FX Hybrid speed affecting BC

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Hanebuth96

Hanebuth96

Member
Mar 10, 2017
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Detroit Lakes, Minnesota, United States
Was shooting my FX Impact at about 965 with .30 FX Hybrid slugs last year but was experiencing a BC of .068, I wasn’t real happy with that so I wanted to try different slugs and see if they would shoot with similar accuracy but a better bc. I tried a couple different options but neither of the would group at 40 even. So I started a chat with @utah_airguns via there texting service asking for suggestions. They had mentioned they usually shoot the FX Hybrids at about 950 with a much better bc than I was experiencing. So I decided to try slowing it down as well and see what happened. Slowing it down to about 955 I got a little better bc, so I slowed it down to about 945 and got achieved a bc of .099. While I was doing my testing I shot for groups as well. The 6 shot group was at 955. The 5 shot group was at 945. The 945 group was larger but the 2 impacts to the left of the group were from the wind changing.
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The M3s launched with 1:22 standard liners and I haven't heard of any changes since then. We should have the same liner.

Based on the numbers I think there's a good chance 965fps is too high of a velocity for that reg pressure. Even if you still had accuracy, I wonder if the slug was somehow getting disturbed and hurting BC?

In my gun at 148bar, max POSSIBLE speed is around 1020fps. That's where it's loud, wasteful, and less accurate due to the valve still being open when the projectile leaves the barrel. I have the hammer spring set to deliver 965fps, which is around 95% of max. I've had good luck at and below 95%. A lot of slug shooters swear by much lower than that (meaning they are running really high reg pressures for the same velocity). Each bar of reg pressure equates to about 2fps of potential velocity in my gun. If we apply that here, that would mean your max at 135 bar would be 994fps. So you are not maxed out but definitely running on the hotter side for that reg pressure if you were tuned at 965fps. Reality could be worse if your reg gauge is off also and it was really 125 or 130bar (my wika gauges were 5-10bar off). I only use digital reg gauges now.

Interestingly 945fps is about 95% of 994fps
 
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Honestly, if it is shooting good, I'd probably just leave it unless you really feel the need for more speed. 20fps isn't much. I'm not lubing hybrids, just shooting them straight from the package. I have done about 15-20 pulls through the liner with JB bore paste patches. This 'polishing' seems to help extend cleaning intervals. It's also functionally a deep cleaning when you do it. Did it help accuracy? Not sure, but it didn't hurt it.

As far as my macro and micro settings - the macro wheel doesn't tell you anything. It just lets you jump the micro position by about 10 clicks for each number on the wheel. The micro position (hammer setting) is going to vary gun to gun so its more important to tune by watching the velocity. The micro position will end up where it needs to be. For reference I think I'm around 4.25. I believe the stock hammer spring runs out in the 160 bar range. If you are only shooting slugs, there is some benefit to adding one of the hammer spring spacers which will increase the hammer mass, and lower the reg pressure needed to achieve the same speeds. I have one of these spacers but it is waiting until I need to tear the gun down to get installed. I'm not going to dig that deep into a gun that's currently working perfectly as-is. The other 'slug upgrades' are mostly not needed. The stock probe works fine (set the depth though). In .30 the standard liner is 1:22 which is semi-fast, and I have yet to see good feedback on where it's slug weight ceiling is. Some people have even reported shooting weights in 60 grain range with it.
 
