 2010 at 2:21 AM
                          Yrrah (Login Yrrah)
                          YF
                          0.25 cal JSB King pellets were shot from the BSA JB OZ 1 Hornet through screens to establish spin rates at 2 feet, 75 yards, 100 yards, 150 yards and 200 yards. Pellet heads were marked with Biro ink to show their orientation as they passed through the two screens and left the ink mark. The angles of rotation were computed and related to the screen separation to resolve the inches per revolution of travel. ... The first range of 2 feet was taken to be a virtual barrel rifling twist rate. The data from the other ranges indicated the increase in spin rate
                          ( revolutions per unit distance traveled ) of the pellets which in turn has relevance to the pellets' static stability and ultimately to the potential for change in dynamic stability and so grouping precision and accuracy.

                          Firstly the range. The white spot near the far fence is the 200 yard marker:

                          200ydrangeandspinratetargetboxAug2010.jpg


                          The target screens pinned with attention to geometric parallel upon the styrene box: (Note the 3 shot test group shot at 200 yards):

                          200yardspinratetesttarget3shotsin18inches086MOA457mm.jpg


                          That "group" measured 1.80 inches or 0.62 MOA at 200 yards! All three shots could have taken out an old buck bunny happy.gif. The sight was as usual set to zero for the range using Dave Eade's (Chairgun author) special extreme range programme written for me following the muzzle to 200 yd velocity/ BC study done some time ago and reported here. This rifle, sight ( 6500 Bushnell Elite 2.5-16X ) and pellets will zero from 10 yards to 225 yards.

                          200yardspinratetestAug2010.jpg


                          Results: In deference to those with slow computers I have not included pics of all the targets or the maths and geometry.

                          Result data points inserted were:
                          2 ft = 1 rev per 18.9 inches ie., 1:18.9 (assumed to equal rifling twist rate of the BSA 0.25 cal barrel).
                          So:
                          2 feet = 1: 18.9
                          225 feet/ 75 yards = 1:15.9
                          300 feet/ 100 yards = 1:14.9
                          450 feet/ 150 yards = 1:13.2
                          600 feet/ 200 yards = 1:12.8

                          Only one decimal place given as the analysis is by nature subjective but all care was taken with the measurements.
                          SpinRateversusrangegraphforJSB25KingsatMV890fps.jpg


                          It can be seen that the spin rate in revs per unit distance traveled climbed from near 1:19 inches (565 revs/sec) close to the muzzle at 890 fps to 1:12.8 inches ( 480 revs/sec) at 506 fps ( my chronographed velocity @ 200 yards at 4,030 ft adjusted to my farm's elevation 3640 ft) at 200 yards of travel.

                          Actual rpm works out at approx 33,900 at muzzle to 28,800 at 200 yards ie., a loss of 15 % in (edit) angular velocity compared to 42.5 % in linear velocity ........... " End quote.

                          Where static overstability and dynamic instability with diabolos may begin down range will depend on the pellet, its BC and shape, quality etc. There may be a criterion spin rate that triggers dynamic instability, not known but let's surmise 1:12" as a guesstimate, after which tractability is lost. "Tractability" being the ability to maintain its lengthwise axis in concert with its trajectory. ... As a guess I think slower mvs from standard twist barrels may help to keep the effective spin rates below that criterion spin rate whatever it is. .............. ?????

                          The eye is clearing a little as the day progresses but I'm told could take two weeks to show its best result. ... Kind regards, Harry.