Airgun Accuracy

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Polygonal and Rifled Barrels compared for BC and Velocity

In the quest for the accuracy of field target rifles and airrifle shooting a lot comes down to finding or selecting the right production batch of pellets. It is known that some rifles seem to produce good accuracy only with one, or a few specific, pellet batches. While other rifles are not “pellet fussy” and shoot well with a much broader range of batches and pellets.

In outdoor shooting, as in field target, a large part of the accuracy is also determined by the wind. And delivering a pellet in a shorter time-span to the target will reduce the effect of the wind. A pellet that loses less velocity over its flight to the target will have less deflection by wind and will hit more accurate.  Losing less velocity is usually expressed as a pellet having a high Ballistic Coefficient (BC)

The factors above (pellet fussy and wind sensitivity) are both influenced by the barrel that is on the rifle. Maybe not only the barrel but at least for a large part. For are rifles there are 4 types of barrels: Smooth bore, Rifled, Polygonal and Smooth Twist barrels.  These four types each have their own effect on the shape of the pellet that leaves the barrel and thereby on velocity and BC. The polygonal barrel leaves a different kind of marking on the pellet. It is less sharp and ussualy less defined compared to the markings that a rifles barrel makes. And if it has an effect on the markings on the pellet is may also be that the polygonal barrels may less pellet fussy.

Pellet shot through an rifled (left) and polygonal barrel (right).

Pellet shot through an rifled (left) and polygonal barrel (right). (picture from Tony on the Yellow Network54 Airgun forum*1)

Rifled (left) and Polygonal barrel (right)

Rifled (left) and Polygonal barrel (right). (picture from CA Varminter on the airgunadvice.net forum *2)

A while ago I found some data of an experiment where a rifled barrel and a polygonal barrel where compared in a direct test. On the same Walther LG300 Dominator field target rifle at 16 Joule / 12 FPE the 4.5mm calibre, 490mm long rifled barrel was replaced by an polygonal barrel of the same length. Fourteen different types of pellets from five different pellet producers were tested in both barrels. On an indoor shooting range the velocity at 1 meter and at 50 meters from the muzzle was recorded with two identical chronographs. All pellets were shot for 10 shots in both barrels and the standard deviation of the velocity was calculated over these 10 shots. This data was used to calculate velocity loss between 1 and 50 meters and the ballistic coefficient for each pellet for each barrel. From what I could find it seems that this experiment was performed on the Walther factory shooting range and probably reported in the German magazine Visier around the year 2010.

Table 1. Test Results

Table 1. Test Results

What do the results tell us? It is a lot of data and everybody may make up their own opinion. Below is my opinion:

The BC found in this test for the JSB Exact (the pellet which is used the most by field target shooters) is with 0.027 in close agreement with what I read on the internet about this pellet. It is also the value for this pellet that I use myself in Chairgun and agrees very well with my observations of the trajectory on this pellet. The Chairgun database has a lower 0.023 value for the JSB Exact but a lot of shooters report that this is too low. So it looks like this test contains valid data.

The one thing that can be concluded safely is that the polygonal barrel usually results in a higher V1 muzzle velocity (average over all pellets +3m/s, + 1 to +7 m/s) . (Except for the Prometheus pellets  which show a slightly lower (-1,3 m/s) velocity when shot from the polygonal barrel).   It looks like the polygonal barrel has less internal friction.

The velocity at 50 meters (V50) is also higher from the polygonal barrel. On average for all pellets this is 4,2 m/s. Excluding the strange behaving Prometheus pellets from the data  shows a velocity that is 4,8 m/s higher.

It also seems safe to conclude that the standard deviations of the velocity are somewhat lower for the polygonal barrel, both at 1 and at 50 meters distance. For the rifled barrel the average over all pellets for the standard deviation of the velocity at 1m is 0.89 m/s. For the polygon it is 0.78 m/s. At 50 meters these figures are 3.41 and 2.95 for rifled and polygonal respectively. But the difference is very small. The standard deviations of the muzzle velocity also show that this Walther Dominator rifle is able to shoot with a very small velocity difference between shots.

About the Ballistic Coefficient

At first I’d say there is a large difference in BC between the different pellets. Secondly: There is very little to no effect of the polygonal barrel on the BC. With exception of the Daystate Select in head size 4.53mm that shows an improvement from 0.021 to 0.029 in BC. The other differences in BC between the barrels are not significant in my opinion. Keep in mind that this is an experiment with a 16 Joule 4,5mm rifle shooting at velocities around 235 m/s. For other airguns, working at higher pressures and velocities the results may be different and a polygonal barrel may show different results there.

Seemingly small differences, as for instance 0.01mm in head size in the H&N FTT, can have a large effect on BC. But in the JSB Exact pellets the headsize has no effect on BC. For the Daystate pellets there is large effect of head size in the rifled barrel and only a minimal effect in the polygonal barrel.

The BC varies from 0.014 to 0.027 for the rifled barrel and from 0.014 to 0.029 for the polygonal barrel. The 0.029 coming form only one observation. Pointed pellets seem to have a very low BC but the dome shaped RWS Superdome strangely also has a very low BC.Looking at the shape of this pellet I would not expect such a large difference with for instance the dome shaped JSB Exact.

Highest BC values are found with the Daystate Select pellet in 4.53 head size in the polygonal barrel (BC 0.029). Closely followed by the H&N FTT 4.51 in the polygonal barrel (BC 0.028).

Where to get a polygonal or polygon barrel?

The Lothar Walther factory in Königsbronn, Germany produces polygonal airgun barrels in 4,5mm / 5,5mm and 6,35mm calibre (.177/.22 and .25).  They can be had with or without choke, in 3 different diameters (10, 12 and 16mm). For the total overview of their very extensive line of polygonal airgun barrels click here: Lothar Walther barrel blanks for airguns. 

In the USA these barrels can be ordered through Lothar Walther Precision Tools in Cumming, Georgia. In the rest of the world they need to be ordered be a registered gunsmith or firearms dealer.

Conclusions:

I’d like to hear your opinions, but in my opinion this test shows that a polygonal barrel has only very little extra to offer for Field Target shooting at 16 Joule / 12 FPE.

I don’t know why, but unfortunately this test does not have any results on shot groups sizes. Would have loved to see all those 10 shot groups. This would give valuable info on how these different barrels react to different pellets, say the pellet fussiness.

I actually own a Walther LG300 Dominator with a polygonal barrel. The third one that was made. The first few Dominators were build by Walther with a polygonal barrel. It was a 21 Joule version when it left the factory and shot really exceptionally good groups with RWS Superdome pellets. 10 to 12mm 5 shot groups at 50 meters indoor bench rested all the time. Then the regulator failed, like on all early LG300’s,  and I asked it to be replaced with a 16 Joule reg. After that it shot it’s best groups with the JSB Exact and was not this mega good anymore. 15 to 20mm groups @ 50 meters. Quite acceptable but not exceptional accuracy. Does this indicate that polygonal barrels shoot their best when the velocity is higher?? I don’t know. It is only one observation. But I heard that in the USA in the air rifle Benchrest comps the polygonal barrels perform up to medal level at 21 to 23 fpe.

*1:[http://www.network54.com/Forum/79537/thread/1258734772/HW+Choked%28.17%29+vs.+LW+Polygon+Choked%28.17%29+Barrel+-+Pellet+Imprint+PICS]

*2:[http://www.airgunadvice.net/viewtopic.php?t=13349]

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