Whats the Science behind shooting heavier projectiles? (like .247, .30, .357)

[39M]

May take me a bit to wrap my head around all that.
But I keep thinking back to a Brenneke type slug. Which is suspiciously similar to the Powershot.

[Bill G]

Its very hard to keep a fair comparison when evaluating different boolits.  I find that if I compare them all at the same velocity, I tend to get a clearer, more fair comparison.  So with the McDrag spread sheet, I set all of my velocities at 950.  SD is of course unchanged with regard to velocity but the BC does change.  Also input the same barrel lengths while comparing.  Thanks to Bob's wonderful work, we can now get a fair estimation of potential velocity based on SD, pressure and barrel length.  If you have a comparison of same caliber projectiles, and the SD exceeds what could possibly reach 950fps, then you know you'll need more barrel length and or more psi.  But knowing how to estimate the fps based on SD is the key.  If the SD is too high to reach the 950fps goal, then you have what I would consider a fair comparison based just of capability of the delivery system.  Often you will see that the BC is higher but since the initial velocity was so much lower, the trajectory just isn't adequate for long range work.
These Tools are on the web and can be fairly easily adapted to our realm of work.  The McDrag spreadsheet is on the web and can be down loaded free.  The Koble web sight has a couple of online calculators that are awesome tools.  Garbage in, Garbage out though.  Keep in mind that bearing length is important and that longer is sleeker.  I think Bob would agree, approximately 60% of caliber is the min bearing surface length, definitely not less.     

Thanks Bob for all the hard work and data building.  You have paved a path to enlightenment.

I've though about the 6.5 also but a reasonably priced barrel isn't really available.  At least not in the realm of air gun building. (opinion).  For the sake of weight management. It seems that the thinner liners, set in CF, are the logical straight forward approach.  Then you get into the liner under tension as the advanced approach.

FWIW
Bill     
 

[39M]

I haven't looked at them all, but Green Mountain has decent prices on barrel blanks. Although a good bit of them are heavy bull barrels that would probably take a lot of machining to fit an airgun.

[39M]

So I came up with a rough sketch of a design that I think could have some promise.

It's a sort of semi-sabot with a teardrop shaped bullet and a sabot with a cupped base that would only cover the tail of the bullet, enabling the front portion to remain in contact with the rifling for better stabilization.
The sabot would disconnect upon leaving the barrel.

Actually, I think they could be loaded in two pieces, or placed together just before loading, in an air rifle. Therefore, eliminating the chance of the sabot staying attatched as they leave the barrel.

Prolly not feasible, but just a thought.

[rsterne]

I sense a slight confusion here between BC and Form Factor.... If you make a bullet less dense, to decrease the SD, you will also decrease the BC, even if the FF remains the same.... The only way to keep the same BC with a lighter SD is to gain the same percentage on the FF....

Bob

[39M]

After thinking that the teardrop bullet might be too heavy, I decided that a semi-wadcutter hollow point might be a better design(Kind of like a crow magnum). And then I read some of Bob's posts saying that for between 800 and 1000 fps, or  somewhere around those numbers, that the raindrop was a poor design. Or at least not necessary, with the (air-cap?) forming on the face of a flat point design. Which is a bit confusing, considering that a lot of airgunners prefer domed pellets, maybe only for their penetration or because the head size is a good match for most barrels.

Anyway, I came up with a similar but new sketch.

[39M]

I sense a slight confusion here between BC and Form Factor.... If you make a bullet less dense, to decrease the SD, you will also decrease the BC, even if the FF remains the same.... The only way to keep the same BC with a lighter SD is to gain the same percentage on the FF....

Bob

Actually Bob, in my sketches I was going by your theory that a long boat tail is the best way to make use of BC at sub-sonic speeds. There's quite a bit of FF behind the idea, as the boat tail in this case comes to a long point after the half-sabot releases as the bullet exits the barrel.
Maybe the tail is a bit extreme in design?

