# SD, or ifSD?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Delmer, Jun 9, 2013.

1. ### DelmerNew Member

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Jun 9, 2013
Hi shooters. I'm new at the cerebral aspect of shooting and hunting (more of a shooter so SD never really entered my mind). I see the definition and I think about one of the weapons I own, and I just wonder if I truly can apply the math and conclude that the setup I'm using is any better than a factory load when it comes to penetration.

Leaving details out that are unimportant (so as to not seem self-serving more than anything.. I'm not here to say I have better stuff than anyone) when calculating SD, I do think that rather than using the diameter of the bullet you seated, you really need to understand that the bore diameter.. more specifically the diameter of the bullet as it LEAVES the barrel is the diameter that should be used to calculate SD when truly assessing/comparing theoretical performance.

For example, the various flavours of Lee Enfield rifles that were chambered for the 303=British commonly vary quite a bit in their measured bore diameters. If you fire a 311 diam bullet through a tight barrel, besides the horrible things going on inside there, the bullet actually exits with a higher SD than it would if it were fired through a worn or loose barrel.

So a question- if I buy 7.62 54r and pull the 180 gr 311 diam bullet out, neck-down the case and press in a 308 180gr bullet.. and fire it, does it exit with a SD that is any better than the factory 311 bullet?

My question stems from the claim one person made that making proper rounds for a tight bore will result in "better ballistics" due to the fact that it is smaller diam. I would agree "better ballistics" due to the loss of unnecessary friction and thus higher MV, but I'm here to argue that the diameter ends up being exactly the same and since the weight is the same the SD's are equal.

Am I off track here? or just thinking too much?

2. ### J E CustomWell-Known Member

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I think you mean BC (Ballistic Coefficient) Instead of SD (Standard deviation).

There is a formula for BCs (G1 and G7) that works best for calculating Ballistic Coefficient based on bullet shape and type.

SDs are based on velocity averages and extreme spread.

Bryan Litz has a great book on Applied ballistics that will help you understand external Ballistics
better and will answer most if not all questions. (Not that I understand all there is to it, but it has helped me a great deal).

J E CUSTOM

3. ### barnesuser28Well-Known Member

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SD also means Sectional density... I think the OP is WAY over thinking it. SD is important but not THAT important.

4. ### jfseamanWell-Known Member

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Thinking too much? Hmm, I believe not possible but that's just me.

Off track. Well it's not how I would do it to achieve better accuracy with a 7.62x54R.

I consider everything a 'target' rifle and want ammo that's fun, fun to me is accurate. Not perfect for a 7.62x54R like I would for a bench rest rifle but good.

At the moment I am shooting Speer Hot-core 150's #2217 Speer Hot-Cor Bullets 303 Cal 7.7mm Japanese (311 Diameter) 150 Grain

The powder could be anything from Trailboss puffer loads to a good stiff loading. Depends on my 'mood'.

I let a friend shoot some of my loads and he was amazed at the difference in accuracy to 'surplus'.

Get some Winchester 7.62x54R brass and make some fun.

5. ### Michael EicheleWell-Known Member

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Sectional density (as we use it in this context) is factored by two things. 1: Bullet weight 2: Bullet diameter. What ever the bullet weight/diameter is when it is in your hands may be slightly different when it exits the muzzle based on barrel dimensions and how bad it rakes copper off the jacket but you are splitting frog hair at this point. Smaller diameter bullets of equal weights of larger diameter bullets have higher sectional densities. The higher the sectional density, the more potential you have for a better BC. At least assuming you also have a good form factor. When it comes to equal weight + equal form factor, the higher the SD, the better the BC.

Shooting a 155 grain palma bullet in a Palma tight bore is not going to increase its sectional density and subsequent ballistic coefficient to any appreciable degree.

In theory, a 155 grain Palma bullet could change it by roughly 5% if it started out as a .308" diameter AND ended up exiting with a .300" dimension. That said, I'm no expert and I think the old Palma barrels (tight bore) were only .306ish. .300 might be a suicide mission.

Welcome to the forum.

M

6. ### J E CustomWell-Known Member

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First- if you ask a question don't get offended, everyone has there opinion and it may be different
than yours or someone else. (It is just a discussion).

With that said, There are two diameters in a barrel, There is the bore diameter(Grove diameter)
and the land diameter (Rifling) Bore diameter is the same (Or Close) to the bullet diameter.

Land diameter is less than the bore diameter so it can engrave the bullet. This diameter may vary based on the barrel makers preference and number of rifling's/lands (2,3,5,6 and 8). they may vary
from .003 to .005 In height. Example: A 308 win would have a bore diameter of .308 and a land
diameter of .300 (.004 land height) +/- .0001 or .0002.

And yes, accuracy if at its best with a good fitting bullet but the ballistic coefficient is not changed
much by the rifling process. Sectional density is changed slightly by the bullet being compressed
in the engraving process but again ,not much.

In general the ballistic coefficient is set by the bullet maker based on the length, weight and diameter.
Shape can alter the length of a bullet of the same weight and caliber of another bullet.

There are lots of things that have to be right to have a good shooting rifle and in the long range
game everything comes into play. High ballistic coefficient's, low Standard deviations, powder densities

Being aware of the quality of your system is paramount to long range hunting /shooting but there
is a point of diminishing returns and some things are not worth the effort and everyone has there
own idea where that is.

