Friction and air leakage

[melloroadman]

Been doing some research . We know that bore seal in important for performance . And that a bullet oversized to groove diameter can cause excesive friction . We also know that our A.R. do not have enough preasure to obturate pure lead to help seal the barrel . Obturation is found buy multiplying the bhn number by 1440 . So 5x 1440 = 7200 psi. What are we left with then other than lube? Following from Ingot to target . Marvin
In the old days, there was a lot of talk about bullet hardness, and how soft bullets could cause leading by having the bullet metal getting scraped off as the overly soft bullet traversed the bore. But keep in mind, in the old days, they considered a pure lead bullet soft (with a BHN of 5) and a 16-to-1 bullet hard (with a hardness of 12 BHN). We cast with harder alloys today, and what is considered hard and soft today is very, very different than in pre-WWII America. The problem is, the Oldtimers spoke in terms of hard and soft, not in terms of measured hardness values, so a new caster going back and reviewing the older casting literature is easily confused about what causes leading (addressed in detail in a later chapter). Commercial casters almost universally exploit this confusion and use it as a part of their sales pitch, touting their hard-cast bullets (commonly with a BHN of 18-22 ) as being the perfect remedy to prevent leading. Taint necessarily so, Compadre. Extra hard alloys can actually cause leading (again, see the chapter on leading for a detailed explanation of this). The bottom-line is if youre casting bullets for typical revolvers (standard and magnum, ignoring rounds like the 454 Casull, which is a case unto itself, see chapter on GC bullets), and you are using an alloy with a hardness of at least 11 BHN, any leading you observe is not caused by the alloy being too soft. Remember, Elmer Keith used the Lyman 429421 cast of 16-1 with a BHN of about 11 for the .44 Magnum. What is surprising is that today is all these newcomers that get all hot and lathered worrying over whether their 20 BHN bullets are too soft!?!
Obturation. OK, if we know that soft bullets with a BHN of 6 can cause problems, why dont we just cast everything out of linotype? If a little hardness is good, then more is obviously better, right? Well, aside from being a really expensive way to make cast bullets, there are some physical drawbacks to this approach. Obturation is the plastic deformation of the bullet metal in response to the applied pressure (from the burning powder). Cast bullet obturation was extensively studied and characterized by Dr. Franklin Mann over a century ago, and summarized in his most excellent treatise The Bullet's Flight from Powder to Target. Using soft cast bullets, he observed bullet swelling from several thousandths of an inch to several calibers, depending on the conditions employed (pressure, barrel condition, etc.). Modern barrels are exceptionally well-made, but there are minor imperfections (one or two ten-thousandths of an inch) in groove diameter, the width of the lands and grooves, minor local variations in twist rate, etc. As the bullet is engraved, these minute imperfections result in an imperfect seal between the bullet and the bore. The defects in this seal will be the same size as the variation in the dimensions. Since the hot gas molecules that are driving the bullet down the bore are less than one ten thousandth this size, gas leakage is a problem. A lot of attention has been paid to groove diameter and hand-lapping or fire-lapping to make this diameter more uniform through the length of the bore. Another issue that is also addressed by such lapping is that of the grooves and lands. If the grooves and lands vary in width, then this seal also is compromised. The forward edge of the land isnt so much of an issue because the bullets forward momentum continuously drives it into this edge, forcing this seal closed. Its the trailing edge where the seal is compromised if the dimensions vary. This is why its not uncommon to see leading follow the rifling, the trailing edge seal was compromised and the gas-leak cut the bullet metal at this point and deposited the metal fouling at its point of generation. By matching the bullet hardness to the pressure of the load, we can exploit obturation to prevent this problematic fouling. By reacting to the applied pressure, the bullet metal can undergo plastic deformation to conform itself to the local profile of the barrel, and help to maintain the seal
It is important to recognize that obturation is not simply an increase in bullet diameter, it is also a backfilling of defects obtained in the engraving process, and therefore plays a role in every shot fired with a cast bullet, even those that are properly (or over-) sized for the bore
Some folks dont like to believe that obturation plays an important role in cast bullet performance. These naysayers like to point out that this mechanism only operates at the peak pressure of the load, which only applies to a short period of time and a small stretch of the barrel. This is not true. The models and correlations that experimental ballisticians have put together to explain the observed behavior generally tend to correlate peak pressure to bullet hardness. This is simply the model that we use to explain the observed data. All metal undergoes some response to applied pressure, the magnitude and speed of that response depend heavily on the hardness of the metal, but lead alloys are soft and the degree of deformation is proportional to the applied pressure. It is important to also note that the rate of gas leakage (and hence gas-cutting) is also a direct function of applied pressure. Thus, peak pressure induces the most and fastest obturation, and enhances the bullet/bore seal when it is needed most, at peak pressure. Lesser pressures at other points along the P vs. T curve induce smaller (and slower) degrees of obturation, that still play a role in maintaining this seal. Obturation is not an on-off switch that only operates at peak pressure, that is simply how the models that have been applied to explain it work.

