Sectional Density and Copper

[stevec]

I have been listening to the excellent interviews with Dr Kevin Robertson on the Big Game Hubting Podcast. He is very entertaining and knowledgeable .

He spoke at length about the importance of sectional density. I think he looks for a minimum SD of .270. Incidentally, he is a big fan of Barnes TTSX and North Fork Bullets.

It is very difficult to achieve this with copper in standard twist rate barrels, particularly given that standard factory ammunition is generally lighter.

Does Sectional Density actually matter, given the superior penetration of copper bullets?

I suppose it is a theoretical point for most of our UK shooting in any case.

 

[jthyttin]

Does Sectional Density actually matter, given the superior penetration of copper bullets?

There's nothing "superior" in copper or other monolithic bullets.

Some people are caught on the residual weight hoax and forget the physics.

Penetration is more or less result of resistance and momentum. Frontal area, medium and velocity affect the resistance. Velocity and mass affect the momentum. These of course change along the terminal path of bullet.

Problem with monolithic bullets is that they usually retain more frontal area than traditional bullets, at medium-to-high impact velocities. They also have somewhat low mass per given bullet length, like you point out. So if the monolithic bullet is fast to expand, mushrooms beautifully and retains 100% weight -> penetration actually requires more mass i.e. SD for given diameter.

In practice you need to test the terminal performance, it could be modeled in theory but there are so many variables that actual testing (ballistic gel, field testing etc) is much faster and cheaper, and provides close enough results. For individual user, it's much easier, faster and cheaper if the manufacturer, other users or some 3rd party has done the bulk of testing and published results.

Conclusion? Ask for field results and look for ballistic media testing for the very bullet you're interested in. The results are not universal for given manufacturer or even given design. And they always depend on target species, shot placement and impact velocity (cartridge and distance).

 

[nun_hunter]

So if the monolithic bullet is fast to expand, mushrooms beautifully and retains 100% weight -> penetration actually requires more mass i.e. SD for given diameter.

Surely retaining almost 100% of say a 120g 30cal bullet Vs 50% of a 180g bullet also in 30 cal means that it'll have more mass and penetration for a given velocity once the bullet has passed more than 5-10cm into an animal?

 

[jthyttin]

it'll have more mass and penetration

Don't forget the frontal area, it has linear inverse correlation (or thereabouts) to penetration. Double caliber expansion has 4x frontal area compared to non-expanded.

 

[nun_hunter]

Don't forget the frontal area, it has linear inverse correlation (or thereabouts) to penetration. Double caliber expansion has 4x frontal area compared to non-expanded.

I suppose some of the copper bullets that the petals peel back and break off would be better as the frontal area for penetration of the shank wouldn't increase but you'd still get additional wound channels and energy dissipation from the expansion.

 

[jthyttin]

Yes and there are more traditional bullets also that are designed to shed the front part and keep penetrating with quite small frontal area. Intentional examples include Winchester FailSafe, and also constructions like Nosler Partition (if driven fast enough).

Also expanding monolithics will shed the front part if impact velocity is high enough (1000m/s or so in my limited experience).

Another thing is, whether the user wants or is prepared to deal with multiple wound channels or the results of high impact velocity.

 

[webley701]

Yes and there are more traditional bullets also that are designed to shed the front part and keep penetrating with quite small frontal area. Intentional examples include Winchester FailSafe, and also constructions like Nosler Partition (if driven fast enough).

Also expanding monolithics will shed the front part if impact velocity is high enough (1000m/s or so in my limited experience).

Another thing is, whether the user wants or is prepared to deal with multiple wound channels or the results of high impact velocity.

Bullets from Yew Tree & Virtus (to pick just two) partially shed their mass at considerably less than 1000m/s.

 

[Selous]

Sectional density becomes less relevant or even irrelevant with bullets of different constructions and was only ever valuable as a very crude measure of penetration performance between similar projectiles. Other factors quite simply have much greater effect. Take another example of Kevin Robertsons where they tested a Dzombo round nosed solid of the same calibre, weight, sectional density and velocity as the Dzombo flat nosed solid. Both were fired into wet newspaper from the same range but the flat nosed bullet penetrated a metre deeper than the round nosed solid with the same sectional density, purely as a result of a change in nose profile.
With non lead ammunition the variation is even greater as some are designed to deform but not fragment (eg Barnes TTSX) , some are designed to fragment into larger pieces and leave a cylindrical bullet base to penetrate (eg Yew Tree, Virtus) and others for the frangible front core to vaporise leaving only the rear core to penetrate (eg Geco Zero). These differences may mean different ideal SD values are needed for each design.
For UK use the 130g TTSX monometal bullets in .308 calibre work very well so you could probably use a sectional density value of 0.196 as being satisfactory for UK deer for this bullet design (expanding but non fragmenting) providing the design deforms rapidly and mass is maintained following impact at a specific terminal velocity.

 

[Heym SR20]

It doesn’t really matter about all the different theories on ballistics.

All that really matters is that there are plenty of very good monolithic bullets that are accurate and provide very good terminal effect on animals.

Like all bullets you do need to match the bullet to your rifle, cartridge and more importantly the intended game animal. A bullet designed to shoot Oryx which are the size of horse, and significantly tougher might be quite so good on a small dainty Roe Buck.

And lets not forget that original goal behind the early monolithic bullets such as Barnes was a bullet that would hold together at magnum velocities when impacting a reasonably large and tough animal.

The fact that most monolithics penetrate very well and cause a good wound channel means that they work. Every else is just down to the shooter.

As to aerodynamic properties, bullet mass is just one factor, shape, in particular length and launch velocity has far more effect on the aerodynamics and trajectory of the bullet.

Besides a flat trajectory, long point blank range etc is in many ways much less important. Trajectory is a known, range is easily determined and ballistics calculations easily done. Wind is a challenge - always has been, always will be. But, truth be told BCs etc only really become an issue over 300m, and the vast majority of hunted animals on which you are using monolithics are shot with 200m, indeed majority will be 100m or less.

 

[jthyttin]

Bullets from Yew Tree & Virtus (to pick just two) partially shed their mass at considerably less than 1000m/s.

I should've been more clear, but I was expanding (sic) on nun_hunter's notion about monolithics that are designed to break up. AFAIK at least Virtus counts among these.

Point being, even monolithics that are designed purely for expansion and not to fracture into separate pieces, will shed the front (up or rather down to the bottom of front cavity) if you drive them fast enough. The rear part will stay at almost the nominal diameter.

 

[Edinburgh Rifles]

It matters as much as Coefficient of Drag does on your 5 series BMW….

 

[Heym SR20]

It matters as much as Coefficient of Drag does on your 5 series BMW….

Surely you mean on your Audi A6 Allroad and how it’s vastly different the 5 Series BMW.

Indeed you can use a nassly voice and start comparing the drag coefficients of all the different BMW 5 Series models. But do not ever ask a Porsche Driver about the difference between the drag coefficients of the various Porsche 911 variants …… vitally important…………….However probably not as bad as asking a golfer about differences between golf balls.