Just my opinion

J E CUSTOM

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8. ### J E CustomWell-Known Member

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OOOOOKKKKKK. What is the point.

I have never seen a mosin Nagant at a precision shoot or any other service weapon.

Lots of the service weapons will shoot very well but not bench rest or long range precision
hunting. They were not made for that purpose and distance.

Just my opinion.

J E CUSTOM

9. ### jfseamanWell-Known Member

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The pure fun of it.

No you won't find it at a precision match but you will find it at the range getting shooting time.

Just because it don't float your boat doesn't mean that someone else wont have a good time with it.

10. ### BengWell-Known Member

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JE Custom said in a polite manner, that Shooting a Mosin Nagant and worrying about the effects of actual barrel diameter on bullet sd just doesn't fit

A 180gr .311 caliber bullet and a 180gr .308 caliber bullet of similar formfactor
would have ballistic coefficients of .5 and .51 respectively.
Launched at 2600 fps in a 10mph 03 o'clock wind you would see a difference in wind drift of 2.5 cm and 6cm in drop [ps: at 1000yards].
That's insignificant, more important might be to what degree the bullet is damaged when launched in a barrel of wrong diameter.

Effects on terminal ballistics are again most propably due to damage to the bullet and not due to a minor changes in sd.
The bullets construction is much more important than initial sd anyway.
That determines the bullet's ability to change sd inside the target medium.

11. ### J E CustomWell-Known Member

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I don't mean to sound like I don't like to shoot one, just that I would not use one for long range.
I am all for having fun.

I love to shoot the old service weapons and have owned many of them but with the ammo situation
I shoot them far less than I use to. Unfortunately, the days of going to the range and burning 500
to 1000 rounds is gone for me, and precision shooting is still a practical/possible way of enjoying
shooting for less ammo expense.

There are however some rules that I wont break. One is shooting the wrong size bullets for a rifle.
If it has a .311 bore dia that's the size of the bullets that I use. some of the ole rifles have different
bore sizes than we use today( .223 instead of .224, .307 instead of .308) are just some examples.
and if I cant find or make the proper size bullets, I sell the rifle and don't worry about it or it becomes
a safe queen.

There have been cases of different size bullets being used with a certain amount of success but the
practice is risky. PO Ackley fired some 8mm rounds through a 30/06 to study the effects. He reported
that there were no dangerous pressure effects but he did not recommend anyone doing
the same thing. I is common practice to use .224 bullets in a 22 hornet with .223 dia bore But that
does not make it OK.

If a person substitutes components they are risking problems that are not necessary.

So just because someone is doing it doesn't make it safe it just means that they are willing to risk
it and in a different rifle it could be fatal.

Be careful and have fun.

J E CUSTOM

12. ### DelmerNew Member

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Jun 9, 2013
First, Thank you Gentlemen. I had no idea that this volume of quality info could be had on any public board in just a single day. I quit them about 10 years ago being just turned off by how people always just degenerated into conversations similar to what's developing here. Hey- we are what we are though.

I'll accept that I'm thinking too much. I'm not at the technical point in my prowess to be worried about those effects translating onto the "target". I have yet to get any trigger work done.

Myself I understand Fred's sentiment. I shoot for fun. I learn for fun(and brain exercise). Let me extend Fred's point a bit with an analogy using my favorite group of backward enthusiasts- bicyclists. They will pay a few thousand dollars so that their exercise can be easier... carbon fibre bike, shave all the hair, teflon stretchypants.. etc. Me I would strap on a huffy and a large windbreaker and get 3x the amount of exercise per mile than them... because it's HARDER.

So I buy a \$5000 marksman rifle and I can hit a target at 800 yards under MOA... but my BUDDY, he can get under MOA at 600 yards with his NAGANT! One of us is truly doing something special.

Funny really, I don't disagree with the counter opinions regarding war weapons.. but one of my 54R's is tight to 308, a late 70's 27" finnish army marksman/sniper trainer, based on the Nagant. fully ALL of my long range target shootin' is strictly with war weapons including Lee ENfield (can't beat that report), 1903A3, M14. Can't beat the prices either. I still get to "tune" them with diminishing returns but improvement is success and it feels just as good.

The M28/76 is floated by the army already.. and that trigger- wow. How cool is the peepsight?? original euro tracks for sling and bipod! There may have been one of 2 of these aimed at terrorists durign the olympic games fiasco back in the day. (See Mr Willard, I CAN combine firearms and history.. you were wrong, and you're still a high school history teacher.)

Some take-aways then...

1. "in-flight" Sectional Density is technically better when a given bullet is fired through a statistically tight bore verses a loose one of the same base caliber... but "not by enough that it matters".

2. a .308 diam bullet fired through a tight-bore .311 rifle does *not* have "better ballistics" than a .311 diam bullet fired through same. They are equal even though they started out at different diameters..

3. SD is calculated using unfired diameter and weight... and I say that method is just a bit flawed technically.

Ok Guys.. thank you sincerely for the input. I plan on learning more about the concepts you have presented by revisiting the text a dozen times as I progress.

Last edited: Jun 11, 2013