Sectional Density and cast bullet alloys

[w30wcf]

About 20 years ago I discovered that I could push a 12 b.h.n. (w.w. + 2% tin) .22 caliber 51 gr. cast bullet (225415) with good accuracy and no leading to 2,600 f.p.s. in the .22 K Hornet.

A few years later I was working with Lyman's 311316 in the .30-30 and was able to push this 120 gr. bullet to 2,300 f.p.s. with decent accuracy and no leading. The alloy was the same 12 b.h.n. w.w. + 2% tin.

These two examples have one thing in common ....lower bullet sectional densities of .15 to .18.

By comparison, using the same alloy and a 188 gr. RCBS cast bullet, about 1700 f.p.s. is all that I can achieve with decent accuracy and trouble free shooting. The sectional density of this bullet is much higher at .28.

Has any one else had a similar experience?

Thank you,
w30wcf

[JohnH]

I've been reading your posts, you seem quite a knowledgeable fella, (certainly more so than I) but I hate the term "sectional density". What the %&^% is that????? Properly, density is a term to describe mass. The ratio of a bullets weight to the caliber has nothing to do with the mass of lead. That does not change, and belongs to another science altogether. I'm not sure, seems to me that Hatcher came up with this (I'm sure I'll be corrected) but whoever it was, I wish they had chosen another name for their formula. In fact, as cast bullet shooters, we have more control over the density of our bullets than any other shooters out there, as true density changes as the alloy changes. "Sectional Density".... How I hate that term...... Worse, I hate the math, it has nothing to do with density or mass or E=MC2 or anything like that.

I have to readily admit I don't have the kind of experience you are looking for in your thread, and I do believe there is something to your observation, but I doubt it has anythig to do with density. Sorry for the hijack, I do appreciate your patience on my rant on this.

A good day to you w30wcf, I hope you have a good sense of humor nad that others have similar experience to share.

JohnH

[waksupi]

I believe with cast bullets, bearing surface, would be a better term than sectional density. Bearing surface would have the effect of guiding the bullet on it's path, And I have a personal preference for longer bullets. I believe Bell of Africa advised 3.5 caliber length for a bullet, and I have been very happy with this type relationship in most hunting bullets in cast. The bearing length tranfers to sectional density, enabling a deep wound channel. What more could a man want? I do understand the desire for a shorter bullet for different velocities, accuracy, and recoil levels. At the shorter lengths, I have not shot enough cast in this size, to know about the relationship in bearing surface,and pressure.
I figure the Oldfeller 6.5 cruise missile is about 12 times length to bore diameter.....Now, THERE, is some sectional density, AND bearing surface!

[JDL]

There was an artical in Handloader some 25-30 years ago about short, hence low SD, boolits being able to withstand higher velocity. Later today I'll dig around and see if I can retreive it.-JDL

[sundog]

Well, I started to repsond, but then trashed it. Then reread the thread and decided to add this. Alot depends on twist. Ric, your "banana'd" Swede boolits are a prime example (and I'm beginning to buy in on that now). The shorter, stubbier boolits are by nature inclined to work better in slower twists, which also make them cast friendly. To much twist and run the risk of stripping and other bad things. One of the easiest gun/boolit combos to get to work is the 32 Win Spl with a 16 twist and a 170 FN (VERY low SD). One of the most difficult -- Karlina.

Now, about those bent 6.5s. I would think that as diameter increases in a proportional shape that it would be more difficult to bend. IOW cast is not the best material to use for the Swede, even though we can get it to work up to about 1650, whereas the 32 WS is no problem at 2000+. Now how to explain something like the 358009. Low SD, but long, and I know that in my 14 twist can shoot 1900 out to 200 yards with no problem. Biggest problem with low SD as I see it is bleeding off velocity to quickly. "Gotta know your limitations," Harry Callahan. sundog

[waksupi]