Obturation is also supported by the sealing effects of the bullet lubricant (see lube chapter). In the absence of obturation, the entire burden of sealing the bullet/bore interface falls on the lube. With a top-notch lube this can be accomplished, but building teamwork between the alloy and the lube is a better way to do things. Is obturation necessary for good cast bullet performance? No. But it IS a tool that we can make use of and make work for us, so why not take advantage of it?


Hardness. So we want to make sure that a bullet isnt too soft, or leading will result through galling and abrasion, and we want to make sure that it isnt too hard so we dont lose the beneficial effects of obturation, and fall prey to leading through gas-cutting. Does that mean that we have to hit a very specific hardness for each cast bullet application? Thankfully, the answer to that question is No. Rather, there are a range of hardness's that serve very well for each pressure/velocity level
Useful
Application Hardness Range
Light target loads (
Standard revolver loads (800-1000 fps, 16,000 psi) BHN 8-14
+P revolver loads (1000-1200 fps, 20,000 psi) BHN 10-16
Magnum revolver loads (1200-1500 fps, 35,000 psi) BHN 12-20
454 Casull (1400-1800 fps, 50,000 psi) BHN 16 and up
The lower end of each of these hardness ranges will expand somewhat in each of these applications. Harder bullets can be used, but they wont obturate meaning that youll have to use a lube capable of sealing the system, since the bullet cannot contribute to this critical job. Hard lubes probably wont work here. Note the recurrence of BHN 12 in many of these ranges, and remember thats what the Oldtimers used to think of as a hard bullet. Well come back to this thought

[lloyd-ss]

Marvin,
Thanks for consolidating such a wealth of information into a concise form.  I find the subject of obturation fascinating, and with some degree of potential for achieving increased velocities in air guns.  When I first saw the "Power Belt" black powder bullets with their plastic sealing cup/skirt, I thought they had potential in air rifles.  I have tried the .45 and .50 cal with air, and there velocity is good, but hard to quantify.  The copper jacket certainly runs counter to usual big bore practice.  A Power Belt type base on a lead bullet could be quite interesting.

Much of the information about obturation in you post seems to be geared toward hand guns, and  most likely bullets with flat bases.  I would think that the 7,200psi figure at which obturation occurs is for flat based bullets.  Diablo style pellets exhibit obturation at much lower spring gun pressures.  I was thinking about what it would take for obturation to occur in a big bore bullet with a hollow base. Using the calculation for hoop stress
P = 2 * Y * t / ( D - 2t)    where P is pressure in psi, Y is the yield strength of the material, D is the outside diameter, and t is the wall thickness.

The yield strength of lead ( the stress at which it deforms and won't spring back) is an elusive number and varies with hardness and temperature.   I think a number of about 4,000 psi for the yield strength is reasonable.

If you choose a bullet dia of .500 and a cup wall thickness of .050, and a yield of 4,000psi, the required air pressure to reach the yield stress and expand the O.D. is 1,000 psi.  Of course, it would take more pressure than that to obturate the hollow base enough to fully engrave it into the grooves.  I think hollow base bullets could be used to reduce barrel friction and augment the normal sealing in the barrel.  Maybe it has been tried and abandoned with air for some reason, but the Minnie ball style base is still around.

BTW, the hoop stress formula given above can also be used to calculate stresses in air tubes, but an appropriate safety factor must be added, i.e., at least 3 to 1, and the appropriate yields or tensile strength of the material must be used.