[rsterne]

For the most part, the choice of shapes for airgun pellets has been flat (wadcutter), pointed, or domed (some domed, the better ones a true hemi-sphere).... For whatever reason, we have not really experimented with the type of nose used in bullets, with a tangent ogive and a flat at the front (Meplat) that is less than the full diameter.... Something like this....



At least in theory, that should have less drag than a round-nose design.... Imagine the stagnation zone and you have an ellipse travelling long-ways, instead of a circle.... However, it still has the skirt, and the associated drag of not only the full caliber diameter base, but the flare in front of that from the waist.... If you could figure out a way to make a saboted bullet and maintain accuracy, it may be worth the trouble.... I don't think you have to go right to a point, they haven't used that idea in racecars since the 1930s (OK, maybe the 50s).... Any bullet has a "wake zone" behind it, the boattail is a matter of trying to shrink it in size.... The ballistics programs break down if you make the boattail less than half the caliber.... I think the ideal shape for what you are thinking might be like a football with both points cut off....

Bob

[39M]

I sense a slight confusion here between BC and Form Factor.... If you make a bullet less dense, to decrease the SD, you will also decrease the BC, even if the FF remains the same.... The only way to keep the same BC with a lighter SD is to gain the same percentage on the FF....

Bob

Also, the idea was not to make the projectile less dense, but to increase BC without adding density.

As by increasing Form Factor.

A short bullet at a certain weight has the same sectional density as a long bullet of the same weight as long as they are the same caliber, even if the materials used to make the projectiles have a different density themselves.

[39M]

For the most part, the choice of shapes for airgun pellets has been flat (wadcutter), pointed, or domed (some domed, the better ones a true hemi-sphere).... For whatever reason, we have not really experimented with the type of nose used in bullets, with a tangent ogive and a flat at the front (Meplat) that is less than the full diameter.... Something like this....



At least in theory, that should have less drag than a round-nose design.... Imagine the stagnation zone and you have an ellipse travelling long-ways, instead of a circle.... However, it still has the skirt, and the associated drag of not only the full caliber diameter base, but the flare in front of that from the waist.... If you could figure out a way to make a saboted bullet and maintain accuracy, it may be worth the trouble.... I don't think you have to go right to a point, they haven't used that idea in racecars since the 1930s (OK, maybe the 50s).... Any bullet has a "wake zone" behind it, the boattail is a matter of trying to shrink it in size.... The ballistics programs break down if you make the boattail less than half the caliber.... I think the ideal shape for what you are thinking might be like a football with both points cut off....

Bob

I guess my thinking of coming to a point wasn't influenced by a race car, which normaly has other needs such as adhearing to length restrictions as well as the need for full width to facilitate downforce on the back of the car. But more akin to the trailing edge of an aircraft wing.

The idea of the non-attatched semi-sabot was to make full use of the limited pressure in an airgun. And the idea that it did not fully encapsulate the projectile to let the 60% of diameter bullet contact with the lands and possibly grooves for greater stability and accuracy.
Definitely an unproven design. But I don't think it has been disproven yet.

[39M]

Another thought about the stagnation zone and it's effect overcoming drag at subsonic velocities.

When you brought up race cars, it got me thinking. Race cars are subsonic. Yet there aren't any that are shaped like a bus in the front. Below are a couple of pictures of the car that set the land speed record, slightly above Mach 1. Note the sharply pointed nose of the car.
I suspect there will come a day when a pellet with a long pointed plastic tip will be used for long range subsonic air rifle shooting.

[rsterne]

Things change completely when you need to approach Mach 1.... The transonic zone is from Mach 0.8-1.2, and things change dramatically in that range.... Here is the drag curve of the G1 projectile....



and here is what it looks like....



