This write-up describes the approach I took to improve my benchrest scores. It might help some of you who want to do the same. I found some interesting ways to tighten groupings. And I went down some rabbit holes that yielded no gains.
I shoot 25 meter USARB benchrest in the heavy varmint class (20 FPE max). I use an unregulated USFT and had been shooting decent scores; my cards averaged 244 out of 250 possible points. But those resulted in average 3-card scores in the 723 to 738 range, good for no better than middle-of-the-pack rankings at our local matches.
I wanted to do better. Last summer, when I switched from a .177 barrel to a .20 barrel, I also started a quest to modify the gun and set-up to get my scores higher.
After trying many things, I now am getting to the point where the gun performs well. At a recent match I shot 248 and 249 point cards. Along with a 244 first card, my score was 741, only the second time I’ve ever reached 740, and the first time with this gun. This level of performance puts me in range of our club’s best shooters.
Below I’ve described some of the changes that improved accuracy and some that didn’t work. I tested by shooting at our local outdoor range, using a Randolph front rest and a Protektor rear bag. I shot each pellet at a separate bull that was printed on millimeter graph paper. I recorded the x, y coordinates for each shot to calculate average spreads, standard deviations, CTCs, etc. I only used data from shots made in windless conditions, i.e., essentially no movement of the streamers on my wind flags. (1 to 2 mph wind can move the POI 1 to 2 millimeters and I’m looking for improvements in the tenths of a mm.) About 1500 shots were used in this work. The process was not planned out at the beginning, so a fair amount of interpretation was needed to reach conclusions from massive amounts of data.
Category 1: Factors that are important and can improve precision. Most of these are typical and are necessary but not sufficient to achieve consistently tight groups. One or two are less conventional but got me over the hump towards tight groups:
++++ Pellet choice. In my .20 caliber LW chrome moly barrel, the JSB 13.7s shoot far better than H&Ns and JSB 15.9s
+++ Ideal FPS, reservoir pressure, hammer spring, transport port restrictor. In my unregulated USFT, I would shoot a string of pellets starting at a high pressure for a given hammer spring tension and fully open transfer port, and plot x and y deviation, and FPS, as pressure fell. I would identify pressure ranges and HS settings where deviations changed very little across 10+ shots, and note the FPS range of these. I found I got the lowest deviations when FPS ranged from 780-800 FPS, and with reservoir pressure starting at around 1250-1300 psi. For accurate shooting I would fine tune the HS tension to get 790 FPS at that starting psi, and refill to that pressure level after every 10-15 shots. I keep the pressure range narrow because I found as pressure fell, the POI’s would move slightly to the right.
+++ Barrel work. I was getting periodic fliers of 5 to 8 mm. This would drop the score on a target to a 9 or 8. It might happen once or twice a card, but it also put a nagging and unhelpful concern in my mind about whether it will happen on the next shot. In another thread, I described the barrel lapping that @nervoustrig did on this barrel that has eliminated these random fliers.
++ Figuring out how to clean and condition the barrel and lube pellets. After nervoustrig’s work, the fliers stopped but the spreads grew. Only after a lot of experimentation did I find that I needed to stop frequently pulling wet patches (I use WD-40) through the barrel. After a wet patch cleaning, I need well over 150 shots before groups retighten. Even a dry patch clean needs 30+ shots. (My first card described above had several 9’s in the first 12 shots due to not getting enough fouling shots/sighters in before starting on the targets. Lesson learned.) And I lube pellets (with WD-40) because without lube I found lead particles in the pull through cleaning patches.
++ Stabilizing the action through the shot. I placed a tube filled partway with mercury on each side of the hammer spring housing. In theory, it reduces the vibration caused by the hammer banging against the top of the valve as the valve stem is depressed. See picture.
++ Taking pre-tension off the hammer to reduce hammer bounce. USFT’s OEM hammer springs are in compression even when the gun is not cocked. I was concerned that this could result in multiple valve openings as the hammer bounces back and forth between being pushed back by the valve/stem closing and the compression from the hammer spring. I replaced the OEM hammer spring with a stronger but shorter spring, so when the hammer is not cocked there are about 4 millimeters of free play between the valve stem and the start of compressing the hammer spring. (Also in that same picture.)
+ Sorting for pellet weight to the tenth of a grain. Tests with other guns showed a couple of tenths of a mm vertical height decline as pellet weight increased by 0.1 grain. I have not repeated this yet with this gun and pellet but for now I sort pellets.
Category 2: Changes that hurt grouping sizes
- Putting weights on the buttstock. I tried 2 and 3 pound weights on top of or under the butt. (I also tried placing the front rest at different points along the forearm. This also hurt groups.) The takeaway is that gun balance in the front and rear rest is important, but I haven’t yet found anything that improves on the base case set-up.
- Attaching mercury tubes to the barrel. But I might look at this some more.
- Putting pellets through a sizer.