Lloyd

[melloroadman]

I tried the hollow base in swaged bullets and found no gain in fps . This was in a .357 caliber . Skirt was .020 thick .5 bhn lead .Marvin

[melloroadman]

Here is another link .Marvin
http://www.lasc.us/FryxellCBAlloyObturation.htm

[JonnyReb]

 Another great informative post Marvin, and hinting torward future airgun projectiles for sure. Lloyd i've experimented with the power belts to, and still feel the general design is the best i've seen fot AG's. The problem with the 170gr. powerbelts, as you mentioned is that they are copper jacketed and for my Sam Yang .45 were slightly to large for the bore. I did achieve good accuracy but my velocity was down by about 10% when compared the excellent 150gr. epp slug, i felt like that was partialy due to projectile weight but for the excellent seal they give, 10% was to much to lose on an already underpowered gun. Perhaps the easiest solution might be separate wad/cups to seat behind the projectile in a single shot, which would allow use of already known accurate AG BB slugs. As the would be quite flexible they might work with a variety of similar bore diameters. To incorperate the powerbelt base on AG sized slugs would require some heavy retooling for any manufacturer, but maybe if it was for a particular gun(like a certain .357) it might be worth considering.  J

[yote300wsm]

Thin the skirt out. Thin enough to expand into the rifling when fired. Use pure lead. That's why the JSB's with their thin skirts shoot so well out out of so many guns. That has also been  my experience with big bore ML's. Jmo.

[lloyd-ss]

I tried the hollow base in swaged bullets and found no gain in fps . This was in a .357 caliber . Skirt was .020 thick .5 bhn lead .Marvin

Marvin,
Were those bullets possibly already tight enough in the bore such that any additional expansion would provide no benefit?
Seems if those bullets were a little loose that the hollow base might have shown some improvement.

Good article in that link. Thanks!
Lloyd

[JonnyReb]

Thin the skirt out. Thin enough to expand into the rifling when fired. Use pure lead. That's why the JSB's with their thin skirts shoot so well out out of so many guns. That has also been  my experience with big bore ML's. Jmo.

  Well no doubt, if you found the right thickness, or if anyone ever had a BB mold built that duplicates the diablo or shuttlecock design of a small bore pellet, this would be completly plausible. The current design of the Eu Jin pellets with their typical skirt and 3 or 4 gas rings, sized exactly to their own guns, work very well unless these lightweight pellets(77gr. 9mm) are overwhelmed by the force of the discharge. Some guy named "Jerry" then barnes,used to make AG specific bullets with a skirt and 6 to 9 of these rings, some quite heavy but unfortunately i can't find where he went to, although i still see his pellets come up for sale on the YF every now and then. He had a good idea with those i think.  J

[yote300wsm]

Anybody want to make a 20ga AG?? Here's your pellet.

http://www.midwayusa.com/viewProduct/default.aspx?productNumber=236595

[lloyd-ss]

Anybody want to make a 20ga AG?? Here's your pellet.

http://www.midwayusa.com/viewProduct/default.aspx?productNumber=236595

It says .576 dia, which I don't think is 20 gauge, but what about in one  of those Jack Haley .58's ?

[rsterne]

20 gauge is 0.612".... I'm guessing that slug requires a sabot for use in a 20 gauge.... but I agree it looks GREAT for a pellet in the right bore air rifle.... 

Bob

[melloroadman]

If you read up on pellets and their skirts . You will find that the pressure really does not expand the skirt . It keeps it pressed against the barrel . Have you ever noticed that the skirt is generally the largest diameter on a pellet . Marvin

[rsterne]

I beg to differ.... I have recovered pellets from a bucket of water and the skirt is expanded so much that it is no longer conical but cylindrical in shape.... Not only did it expand.... but it permanently distorted.... 

Bob

[melloroadman]

After it has hit a object . I would expect it to expand yes . The men that wrote this article knew what they were talking about . Remember that this all takes place is a millisecond of time . It has to start to take place before the bullet begins its movement . When it is in the chamber or throat of the rifle barrel . With resistance from the lands . Marvin

[yote300wsm]

That slug uses a standard Winchester AA trap wad. My Father and I load the 12 ga version for deer hunting.

[JonnyReb]

 Here are the .356 eu jins, a true bigbore "pellet"  .. 46 of em to a tin