Obturation

[GregLaROCHE]

If we over size our boolits two thousandths over bore size, is obturation still a factor? If so how much and how does it effect our choice of boolit hardness, when we try to push them at higher speeds?

[la5676]

I will wait for the responses to this also.

[John Boy]

A good read to answer your questions ... http://castboolits.gunloads.com/showthread.php?346147-Minimum-Pressure-for-obturation
PS: You made no mention in a handgun - rifle - powder choice (Black or smokeless) and "when we try to push them at higher speeds?"

Gun crank, Dan Theodore, also tested & recommends 1:16 alloy. He prepared an excellent article in the Black Powder Cartridge News with pictures of obturation of 1:16 bullets

[GregLaROCHE]

[QUOTE=John Boy;4827562]A good read to answer your questions ... http://castboolits.gunloads.com/showthread.php?346147-Minimum-Pressure-for-obturation
PS: You made no mention in a handgun - rifle - powder choice (Black or smokeless) and "when we try to push them at higher speeds?"
.

Gun crank, Dan Theodore, also tested & recommends 1:16 alloy. He prepared an excellent article in the Black Powder Cartridge News with pictures of obturation of 1:16 bullets[/

For the most part I’m interested in BP rifle, but I was asking the question in general.

I would like to read the article you mentioned. Do you know where I can find it? Thanks

[Larry Gibson]

To obturate essentially means to swell, close or obstruct.....[Webster].

In firearms jargon "obturate" is most often meant to refer to the swelling of the bullet under acceleration [sometimes referred to as "bumping up"] to seal the bore in a bore/groove diameter larger in diameter.

Since we use, or at least most use, cast bullets at or larger than groove diameter the bullet does not "swell", it does "close" and, hopefully, it does not "obstruct".

[243winxb]

If there is a tight spot in the middle of the barrel. A smaller groove diameter, something like .0005", then the sized down bullet may expand (obturation) under pressure using a softer alloy.

Soft alloys will allow bullets to skid and/or slump from high pressure.

Cast bullets are swagged into the barrel on firing, when oversize.

[John Boy]

Greg, instead of me rooting through my BPCR news ... read this thread. http://castboolits.gunloads.com/showthread.php?270490-Bullet-hardness-and-obturation

[1hole]

I've always been puzzled by the "common knowledge" idea that cast bullets "should be 2 thou over groove diameter". I ask why? no matter how much larger a bullet is when fired it will be exactly the size of the bore when it's fully in the bore.

Thus, to me, the question becomes, "Which is the best way to size a bullet to fit, a sizer die or the gun's throat?" Given the violence of a bullets exit from a case my bet is a sizer die is likely to be more consistent. To back that guess, I've never seen accuracy improvement with any over size bullets.

[35remington]

If the bullet fits the throat of the firearm in question the bullet does not have to undergo any changes to have the potential to shoot well.

I suppose the idea that a bullet needs to be a couple thou over groove diameter may be because many throats approximate this size. Or maybe not.

[lightload]

I was given a box of hard cast commercial 158 grain bullets. Diameter is .357. Lube is a hard type. I want to shoot these through my Smith revolver with .358 diameter barrel. I will tumble lube with Ben's liquid lube on top of hard lube and load with a 3.5 gr Bullseye charge in .38 Spl cases.
Am I likely to have leading since bullet is undersize?

[Outpost75]

PLEASE folks understand that BORE diameter is the smooth hole BEFORE it is rifled and reflects the diameter measured across the tops of the rifling lands. After rifling the GROOVE diameter is what most cast bullet shooters are interested in, but in general you want cast bullets to reflect the THROAT diameter, which is represented by the unrifled portion of the chamber ahead of the cartridge case before the rifling starts.

In revolvers the cylinder throats ahead of the chamber are generally groove diameter or a bit larger. In cases where the cylinder throats are smaller than barrel groove diameter, accuracy will generally suffer, although in black powder calibers used with soft alloy and fast-burning powders in which the bullet OBTURATES in transitioning from the cylinder into the forcing cone of the barrel, accuracy will generally be satisfactory. Soft bullets which "fit" properly do not "slump" in correct loads of suitable pressure and velocity level for the alloy used. Elmer Keith used 1:16 tin-lead for the great majority of his load development in .357 and .44 Magnum.

[GregLaROCHE]

Elmer Keith used 1:16 alloy. Anyone know if he oversized the boolits? If so how much?

[DonHowe]

Take a cast bullet and measure its diameter. Squeeze it lengthwise in a vise or set on a hard surface and bump the nose with a hammer then measure it again . It will be larger in diameter. This is simplistic but this is what happens to a bullet when kicked in the but by gas pressure from burning powder. This will try to happen anytime chamber pressure exceeds compressive strength of bullet alloy. The mechanics of this are at work whether the bullet is dead soft lead or solid copper. If the bullet is hard enough and pressure is high enough the shortening/expanding bullet while expand the barrel itself until pressure decreases.
Given "normal" pressures for cast bullet loads, a bullet starting out over groove diameter will be swaged down to fit but it will TRY to obdurate. Good reading on this subject can be found in The Bullet's Flight by FW Mann.

[Gohon]

I was given a box of hard cast commercial 158 grain bullets. Diameter is .357. Lube is a hard type. I want to shoot these through my Smith revolver with .358 diameter barrel. I will tumble lube with Ben's liquid lube on top of hard lube and load with a 3.5 gr Bullseye charge in .38 Spl cases. Am I likely to have leading since bullet is undersize?

Are you sure about the .358 groove diameter of your revolver? The 357 magnum saami specs are .346 bore diameter and a .355 groove diameter for a 357/38 barrel.

[Outpost75]

Elmer Keith used 1:16 alloy. Anyone know if he oversized the boolits? If so how much?