Now, about those bent 6.5s. I would think that as diameter increases in a proportional shape that it would be more difficult to bend. IOW cast is not the best material to use for the Swede, even though we can get it to work up to about 1650, whereas the 32 WS is no problem at 2000+. Now how to explain something like the 358009. Low SD, but long, and I know that in my 14 twist can shoot 1900 out to 200 yards with no problem. Biggest problem with low SD as I see it is bleeding off velocity to quickly. "Gotta know your limitations," Harry Callahan. sundog

Definitely harder to bend, as they get thicker. Think of two short pieces of wire, of proportional differences relating to bullet diameter. The thin one you can easily bend in your fingers, the thicker one, takes more effort.
I would think the 358009 would have fairly high SD? I know I can hit pretty regulary off hand to 250 yards with the .358, with the kissing cousin of the 358009, the Bator Heavy.

[w30wcf]

Thank you all for the input.

As we know, sectional density is simply a ratio between the weight of the bullet and it's area. Simply put, the longer the bullet, the higher the density,
the greater the bearing surface and thus the more the resistance.

Bullets that are lighter for the caliber that have enough bearing surface to shoot accurately at higher velocity, have less resistance and thus are able to utilize softer alloys, at least in my experience.

The testing continues........

w30wcf

[sideironjohn]

John, I know that was tongue-in-cheek, but surely you know there are other uses of the word "density." (E.g., The population of Los Angeles is quite dense.)
Take a frontal view of a bullet, and for any given section (say the top right quarter) it will be denser the heavier the bullet.
As mentioned, it doesn't say everything about length, except in same caliber, same shape bullets, nor about bearing surface.
[Now THERE'S a term to get annoyed at! Bearing surface of a bullet is an area, not a length! And that surface is greatly affected by depth of rifling.]
A Postell or similar will have the same SD as the same weight/caliber of a RN or FP, but not necessarily the same bearing surface.
I have very little experience shooting cast (though I look forward to more!) but it's intriguing to think about the interplay of pressures, twist rate, bearing surface and speed. You need the faster twist for the longer pill to stablize, regardless of SD, bearing surface or anything else. You need the greater bearing surface to mitigate stripping. If you could do with less bearing surface - ignoring stripping - then you might get much higher speeds at a given pressure. If you could get away with a slower twist then you ought to be able to get away with a smaller bearing surface, no? And at a higher velocity (increased RPM), your bullet will stablize better at a given twist than if slower.
Sundog, how come you say the 358009 has low SD? It is right there with a 200g .308" or a 450g .458" bullet. When you say "bleeding off velocity too quickly" sounds like you're thinking of something else. The greater the SD the greater the inertia, AEBE. As for why yours shoots so well, you do have that 14" twist, opposed to my 12" twist, so AFATG, it could be worse. Now, I don't have one in front of me, but IIRC it has no shortage of lube grooves. I'll bet a nickel its bearing surface length is about .75" or so.
[Bell's equation - if it's his - makes no sense without a factor for caliber; you'd end up with a 1.75" long .458" RN! (What's a 500g? 1.25"? Even with a .308" dia bullet you're looking at 190-210g!]
I shot a few of these at about 2400 fps, and though there was definitely a leading issue (scrub the bore every round) they were poking where I was pointing, and that's in a 12" twist bbl.
Maybe there is some magical obturation-to-SD ratio, or maybe it's obturation-to-caliber on a curve with pressure. That makes sense to me. A given cartridge will require a commensurately greater pressure to push a heavier bullet at the same speed, and therefor exert more stress on the bullet, requiring more "bite" into the barrel. Keep your rifling shallow (a la microgroove) and your twist rate moderate and your bearing surface great (to mitigate the shallow rifling yet still achieve target obturation factor) you may be able to get away with some higher speeds without negative effects. Maybe I don't know a lick of what I'm talking about! (Isn't the internet fun, boys and girls?)
w33wcf, I'm not sure if any of this is relevant. I need to go to bed; I work in 5.5hr! But maybe it's just that your gun don't like that 188g-er!
;-)

[w30wcf]

sideironjohn,
Thank you for your input. As many others here, I have been shooting cast bullets for 30+ years and I'm still learning.