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Did UA tell you that shooting slugs at their elevation is likely to produce quite different results? As altitude increase and air pressure decreases, slugs are stabilized with less twist and their BC increases due to lower friction from the thinner air. Having a reduced BC is a symptom of having marginal stability and reducing the speed can enhance the stabilization. Lots of people assume, incorrectly, that slug accuracy and BC will increase with muzzle velocity. You’re approaching this in the right way by finding the speed that gives the best BC. Having an extra 20-30fps is pointless if it bleeds off in the first 20 yards due to lower BC. Also, if the slug is sufficiently spin stabilized to provide its optimal BC, it’s often more accurate and resistant to destabilization from the muzzle blast. That’s not to say that you shouldn’t tune to get the least excess air, but it’s good to find the optimally stable speed first and then focus on getting the best combination of pressure and hammer to shoot at that speed. As TDK stated, many people find that running a high reg pressure and insufficient hammer to open the valve fully (known as partial valve lock or tuning below the knee) is beneficial because it generates a pulse of high pressure air of short duration. Tuning below the knee was discouraged in the past because it is subject to higher ES from variation in hammer strikes and/or reg creep. However, FX guns are very resistant to this effect due in part to the excellent hammer system design, The M3 also benefits from having its semi balanced valve. For example, I can adjust the macro on the fly and dial the hammer from a level needed to shoot a 34 grain slug at 980, down to a level needed for a Hades pellet at 15.85 grains. The ES for the pellets is still single digits, indicating that the system is still very consistent with >50% valve lock.

Here’s a good article from airgun guru Sterne explaining some of these “issues”:

hardairmagazine.com

Pellet Twist Rate and Stability - Hard Air Magazine

Today we’re investigating pellet twist rate and stability. I must say that I am not aware of any twist calculators that will work for Diabolo pellets!
hardairmagazine.com hardairmagazine.com
 
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BC should not change substantially with altitude. The effect of altitude is to reduce the air density, so the drag will reduce but not the drag coefficient and hence not the form factor in the BC. If the correct calculation of the BC is not used, i.e. the atmosphere is not corrected to sea level standard values, then it will appear that BC increases with altitude. Because, as you say, stability is increased at altitude, there may be a small change in BC due to different yaw effects. It could increase or decrease.

The Kolbe calculator is not accurate between about 1000 ft/sec and 1200 ft/sec. This is because the equations Bob McCoy produced are beyond their limits of applicability at speeds close to Mach one. Bob probably never intended them to be needed between the above speeds, as the calculator was intended for bullets. Bob McCoy and I, when we were working for our respective government research organizations, exchanged software. The software I sent to Bob was for calculating the aerodynamic coefficients needed for stability calculations and Bob's equations, while not identical, are very similar in form and Mach number division, possibly based on the RARDElift version of the original software.

Bob Sterne is aware of the limits to the Kolbe calculator, as he has seen for himself that it loses accuracy near to Mach One. Unfortunately, the only software I am aware of which can calculate values at Mach one is not generally available.
 
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TDK said:
Honestly, if it is shooting good, I'd probably just leave it unless you really feel the need for more speed. 20fps isn't much. I'm not lubing hybrids, just shooting them straight from the package. I have done about 15-20 pulls through the liner with JB bore paste patches. This 'polishing' seems to help extend cleaning intervals. It's also functionally a deep cleaning when you do it. Did it help accuracy? Not sure, but it didn't hurt it.

As far as my macro and micro settings - the macro wheel doesn't tell you anything. It just lets you jump the micro position by about 10 clicks for each number on the wheel. The micro position (hammer setting) is going to vary gun to gun so its more important to tune by watching the velocity. The micro position will end up where it needs to be. For reference I think I'm around 4.25. I believe the stock hammer spring runs out in the 160 bar range. If you are only shooting slugs, there is some benefit to adding one of the hammer spring spacers which will increase the hammer mass, and lower the reg pressure needed to achieve the same speeds. I have one of these spacers but it is waiting until I need to tear the gun down to get installed. I'm not going to dig that deep into a gun that's currently working perfectly as-is. The other 'slug upgrades' are mostly not needed. The stock probe works fine (set the depth though). In .30 the standard liner is 1:22 which is semi-fast, and I have yet to see good feedback on where it's slug weight ceiling is. Some people have even reported shooting weights in 60 grain range with it.
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If you are using FX/RMR hybrids, I'd strongly suggest cleaning them and cleaning out the tips. They are a bit dirty and fair amounts of lead chunks.
 
