Shot cycle Dynamics in 3 Spring-Piston Airguns Chap 5

​Does lubrication with Krytox improve performance?

There are lots of excellent articles on the benefits of lubricating spring piston rifles with Krytox, but I’ve never been able to find a detailed, quantitative before/after evaluation of Krytox in these rifles. There also are some excellent posts on Krytox in GTA:

https://www.gatewaytoairguns.org/GTA/index.php?topic=141898.0
 
I first heard about Krytox from Leo Gonzales. Leo is a fantastic WFTF competitor (he placed 4th in the last WFTF World Championship!) and he knows a lot about springers, so when he makes suggestions, I listen very carefully! 
​In this chapter I switched my LGU and FWB 124 to Krytox and took a look at their performance before and after the switch.
Before Krytox, both rifles were sparingly lubricated with a combination of moly and Superlube, with the moly mainly for the metal parts and Superlube for the synthetic piston seal. This was the lubrication used in Chs. 1 and 2 of this blog.
​Before lubricating with Krytox, I first thoroughly degreased the mainspring, piston, piston seal, and compression chamber. Then I applied GPL 205 very sparingly, wiping off after applying. Krytox is much more expensive than molybdenum disulfide or Superlube, but Leo pointed me to an online store with pretty reasonable prices:

https://www.amazon.com/Krytox-Grease-Pure-PFPE-PTFE/dp/B00MWLDALQ/ref=sr_1_16?keywords=krytox&qid=1580829181&sr=8-16
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Figure 5.1 shows how Krytox affected the accuracy of my LGU. Accuracy was about the same after Krytox was applied, where four 10-shot groups were made off the bench at 20 yards before Krytox, and another four 10-shot groups were made after Krytox. No warm up shots were taken after the Krytox lubrication and the last 10-shot group with Krytox is pretty nice, with a ctc distance of 0.090”, so maybe accuracy is improving as the rifle starts settling with the new lube? As I discussed in Ch. 3, this group needs to be taken with a grain of salt! 

Fig. 5.1 LGU accuracy with 10-shot groups off bench at 20 yards with moly/Superlube (top four groups) and Krytox (bottom four groups).

​Accuracy with the two types of lubes was similar, but there was a BIG difference in the fluctuations in the MV. Figure 5.2a) shows the MV for each of the first forty shots in Fig. 5.1, before Krytox. Figure 5.2b) shows the MV for each of the last forty shots in Fig. 5.1, after Krytox, plotted with the same vertical scaling. The standard deviation and extreme spread of the MV both dropped by about a factor of two when the moly/Superlube was replaced by Krytox! Again, this happened right after the Krytox was applied; there were no shots to get the rifle settled in with the new lube. My LGU shoots lots of different types of pellets very well with similar POI, but one of my concerns has always been that the MV can be pretty erratic with some brands/dies of pellets. At 20 yards, fluctuations in MV aren’t that critical, but at longer ranges these fluctuations could cause big POI shifts. The Krytox did an amazing job of stabilizing the MV using pellets from the very same tin.

Fig. 5.2 LGU MV with a) moly/Superlube and b) Krytox for the groups on shown in Fig. 1.

​Krytox made no discernable difference in the recoil of the LGU, as can be seen in Fig. 5.3, which shows recoil traces before (blue) and after (orange) Krytox.

Fig. 5.3 LGU recoil traces showing the position, velocity, and acceleration of the sled-mounted rifle over 250 ms for moly/Superlube and Krytox lubrication of piston seal and mainspring. Note that velocity was measured using the soundcard on a pc, so the signal is ac-coupled and the slower rearward drift of the sled is not captured, as explained in Ch. 1.

​Later on the same day, I shot eight 10-shot groups off the bench at 20 yards with my Krytox-lubed LGU, as shown in Fig. 5.4. These are the best 10-shots groups that I’ve ever shot at 20 yards with any air rifle, with an average ctc of 0.23”! MV fluctuations were a bit higher than I would have hoped, but I expect that if I had been using the original moly/Superlube on that day with those pellets, the MV fluctuations would have been even worse

Fig. 5.4 LGU accuracy with 10-shot groups off bench at 20 yards with Krytox.

​Since Krytox did such a good job of taming MV fluctuations, I did some accuracy testing at 52 yards where MV fluctuations make a much bigger difference than at 20 yards. At 50 yards, a decrease in MV from 800 fps to 760 fps translates into a drop of around 0.75”! The extreme spread in MV using my LGU with Superlube/moly is around 37 fps, so one will get vertical stringing on the order of 0.75” at 50 yards just from MV fluctuations. Figure 5.5 shows typical groups at 52 yards off the bench before Krytox. Looking at the bottom row we can see that some 5-shot groups were pretty small, around 0.40”! However, there was a lot of vertical drift in the first two groups, shot from left to right, so I only counted the last three 5-shot groups. Again, choosing the smallest group would not be very representative of how the rifle was shooting. Putting the last three groups together resulted in 15 shots inside a 1” circle.

Fig. 5.5 LGU accuracy with 5-shot groups off bench at 52 yards with moly/Superlube (before Krytox). Here I’m focusing on the last three groups on the right in the bottom row.

​Figure 5.6 shows how the Krytox-lubed LGU did at 52 yards. Unlike Fig. 5.5 where I shot three 5-shot groups, in Fig. 5.6 all the groups were 10 shots. I was hoping that the Krytox would results in a more dramatic improvement at 52 yards, but the groups are still pretty good for a springer at 52 yards! Most 10-shot (not 5-shot!) groups were under 1” and 70 shots landed in a circle just over 1” in diameter.

Fig. 5.6 LGU accuracy with 10-shot groups off bench at 52 yards with Krytox. The first group in upper left hand corner was for warming up and wasn’t counted.

​My LGU’s POI has always been pretty consistent and I think part of this is because I use very little lubricant. I’m not sure if the consistency is much better with Krytox, but I’m finding that over several months under different conditions, the POI has not changed much.
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Encouraged by what Krytox did to my LGU, I decided to try it in my FWB 124. Surprisingly, and unfortunately, Krytox had a very different impact on my FWB 124! With the Krytox-lubed LGU, MV remained about the same but MV fluctuations dropped by a factor of two. With the Krytox-lubed FWB 124, MV dropped by about 35 fps and MV fluctuations stayed about the same, as can be seen in Fig. 5.7. Figure 5.7 a) and b) show that accuracy was about the same or perhaps a bit worse with the Krytox. Maybe it will take some time for the rifle to settle in with the Krytox? One possible explanation is that my FWB 124 was lightly dieseling with the moly/Superlube, which have a lower ignition temperature than the Krytox, so that pushed the MV up a bit? That’s just one guess.

Fig. 5.7 a) FWB 124 accuracy with 10-shot groups off bench at 20 yards with moly/Superlube (top four groups) and Krytox (bottom six groups), b) more 10-shot groups with Krytox. Average MV dropped about 35 fps but the standard deviation and extreme spread stayed about the same when Superlube/moly was replaced with Krytox.

​So is Krytox worth it? If I had only tested my LGU, I would answer with an emphatic YES! However, the Krytox experiment on my FWB 124 showed that things are a bit more complicated, as they tend to be in real life!
​I’m very intrigued how/why Krytox could affect two 12 ft-lbs springer so differently. Maybe it has something to do with the differences in piston seal material? I would very much like to understand the cause(s) of these differences and believe that will teach me something new about springers. If you have any ideas, please let us know.