A couple of things:
Yes, "The bearing surface of a bullet is an area, not a length."

After the bullet has started into the barrel, there is no further resistance from the depth of the rifling since the bullet has been form fitted to the barrel.

"If you could do with less bearing surface - ignoring stripping - then you might get much higher speeds at a given pressure. If you could get away with a slower twist then you ought to be able to get away with a smaller bearing surface, no?" That makes perfect sense, but in my experience, it hasn't proven to be the case..........

I think the "stripping" of cast bullets happens almost never, unless one is using an undersized bullet in a very worn barrel at higher velocities. As mentioned above, I once shot Lyman's 311316 g.c. bullet (low sectional density / low bearing surface / 120 grs. / soft 12 b.h.n.alloy / higher velocity / faster 10"twist ) at 2,300 f.p.s. in both a 6 groove and micro-grooved .30-30 barrels with very good results.

Since 1968, micro-grooved .30-30 barrels have rifling the same depth as conventional 6 grooved barrels (.003" -.004" deep). Prior to 1968, the grooves were shallower.

Welcome to cast bullet shooting!
w30wcf

[sideironjohn]

Thanks for the welcome!
I didn't catch the bit about the 120g. That would definitely go contrary to my theory! Short, little bearing surface, fast in a ffast twist. Oh well.
About bearing surface, what I meant wasn't about the length of the barrel, but about the circumference of the bullet, the width of the bullet bands, and the depth of the rifling. Once the bullet is spinning in there, freezeframe, and all the parts of the bulelt contacting the barrel: the flats of the lands and grooves, and the walls of the rifling, for the length of the so-called "bearing surface length" is all the bearing surface. In a micro-groove you won't have as much bearing surface, and seems you might get into stripping problems. No?
Actually, it might make more sense to consider the bearing surface only the forward edge of the rifling into which the bullet is pushing, since the flats (once obturated) and the rear don't actually "bear" anything to speak of.
It would be interesting to compare an old micro-grove 45/70 and one of the newer prod models, with a 405g, minimal bearing length, fast as comfy. Of course it's not fair; you'd need maybe six of each. ;-)
Anyway, dont misinterpret my free rambling for any confusion abou your proficiency in the world of casting. I'm completely green, and just love lurking on this board and its archives. Some great stuff around here....

[waksupi]

I'll toss another log on the fire. Has anyone shot a plain base PAPER PATCHED bullet at higher velocities? If so, with what results?

[45 2.1]

I'll toss another log on the fire. Has anyone shot a plain base PAPER PATCHED bullet at higher velocities? If so, with what results?

Excellent! PP Lyman 457191 at over 2000 fps. Kinda stiff in an 1895, cast of pure and still blew up at 300 yds.

[sideironjohn]

My father was doing quite a bit a couple years ago. IIRC he breached 2600 in hih H&H with no leading. I'll ask him for more info and report back.
I always like to see his stash of PP 45/70's when I visit. They just look so darn authentic!

[JohnH]

sideironjohn, I started to go into a longer diatribe about why I dislike the term "sectional density", then I realized what your explaination meant. Duh!!!! That only took thirty years......Thanks for the insight, JohnH

So another point on this, given that our cast bullets change in section density with every lube groove, does the SD formula accurately describe a cast bullet? 'Course like many things, it may be the only thing we got.

[sideironjohn]

Well, sure. It's just the mean of the whole bullet. Same with spitzers or any other shape. They're going forward, at (e.g.,) 300g per .358", and there you have it.
Depends on what you're trying to glean from the measurement. It doesn't really tell you much about penetrationm except in comparison against similar bullets with different SD, and it doesn't really give a fair comparison between different calibers, since a 225g .358" anda 165g .308" have the same SD, but little else in common. I guess what it does is give a standard measure for how "long" a bulelt weight is in a particular caliber without concern for shape.