Current airgun pellets have an even steeper drag curve in the transonic region, which is why we keep them subsonic (if we have any sense), which is in the yellow area on the graph above.... Incidently, wind drift follows the drag curve, so pushing near Mach 1 may gain you a flatter trajectory, but the wind will push the pellet even further off course.... Last time I looked, gravity was a constant, but the wind was not.... Add to that the inherent internal inefficiencies of trying to push a pellet near Mach 1 using air and you get a double whammy.... You burn more air to get there, and then when the pellet exits the muzzle it gets slammed with horrendous drag and loses that extra hard-earned velocity in the first few yards.... LITERALLY.... Stay below Mach 0.85 or so, and certainly below Mach 0.9 (1000 fps) and you will be a LOT happier with the results....

Bob

[K.O.]

Bob on the .224 that you and Sweede came up with is 43g and wants a pretty quick twist compared to the easily available barrels that are 14 twist..?

What guided that choice?  can he cut a .6-.8 body (the .216 section) instead of a .128

I was so hoping for a O.A.L. closer to .38-.40 or so and a weight of 38-40g... then I could hollow point to about 35g..

http://noebulletmolds.com/smf/index.php/topic,1132.0.html?PHPSESSID=6c6a4q5m9rrrtksbs4hkdtccq2

[39M]

I've never suggested that the reason for attaining a higher BC was to obtain more speed, but instead to carry the speed available more efficiently. I would suggest that a bullet with a higher BC would paint a different picture in your drag coefficient arcs.
And with the looks of that graph, with that bullet, I'm not sure why 892 fps shouldn't be the goal.

As far as gravity and wind, time and distance play a major factor also.
It is true that gravity is a constant, in a progressive sense. Some might confuse a gravitational rate of 32 fps/s as equaling 16 feet in a half second, but due to gravities progressive nature an object will fall far less than 16 feet in it's first half second of free fall.

Similarly, while an object traveling at a rapid rate might have more pressure exerted on it by wind, it will have traveled a farther distance during the time the wind has to push it than the same object traveling at a slower pace. And an object with a lower BC will be blown further off course than one with a higher BC ober the same distance.

Again, I wasn't trying to convince anybody to try and attain a speed nearing mach 1. I just thought that a more efficient projectile with a higher BC would be of some value to whoever mihht be trying to push the limits of extreme long range shooting with a 950 fps air rifle of any caliber.

[rsterne]

That design is 2 calibers long, and I didn't design a lighter one because I couldn't imagine anyone would want one.... There is a .217 cal 37 gr. that I did make shorter, and the reason was to fit Hatsan magazines.... Even that requires a 12" twist.... Simply shortening the middle of the bullet would shorten the support in the rifling, and I would not recommend that.... It's not something Al could do anyway, a different cutting tool is made for every design.... If the bullet was done at 1.5 calibers long it would be about 33 gr. and require a 17" twist.... At 1.75 cal long it would be 38 gr. and require a 14" twist.... It sounds like that is what you are looking for, and I could add that to the list, whether it would ever reach production or not would depend on demand.... You can see exactly what it would look like by looking at the .308 cal 100 gr.... scale that down to .224 and it would be 38 gr....

http://noebulletmolds.com/smf/index.php/topic,1121.0.html

The good thing is that it would work fine in the TJ's 14" twist barrel.... but then the 43 gr. may as well, only trying it would find out for sure....

Bob

[rsterne]

I misunderstood.... your reference to the pointed nose on the Thrust SSC which was designed to break Mach 1 confused me.... Yes, Mach 0.8 (~900 fps) is a great place to be with an airgun.... You must realize I have been studying BCs for a while.... do a search on Bob's Boattails, or check out the Engineering Gate for a post on using the "McDrag" program to optimize airgun bullets.... As far as wind deflection goes, the key is the DIFFERENCE in the time of flight between a vacuum and in the real world, not the time of flight per se.... That is why the Drag Coefficient is so important.... BC takes into account the curve in the G1 profile (or another, similar one), and a constant BC does NOT mean constant drag or wind deflection.... As I said, the wind deflection follows the drag curve above.... Check out the JBM Drift calculator, and input various values for the muzzle velocity and you will be surprised by the results.... Here is an example, actually three examples....



