Is SD (Sectional Density) by itself Worthless?

[MMACFIVE]

After many decades of reloading and hunting I am of the opinion that SD in and un its-self is worthless.

 

For example I believe a 7mm 120gr Barnes TTSX with a SD of .213 will out penetrate a 180 Berger 7mm with a SD of .319. What is important is the Post Expansion (PE) SD and the Berger looses so much weight and has a larger diameter which gives it a lower PE SD than the Bartnes 120.

 

Any Comments.

[308Nut]

1st, why do you say that the barnes will out penetrate the berger? It may in some cases and not in others.

 

If all else were equal (ie construction, expansion ratio, material, etc...) the bullet with the higher SD wins. Period.

 

Also, 2 bullets that have equal caliber and form factor but have different SDs, which do you think will penetrate more?

 

I think what you're looking at is a barnes 120 blowing through a section of ballistic gelatin where the berger might have exploded and didn't exit. That has nothing to do with SD but everything to do with construction and expansion properties.

 

SD is also a direct contributing factor to ballistic coefficient. That is part of the reason barnes bullets have such low BCs. Equal weight and form factor, the jacketed lead bullet will have a higher BC. Why? Because (among other contributing factors such as specific gravity and others) the all copper bullet has more mass which produces more air drag and doesn't weigh more to be able to offset the added drag. Which will penetrate more? That depends on how much each will expand.

 

SD is not worthless. You are looking at your example without the right context.

[.270]

All that stuff is worthless to me. Gives me a headache. Except velocity. I like velocity. Barnes bullets also have a bunch more barrel drag, because they are considerably longer than a like weight copper/lead bullet. Causes pressure to go way up. I tried some once in my .270. Had to load em 400 fps slower than a like weight regular bullet to keep from blowing primers because of the extra bullet length in the lands. Said right on the box to load em at least .040" off the lands to keep pressures down. They've improved em some with some design changes. Still hard to get a lot out of em. All that stuff, bc, sd, fps, gfy, means something but it's too hard to remember. Lark

[MMACFIVE]

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[MMACFIVE]

All that stuff is worthless to me. Gives me a headache. Except velocity. I like velocity. Barnes bullets also have a bunch more barrel drag, because they are considerably longer than a like weight copper/lead bullet. Causes pressure to go way up. I tried some once in my .270. Had to load em 400 fps slower than a like weight regular bullet to keep from blowing primers because of the extra bullet length in the lands. Said right on the box to load em at least .040" off the lands to keep pressures down. They've improved em some with some design changes. Still hard to get a lot out of em. All that stuff, bc, sd, fps, gfy, means something but it's too hard to remember. Lark

Its too bad you've have such a bad experience with the pressure with Barnes. Since the put grooves on the and called the TSX I have never seen this. I drive them at the same velocity with about the same powder charge as conventional bullets.

[MMACFIVE]

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[MMACFIVE]

1st, why do you say that the barnes will out penetrate the berger? It may in some cases and not in others.

I am not trying to make this an argument between Berger and Barnes; Its just that they are kinda the extremes in construction right now. I'm just try to say that technology and changed so much over the last 50 or so years that SD does not have the meaning it once did. When all bullets were a form of cup and core a person could use SD as an estimation of penetration potential. That's not the case anymore.

If all else were equal (ie construction, expansion ratio, material, etc...) the bullet with the higher SD wins. Period.

Exactly, If all else is equal the bullet with the heavier SD will penetrate more. I can't not count the times I've read on these forums "don't use that 145g LRX cause it just wont have enough penetration compared to the 190 Berger...." B.S.

Also, 2 bullets that have equal caliber and form factor but have different SDs, which do you think will penetrate more?

I agree but you missed equal construction....I think you were implying that though?

I think what you're looking at is a barnes 120 blowing through a section of ballistic gelatin where the berger might have exploded and didn't exit. That has nothing to do with SD but everything to do with construction and expansion properties.

Once again I agree...I think this actually supports my theorem that "SD in and un iits-self" is worthless.

SD is also a direct contributing factor to ballistic coefficient. That is part of the reason barnes bullets have such low BCs. Equal weight and form factor, the jacketed lead bullet will have a higher BC. Why? Because (among other contributing factors such as specific gravity and others) the all copper bullet has more mass which produces more air drag and doesn't weigh more to be able to offset the added drag. Which will penetrate more? That depends on how much each will expand.

This statement I might have more problems with. SD is not a direct contributing factor to BC. Only if it has the same form factor (but you might have meant that too?) e.g. a cup and core 220g round nose and a cup and core 220g VLD form factor with have a very different BC. And BTW copper has less mass than a lead bullet not more. And mass does not produce more air drag; form factor does. That's why a 6.5mm 140g berger with a SD of .287 has about the same BC and SD and thier 7mm 168gr bullet. If they have the same velocities won't they have the same trajectory?

[ThomC]

The late Jack O'Connor said "You need to take a little salt with the figures".

[.270]

The pressure is just part of the problem. Every time I've ever been with anyone using copper bullets it has turned into a tracking job. Every time. No exceptions. With well placed shots from enormous rifles. In fact, I won't allow em around me. My load development program consists of using a bullet I know from experience will work, every time. Tinker a little until it shoots good groups, chronograph it and use it. Couldn't care less about all the numbers on paper. Seems to work fairly decent. You can do all the fancy calculating and stuff you want. None of it means much but on paper or targets. The biggest factor in everything is the guy holding the rifle. If he can hunt and then shoot when he finds what he's after is what matters. Lark