- Treating pellets. Poor group sizes were found by:
--- Applying HiTek thermoset coating to the pellets. The coating thickness likely varied pellet to pellet.
--- Applying PTFE powder or HBN powder to the outside of pellets. Powder treatments were applied using a rolling ball mill. These resulted in finding lots of lead particles on wet patches when cleaning the barrel.
--- Applying Nanoslick (tungsten disulfide) to the pellets. This resulted in wide groups but also low FPS (as PTFE and HBN also did). But the high probability of inconsistent application will keep me from doing any more work with all pellet treatments.
Category 3: Factors that seemed to make little or no difference or did not work (yet). I might look at these further but no success so far. Over 150 shots are needed to be able to conclude whether an improvement of 0.5 mm in grouping size as a result of a change is statistically significant. So it might not be worth investing a lot of time to further pursue these.
o Adding weights to the barrel. I calculated the location of antinodes for the barrel and put 50 gram collars in those positions. I used up to 12 collars. It appears that the use of the weights tighten vertical spread a bit but they widen horizontal spread.
o Using a tuner or moderators at the muzzle. I tried an FX tuner and several different moderators.
o Using lighter or heavier pellets in the range of 13.6 to 13.9 grains.
o Changing depth of pellet insertion into the barrel. I got slightly better groups when I pushed the pellet about 5 mm into the barrel compared to shorter distances, using an adjustable TR Robb pellet seater. So I use that distance for now but want to check alternative distances.
o Applying graphite powder to the outside of pellets or the inside of the barrel. Since carbon often builds up from shots and doesn’t normally hurt groupings, I thought maybe a graphite treatment of the barrel and/or pellets would be helpful. FPS fell a lot (as it did with the PTFE and HBN) so more HS would need to be tried to give this a chance. But it is so messy and likely difficult to apply consistently that I decided to go no further.
o Rotating the barrel 180 degrees. The barrel was indexed and placed to shoot to the 12 o’clock position on the shot POI circle created as the barrel was rotated.
o Padding the top of the valve. I placed a few types of rubber and plastics around the valve stem so that the hammer would be cushioned after it pushed the valve stem down. This was an alternative or additional way to reduce vibrations from the hammer blow. But I haven’t yet found a way to get consistent FPS using this.
The journey is, of course, not finished. There are several things to look at further, as mentioned above. But fortunately the gun has reached a pretty good performance level so I’m going to proceed carefully with future changes. My biggest current concern is that the gun seems to need a lot of shots to “warm up” at the beginning of a session, so I might try replacing the Delrin poppet with a PEEK poppet which is a less malleable material.
I shoot 25 meter USARB benchrest in the heavy varmint class (20 FPE max). I use an unregulated USFT and had been shooting decent scores; my cards averaged 244 out of 250 possible points. But those resulted in average 3-card scores in the 723 to 738 range, good for no better than middle-of-the-pack rankings at our local matches.
I wanted to do better. Last summer, when I switched from a .177 barrel to a .20 barrel, I also started a quest to modify the gun and set-up to get my scores higher.
After trying many things, I now am getting to the point where the gun performs well. At a recent match I shot 248 and 249 point cards. Along with a 244 first card, my score was 741, only the second time I’ve ever reached 740, and the first time with this gun. This level of performance puts me in range of our club’s best shooters.
Below I’ve described some of the changes that improved accuracy and some that didn’t work. I tested by shooting at our local outdoor range, using a Randolph front rest and a Protektor rear bag. I shot each pellet at a separate bull that was printed on millimeter graph paper. I recorded the x, y coordinates for each shot to calculate average spreads, standard deviations, CTCs, etc. I only used data from shots made in windless conditions, i.e., essentially no movement of the streamers on my wind flags. (1 to 2 mph wind can move the POI 1 to 2 millimeters and I’m looking for improvements in the tenths of a mm.) About 1500 shots were used in this work. The process was not planned out at the beginning, so a fair amount of interpretation was needed to reach conclusions from massive amounts of data.
Category 1: Factors that are important and can improve precision. Most of these are typical and are necessary but not sufficient to achieve consistently tight groups. One or two are less conventional but got me over the hump towards tight groups:
++++ Pellet choice. In my .20 caliber LW chrome moly barrel, the JSB 13.7s shoot far better than H&Ns and JSB 15.9s
+++ Ideal FPS, reservoir pressure, hammer spring, transport port restrictor. In my unregulated USFT, I would shoot a string of pellets starting at a high pressure for a given hammer spring tension and fully open transfer port, and plot x and y deviation, and FPS, as pressure fell. I would identify pressure ranges and HS settings where deviations changed very little across 10+ shots, and note the FPS range of these. I found I got the lowest deviations when FPS ranged from 780-800 FPS, and with reservoir pressure starting at around 1250-1300 psi. For accurate shooting I would fine tune the HS tension to get 790 FPS at that starting psi, and refill to that pressure level after every 10-15 shots. I keep the pressure range narrow because I found as pressure fell, the POI’s would move slightly to the right.