Note, there are for bullets, not pellets, including the super-slippery Sierra Matchking.... To achieve the same wind drift as that bullet has starting at 800-900 fps you have to push the muzzle velocity all the way to 2600 fps (way out of airgun territory).... Any velocity in between will have greater drift....

Bob

[39M]

Bob, I do have a lot to learn. It's an ongoing struggle

The graph is similar to yours with the G1. But it does show that the heavier, higher BC bullet has less drift across the board than the lighter blockier bullets.

And it brings up another question in my mind. This one regarding K.O. and his 43 grain .217 pellet.
It seems to me that the reason for the 950 fps for small caliber air rifles is due the the greater effect outside forces have on the lightweight low SD & BC pellets and the fairly short distance that targets are set at.
And when moving up in weight to around 40 grains, there may be some benefit to a slightly higher velocity. Say the 1060 fps that .22LR target ammo uses? I would imagine that the larger forces that are placed upon the bullet would be short lived as it loses initial velocity quickly and drops into the sweetspot over the longer distsnces that would most likely be targeted.

[39M]

The Blue Flame also has a fairly pointed profile. It only went 924 feet per second.

[K.O.]

well I am looking at it from the view of what many have(a stock .25 Mrod) honestly an easier path to a higher B.C. round than  Mods for high power...

with a 10-15 pound spring and a 24" barrel I figure a stock Mrod could hit 65 fpe for near a mag with a 2%-4% es...

that would not require being tethered or a tanker conversion in other words easy and assessable to those that use a hand pump...

About the best you could get at that level but would still be useful to those that have more fpe on tap..?

I did notice that the boat tail is .1 long on it and on the .250 and .217 it is .89 so I guess it could be shortened a touch(could be done by hand after casting) (with slight increase in drag?) 


Now 65 fpe with 43g is 824 fps just a bit slower than I want to go my target is 850-950 fps so losing the weight  would be a plus but mainly it is the twist rate... because as drawn just a hollow point would bring it down to 39-40g and should get about 850 fps...


but hollow pointing the 14 twist round to about 34-35g well that would be just about perfect for 950 fps...

Now me I am nuts enough to send it out of the 1:16 Marlin .22WMR Barrel but that is besides the point...

The only other .224 right now is he that opposes the Puppeters(Doug) and he has power to spare but a 1 :14 twist...

Me I like the idea of being able to switch between  the stock .25 green mountain pellet barrel and a .224 barrel with maybe a spring and tport change...

.25 for shooting up into the trees and .224 for the open desert... 

oh I think the B.C. of the BBT .224  listed has  been estimated a bit low  a 30g .22 at 900fps has a B.C. of about 1.4 iirc...

But anyways I do think designing for a 14" twist would be attractive to many just because it would not require a $$ custom barrel... just a $ barrel... and is more in reach power/shot count wise...

Thanks Bob and I would be interested but it would be about 3 or 4 months before I could actually purchase a mold... expecting my first grandchild on the 28th  and then Christmas and the birthdays of my Kids are one a month till Feb...

plus I just could not resist a brand new Bsa Buc for what it was going for and have to finish paying it off...

[K.O.]

"
And it brings up another question in my mind. This one regarding K.O. and his 43 grain .217 pellet.

"
first is not my 43 grain .217 pellet...

I am talking about a .224 bullet... of Bobs Design...

second he did design a  37g .217 bullet (not pellet)... and not 43g...
http://noebulletmolds.com/smf/index.php/topic,1127.0.html

But as much as I would like to see a .217 BBT (Bobs Boat tail)  I once again wonder if the twist required is a bit to much requiring a custom barrel...

Me I am looking at it from the view of broadening the amount of rifles capable of sending the round out..

Now at .250 and above well the amount of air used needs a big tank even with lightening the round...