Pellet design question (.177)

[ptthere]

Bear with me... lol

I have found that my favorite overall pellet (for shooting past 15 yards) are the domed ones. I'm particularly high on H&N FTT's, they are giving me great results.

However, I can't help but wonder "what if?" in terms of all the learning I did years ago when the rimfire .17 HMR and .17 HM2 debuted. For those of you who have owned or shot either, you know exactly just how INSANELY accurate these rifles/rounds can be. The HM2 is all but dead now, but has an interesting story. At one time, it was considered one of the most accurate target guns at 50 yards. Unfortunately for the slower-shooting HM2, popularity favored the HMR because of the much higher velocity (Mach 2.3+), and it was really splitting hairs between the two anyway as owners found the HMR to be almost as insanely accurate as the HM2, even with the higher velocities (and the frankly explosive impacts with frangible Hornady rounds).

So what does this have to do with air rifles? Well, apparently not much... But the question is, why doesn't it? Back when the HM2/HMR rounds were designed by Hornady, the bullets were identical, but the HM2 was the most accurate due to slower velocity (as we see with pellets). However, even the "slower" HM2 was still flying well above the sound barrier from the muzzle (Mach 1.6+ if my memory serves me right).

So here is where things get interesting: Hornady specifically designed that 17-gr bullet as a long and slender tipped boattail because it "stabilized flight" the best after several re-designs and testing of numerous concepts. Now, we all know that shooting accurately at subsonic, transonic, and supersonic velocities are three completely different beasts. But there was obviously SOMETHING that the good people at Hornady saw in their testing that made the pointed boattail design a winner for their ballistic tests.

But safely removing a .17 HMR bullet from an assembled rimfire casing would not be for the faint of heart (don't want to ignite that primer in the rim BOOM). And even if one did, IIRC, the .17 HMR bullet couldn't be test-shot well enough out of a magnum springer or NP2 etc accurately, because the .17 HMR round is technically .172 if my memory serves me right. Even if it were .177/4.51, copper jacketing could be problematic in certain types of airgun barrels from what I have learned.

So, now that I covered some of the bases to preface my question: Has anyone attempted to make a pellet that closely resembles the .17 HMR rimfire bullet? I understand that a bullet moving at Mach 2.3+ will enjoy much more stability than a bullet/pellet traveling right at Mach 1, but Hornady was anal about fine-tuning design for a reason.

So, why don't we see anyone selling a polymer-tipped smooth boattail pellet for air rifles? My first thought was "it's because of lack of a skirt," but we see Piledrivers and others being sold without skirts, so... hmm. Did anyone ever attempt anything like what I am getting at here?

 

[brink]

Do you have any pictures of the Hornaday bullet? 

 

[leadfoot]

You can push a Diabolo or wadcutter type pellet through an air gun bore pretty easily because they are soft lead and only have minimal contact with the bore at at two areas. A bullet as you described would have full contact over a long surface area and would need a lot of "oomph" to push it through. A "magnum springer or NP2" would not produce enough foot pounds of energy to do the job. Some PCP airguns are designed to shoot slugs, but the weight, softness of the alloy, diameter, and velocity requirements exist in some fairly narrow limits.

 

[Arzrover]

In addition to "leadfoot"s statements, the design of projectiles for subsonic and supersonic are radically different. If you look at match rimfire ammo , the overall shape is more appropriate to pellets because a LOT of research has been done on subsonic projectiles for the rimfire world. It's quite difficult to push projectiles with air supersonic and the would need to be well above to be useful because the transition to subsonic destabilizes them. Look at the pressures required to push a rimfire bullet supersonic. The Hornady copper clad would be even higher. I won't say impossible, but not practical with our relatively low airgun pressures .

Bob 

 

[ptthere]

Do you have any pictures of the Hornaday bullet?

There's a picture of a factory load of .17 HMR, to give you an idea of what the bullet looks like. The part that you cannot see is the rounded boattail on the back of the bullet.

To the other responses, my follow-up question in terms of "oomph" required by an air rifle to shoot a projectile of this size: The picture above is a non-lead 15.5 gr offering. Standard lead Hornady rounds are 17 gr.

So is the conjecture that a 20+ FPE air rifle still is not enough to push a 15.5 or 17 gr round down the barrel reliably, or that the shape and copper-cladding of the bullet is not ideal for air rifle pressures in the barrel, or that the round in general is just not ideal for flight under 1150 fps? Or maybe a combination of all?

PT

 

[cosmic]

15.5 nsa swaged slugs are the only .177 I know of.. And they are neither boat-tailed or poly tipped ..

 

[steve-l]

The principle of flight stability for an air gun is completely different to a powder burner. Flight stability of an air gun pellet is provided by the Diabolo shape and that shape induces drag stability, like feathers on an arrow. A bullet or slug's flight stability is provided by spin and the rate of spin required is determined by its sectional density or length , if you prefer. Where the greater the length the greater the RPM required to stabilize it. Too much spin and the bullet will not pitch over at the apogee of its flight path and tend to pancake and skip as the bullet encounters air of different densities. If the bullet is under rotated for its length it will wobble and even tumble in flight. This over/under rotated principle can be demonstrated with a thrown football. With the velocities possible with an air gun, the required spin rates of long boat tail bullets would be very hard to achieve because of the soft lead alloy, the shallow bite of the rifling and lower pressures available.