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I finally got out to do some more testing. I turned up the reg to just under 150 on the gauge went out and shot and got it shooting very consistent over a 5 shot string I had a ES of 2 and a SD of 1.1 then I went back into the house to fill the gun. Noticed the groups opened up a bit compared to before, so while i was filling with my Novair compressor I decided I should clean the barrel to eliminate that as a variable. So I cleaned it put the liner back in, when I got back out to shoot for some reason I had a jump in FPS without changing anything on the gun other than cleaning the barrel. It’s now shooting close to 980 with an SD of 3.4. Could the jump be from temperature change outside or cleaning the barrel? However zeroing at 40 and the shooting out to 100 I now am getting a BC of .120.

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Alright waited till it cooled off a little this evening and went back out. I think a combination of the heat and the FX Radar chronograph having interference with the surroundings while I was shooting prone caused the fluctuations in speed. I verified zero at 965 again and then moved the target back to 100 shot again and my experienced bc was still G1 .120. When doing my shooting at 100 I shot a few different groups including this one that I’m very happy with.
Ballistic-X-Export-2023-06-04 20:27:30.972594.jpg
 
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A couple of points.

It looks like you are calculating the BC from the POI of the slugs, rather than using velocity drop or time of flight. Using the POI method is the least accurate method of all, as there are too many other factors which can affect your slugs drop over range.

The G1 drag law is known to give false high BC values at higher speeds due to the drag law shape being nothing like that of a large meplat airgun slug. Bob Sterne discussed this both in the Hard Air online magazine and in this post. https://www.gatewaytoairguns.org/GTA/index.php?topic=203542.msg156380437#msg156380437

We are attempting to produce a better slug based model, though it is doubtful if many manufacturers would want to adopt it as by using the G1 drag law for high speeds they can legitimately claim measured inflated BC values, a valuable selling point.

The new drag law, for anyone who wants to test it, can be found in the free software on this site. https://gpc.fotosoft.co.uk/
 
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Would I be right in thinking that the following extract would also apply to pellets ?
"Remember this.... If the drag model used matches your slug, your BC will be constant, regardless of the velocity.... If the BC varies with the velocity, the drag model is not perfect for your slug.... The more it varies, the worse the fit to your projectile.... It may become a serious enough problem that you have to use a Ballistics Calculator (such as Strelok Pro) that allows you to input different BCs for different velocities...."
 
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Bluestone said:
Would I be right in thinking that the following extract would also apply to pellets ?
"Remember this.... If the drag model used matches your slug, your BC will be constant, regardless of the velocity.... If the BC varies with the velocity, the drag model is not perfect for your slug.... The more it varies, the worse the fit to your projectile.... It may become a serious enough problem that you have to use a Ballistics Calculator (such as Strelok Pro) that allows you to input different BCs for different velocities...."
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Yes, very much so as there are probably more different shapes to pellets than there are to slugs which, at the moment, are very similar in shape in the majority of designs. Wadcutter pellets are one design which may require multiple BC values if used at higher speeds. A purpose drag law for wadcutters is difficult, as any rounding on the shoulders can have a large effect on the shape of the drag curve. Pointed pellets as well can have a different shaped drag law compared to round head pellets which have the GA and GA2 drag laws.
 
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Hanebuth96 said:
Would be nice if the common ballistics apps would update to have the new drag laws
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The GPC software is rapidly growing in popularity, both here in the UK and in the US. George, the creator of the programs, is often issuing updates and can be contacted directly. He is always keen to add new drag laws for airgun projectiles when we can create them, and is a member on a number of forums. The software is airgun orientated.
 
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Ballisticboy said:
The GPC software is rapidly growing in popularity, both here in the UK and in the US. George, the creator of the programs, is often issuing updates and can be contacted directly. He is always keen to add new drag laws for airgun projectiles when we can create them, and is a member on a number of forums. The software is airgun orientated.
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Hello: Mr. Ballisticboy, I PM you, I would like to discuss with you about the work of pellets/slug, I am a mold maker, if you are interested, please reply me, thank you
 
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