FX Slug BC - Revised at Reply #49

[JBinCO]

I love this place
What causes wind drift to increase at that 950-1200 FPS?
In a slug ?

[BackStop]

I love this place
What causes wind drift to increase at that 950-1200 FPS?
In a slug ?

Bob will probably spank me with his answer, but I would guess that at over 1000fps, you are approaching/treading in Transonic range.   Weird things happen there and anywhere near there.   

[subscriber]

What causes wind drift to increase at that 950-1200 FPS?
In a slug ?

The BC drops and does not fully recover until you reach Mach 2...

[JBinCO]

why?....what does that look like in slugs?   I know this is a huge issue with pellets....but though slugs and twists would help?

[subscriber]

It has to do with the formation, shape and position of the shock wave at different projectile velocities.

[JBinCO]

hence the need for air strippers?
Just double checking my understanding.....I love this place!

[BackStop]

hence the need for air strippers?
Just double checking my understanding.....I love this place!

I'm not sure air strippers do all that much at lower velocities.   Maybe they do, but I am the biggest variable when I shoot.   

[JBinCO]

well I sure understand and live that every shot!
+1 LOL

[subscriber]

Air strippers reduce muzzle pressure and the tendency for the projectile to be buffeted directly after it leaves the muzzle.  The effectiveness of a stripper is related to how low the air pressure is on the back of the projectile, and how little air flows over it from behind just after it leaves the end of the barrel; not on the speed of the projectile.

Diabolo pellets benefit more from an air stripper because they are nose heavy and drag stabilized.  Muzzle blast actually tries to flip the pellet over for an instant.  The benefit still exists for slugs, but buffeting has less effect on a slug due to its shape and its rear biased center of gravity.

[subscriber]

I'm not sure air strippers do all that much at lower velocities.   

It is not the velocity that matters, but the muzzle pressure.

Now, with high velocity, you often have high residual muzzle pressure; so in that sense, more powerful airguns tend to have air strippers.

Short barrel airguns at moderate velocities will also benefit from them because they have higher muzzle pressures than if their barrels were longer.

[subscriber]

[JBinCO]

Man I may want to retake my high school math classes again....with this type of relevancy I think I would Pass them!

[BackStop]

I'm not sure air strippers do all that much at lower velocities.   

It is not the velocity that matters, but the muzzle pressure.

Now, with high velocity, you often have high residual muzzle pressure; so in that sense, more powerful airguns tend to have air strippers.

Short barrel airguns at moderate velocities will also benefit from them because they have higher muzzle pressures than if their barrels were longer.

Yes, I should have included pressure, but lower velocities *usually* mean lower pressure in AGs.   Not always, but a lot of the time. 

[skorec]

  Good description

https://sites.google.com/site/technicalarchery/technical-discussions-1/drag-coefficients-of-bullets-arrows-and-spears

[wahoowad]

My Chairgun stopped working or I would do this myself. I'd love to see a basic drop chart comparing the FX slug to the FX pellet since we have the data. Curious how much flatter it really shoots.

[skorec]

18gr JSB Jumbo Heavy/BC=0,03  vs. 18gr/22gr  FX hybrid slug/BC=0,062  at 950 FPS

[Ribbonstone]

So far, every time I've tested pellets or slugs,the BC works out WOSE at short range than it does when tested at longer ranges.

(this would by by 2 chronographs, measured from the muzzle to the middle of the chonograph for each).

So the BC numbers for something like 10 to 30 yards is always lower than the BC for 30-60 yards.

Without  the radar type crono, it's all I had.  "Had" rather than "have". YOu'd likly have guessed that at long range,  killing the far chrongraph is  a likely event.

Bob will likely tell you why they sluff off velocity faster in the first couple of yards.....I'm just going to assume that faster (closer) and the little "wiggle"many projectiles have at the muzzle is part of it.

The practical side thinks that if you are measuring BC at close range...they you are really just gathering random data.  If it's short range,then the BC really isn't all that important for anything you are actually doing; what high BC slugs do well isn't up close.

.06-.08 is still a BC about 2X better than the good pellet-pellets.  Not that it really makes a rats-rump of differnce at short range.  Nothing I've shot at 25-30 yards with a lower BC 34gr. pellet-pellet  would have notices if i used a high BC 34gr. slug-pellet....but I do think that it would make a differnce at +60 yards.


Something weird doers happen as you approach transonic speeds...every graph/test shows a freakish  jump in drag as you move into the Mach ! area (yet not at higher levels).    I'll leave it to the engineers to explain why...just enough to know that the "sonic wall" of 1000-1200 fps does absolutely "stupid stuff" to windage.

If you get just barely above transonic at launch...you have to go back though that area down range as velocity declines back to transonic down range....you get a double-dose.


So for lead bullet/slug shooters (rim fire/ center fire/ air gun) it turns out that the best choice is to EITHER keep the speed transonic all the way to the target...or keep it sub-sonic all the way to the target.

[rsterne]

My upcoming articles in HAM will deal with this topic.... but here is a summary.... The drag of ALL projectiles has a huge increase in the Transonic range (Mach 0.8-1.2)…. Here are some typical drag curves....



The faster a projectile slows down, the more it drifts in a crosswind.... It is the "lag time" (difference between real world and in a vacuum) that matters, not the "time of flight".... Therefore, regardless of whether it is a pellet or slug, it will drift a LOT more at Mach 1.2 than at Mach 0.8.... This causes the minimum drift to occur at around 850-950 fps muzzle velocity, the exact value dependent on the range to target and the BC.... Here is what happens for a typical pellet at 50 yards....



and here is what happens for various slugs at 200 yards....



If you look at the curve for a BC = 0.10 (the blue line), you will notice that the drift over 200 yards in this wind strength of 10 mph is 20" at a MV of 900 fps.... That increases to nearly 30" at 1600 fps, and does not fall back to 20" until the MV is about 2700 fps.... 

Ribbon's observation about the BC being lower at close range is due to the pellet slowing quicker because it is travelling faster.... and a mismatch between the actual drag of the pellet and the drag model used to calculate the BC....

Bob

[canadian_shooter]

My upcoming articles in HAM will deal with this topic.... but here is a summary.... The drag of ALL projectiles has a huge increase in the Transonic range (Mach 0.8-1.2)…. Here are some typical drag curves....



The faster a projectile slows down, the more it drifts in a crosswind.... It is the "lag time" (difference between real world and in a vacuum) that matters, not the "time of flight".... Therefore, regardless of whether it is a pellet or slug, it will drift a LOT more at Mach 1.2 than at Mach 0.8.... This causes the minimum drift to occur at around 850-950 fps muzzle velocity, the exact value dependent on the range to target and the BC.... Here is what happens for a typical pellet at 50 yards....



and here is what happens for various slugs at 200 yards....



If you look at the curve for a BC = 0.10 (the blue line), you will notice that the drift over 200 yards in this wind strength is 20" at a MV of 900 fps.... That increases to nearly 30" at 1600 fps, and does not fall back to 20" until the MV is about 2700 fps.... 

Ribbon's observation about the BC being lower at close range is due to the pellet slowing quicker because it is travelling faster.... and a mismatch between the actual drag of the pellet and the drag model used to calculate the BC....

Bob

I look forward to the upcoming series.

[JBinCO]

So I am going to go ou on a limb here and say I had a good question lol