+++ Barrel work. I was getting periodic fliers of 5 to 8 mm. This would drop the score on a target to a 9 or 8. It might happen once or twice a card, but it also put a nagging and unhelpful concern in my mind about whether it will happen on the next shot. In another thread, I described the barrel lapping that @nervoustrig did on this barrel that has eliminated these random fliers.
++ Figuring out how to clean and condition the barrel and lube pellets. After nervoustrig’s work, the fliers stopped but the spreads grew. Only after a lot of experimentation did I find that I needed to stop frequently pulling wet patches (I use WD-40) through the barrel. After a wet patch cleaning, I need well over 150 shots before groups retighten. Even a dry patch clean needs 30+ shots. (My first card described above had several 9’s in the first 12 shots due to not getting enough fouling shots/sighters in before starting on the targets. Lesson learned.) And I lube pellets (with WD-40) because without lube I found lead particles in the pull through cleaning patches.
++ Stabilizing the action through the shot. I placed a tube filled partway with mercury on each side of the hammer spring housing. In theory, it reduces the vibration caused by the hammer banging against the top of the valve as the valve stem is depressed. See picture.
++ Taking pre-tension off the hammer to reduce hammer bounce. USFT’s OEM hammer springs are in compression even when the gun is not cocked. I was concerned that this could result in multiple valve openings as the hammer bounces back and forth between being pushed back by the valve/stem closing and the compression from the hammer spring. I replaced the OEM hammer spring with a stronger but shorter spring, so when the hammer is not cocked there are about 4 millimeters of free play between the valve stem and the start of compressing the hammer spring. (Also in that same picture.)
+ Sorting for pellet weight to the tenth of a grain. Tests with other guns showed a couple of tenths of a mm vertical height decline as pellet weight increased by 0.1 grain. I have not repeated this yet with this gun and pellet but for now I sort pellets.
Category 2: Changes that hurt grouping sizes
- Putting weights on the buttstock. I tried 2 and 3 pound weights on top of or under the butt. (I also tried placing the front rest at different points along the forearm. This also hurt groups.) The takeaway is that gun balance in the front and rear rest is important, but I haven’t yet found anything that improves on the base case set-up.
- Attaching mercury tubes to the barrel. But I might look at this some more.
- Putting pellets through a sizer.
- Treating pellets. Poor group sizes were found by:
--- Applying HiTek thermoset coating to the pellets. The coating thickness likely varied pellet to pellet.
--- Applying PTFE powder or HBN powder to the outside of pellets. Powder treatments were applied using a rolling ball mill. These resulted in finding lots of lead particles on wet patches when cleaning the barrel.
--- Applying Nanoslick (tungsten disulfide) to the pellets. This resulted in wide groups but also low FPS (as PTFE and HBN also did). But the high probability of inconsistent application will keep me from doing any more work with all pellet treatments.
Category 3: Factors that seemed to make little or no difference or did not work (yet). I might look at these further but no success so far. Over 150 shots are needed to be able to conclude whether an improvement of 0.5 mm in grouping size as a result of a change is statistically significant. So it might not be worth investing a lot of time to further pursue these.
o Adding weights to the barrel. I calculated the location of antinodes for the barrel and put 50 gram collars in those positions. I used up to 12 collars. It appears that the use of the weights tighten vertical spread a bit but they widen horizontal spread.
o Using a tuner or moderators at the muzzle. I tried an FX tuner and several different moderators.
o Using lighter or heavier pellets in the range of 13.6 to 13.9 grains.
o Changing depth of pellet insertion into the barrel. I got slightly better groups when I pushed the pellet about 5 mm into the barrel compared to shorter distances, using an adjustable TR Robb pellet seater. So I use that distance for now but want to check alternative distances.
o Applying graphite powder to the outside of pellets or the inside of the barrel. Since carbon often builds up from shots and doesn’t normally hurt groupings, I thought maybe a graphite treatment of the barrel and/or pellets would be helpful. FPS fell a lot (as it did with the PTFE and HBN) so more HS would need to be tried to give this a chance. But it is so messy and likely difficult to apply consistently that I decided to go no further.
o Rotating the barrel 180 degrees. The barrel was indexed and placed to shoot to the 12 o’clock position on the shot POI circle created as the barrel was rotated.
o Padding the top of the valve. I placed a few types of rubber and plastics around the valve stem so that the hammer would be cushioned after it pushed the valve stem down. This was an alternative or additional way to reduce vibrations from the hammer blow. But I haven’t yet found a way to get consistent FPS using this.
The journey is, of course, not finished. There are several things to look at further, as mentioned above. But fortunately the gun has reached a pretty good performance level so I’m going to proceed carefully with future changes. My biggest current concern is that the gun seems to need a lot of shots to “warm up” at the beginning of a session, so I might try replacing the Delrin poppet with a PEEK poppet which is a less malleable material.
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