Keith understood the relationship between cylinder throats and correct bullet diameter. He didn't slug barrels. I discussed this at length with him and Skeeter Skelton at Bill Ruger's New Hampshire home when I worked for the company back in the 1980s. Elmer was getting on in years then and a bit feeble, but his mind was still quick as a steel trap and he was an entertaining old cuss. I am glad to have known him.

[cupajoe]

In "Sixgun Cartridges And Loads", Chapter 5, Elmer wrote that he slugged the barrel and also the cylinder chambers. He preferred .001" over groove for maximum or magnum loads. With light loads using short, light bullets, up to .003 over groove is acceptable. In any case " care should be taken to see that your sized bullet will always shove through the throat of the cylinder chambers of the gun by hand using but little effort."

[gwpercle]

In "Sixgun Cartridges And Loads", Chapter 5, Elmer wrote that he slugged the barrel and also the cylinder chambers. He preferred .001" over groove for maximum or magnum loads. With light loads using short, light bullets, up to .003 over groove is acceptable. In any case " care should be taken to see that your sized bullet will always shove through the throat of the cylinder chambers of the gun by hand using but little effort."

A reprint of this book is available for under $20.00 from Amazon and if you cast boolits will open your eyes to a lot of knowledge many have already forgotten . He goes into detail about hardness , alloy and sizing. Most people use boolits much harder than they need to be .
Good read with some good photo's for a good price .
Gary

[facetious]

Obturation isn't going to be the same all the way up the boolit. When you put pressure on the base of the boolit it is going to try and expand before the front, so the pressure outword from the boolit on the barrel sidewalls is going to be more at the base than the nose. This is the idea behind gaschecks and long shank's, the part getting hit the hardest is the check and the long shank can expand out with out putting as much pressure on the side walls.

I think the idea for sizing over size is that in a perfect world you should be able to size to the exact size of the barrel and all would good but in the real world nothing is perfect, from the chamber to the muzzle things can be out of line, have chock's and thight spots. No matter how hard you try it will never be perfect. So by sizing over and letting it swage down the last bit it helps compensate for some of the variances that we can't controle .

[GregLaROCHE]

I started this thread thinking that maybe we need to size over bore size, especially if using hard alloys. Pure lead may easily obturate, where as a hard alloy may not or not enough.

I just bought a Chassepot rifle from someone who has been target shooting with it for forty years. He has extensive documentation of everything he tried. He settled on a pure lead greased boolit (not the traditional load). The boolit was sized to the bore. His opinion was that obturation would take up extra space and engage the boolit. I think he was right because he has some very impressive targets in his files.

What I am wondering about is, I have a good supply of lead from an indoor range. Jacked bullets are not allowed and most lead comes from hand guns. The boolits are mostly bought and not hand poured, so the alloy tends to be pretty hard on average. I average around 16 BHN. If I use my range scrap alloy, will it obturate enough when at bore diameter (that the mould produces), or will I have to add a few thousandths to the diameter?

I know the answer is in my doing my own testing to find out what works best for me, but just wondering what others thought about it.

[Lloyd Smale]

there you go. A bullet bumping up or changing its shape in anyway is a bad thing not a good thing. Yup its might help in a poorly built gun with poor tolerances but in a good gun why would you want your bullets to swell or change shape knowing they will never do it the exact same way every time. Its why in a good gun hard is always better then soft when choosing alloys. Bumping up or the more fancy word obturation is something that came from back when guys shot guns like colts and smiths that had miss matched parts and throat sizes and even bore sizes weren't a big concern by the manufactures. For the most part today it should rarely be discussed other then as a history lesson when casting is the subject. If soft bullets that bumped up were the best bullets for your guns why would they shoot hard jacketed bullets as well as they do. Bottom line is most of us casters are just chasing jacketed accuracy.
If the bullet fits the throat of the firearm in question the bullet does not have to undergo any changes to have the potential to shoot well.

I suppose the idea that a bullet needs to be a couple thou over groove diameter may be because many throats approximate this size. Or maybe not.

[243winxb]

I started this thread thinking that maybe we need to size over bore size, especially if using hard alloys. Pure lead may easily obturate, where as a hard alloy may not or not enough.

I just bought a Chassepot rifle from someone who has been target shooting with it for forty years. He has extensive documentation of everything he tried. He settled on a pure lead greased boolit (not the traditional load). The boolit was sized to the bore. His opinion was that obturation would take up extra space and engage the boolit. I think he was right because he has some very impressive targets in his files.

What I am wondering about is, I have a good supply of lead from an indoor range. Jacked bullets are not allowed and most lead comes from hand guns. The boolits are mostly bought and not hand poured, so the alloy tends to be pretty hard on average. I average around 16 BHN. If I use my range scrap alloy, will it obturate enough when at bore diameter (that the mould produces), or will I have to add a few thousandths to the diameter?

I know the answer is in my doing my own testing to find out what works best for me, but just wondering what others thought about it.

Use the extensive documentation. Simple

Are you going to use Black Powder?

[Gohon]

Its why in a good gun hard is always better then soft when choosing alloys

What is your definition of hard?

[GregLaROCHE]

I will be using BP and. BHN of around 16.

[Outpost75]

I will be using BP and. BHN of around 16.

16 BHN is unnecessarily hard for BP, 8-10 BHN is fine for holy black.

[GregLaROCHE]

16 BHN is unnecessarily hard for BP, 8-10 BHN is fine for holy black.

That’s my problem. That’s the way I get it and am wondering if I can make it work.

[Lloyd Smale]

hard starts at air cooled ww to me and even that in a good gun is soft if you want the ultimate in accuracy. When I shot comp I casted most of my bullets for matches out of 5050 ww/lino that tested out at 17-18 bhn and that was for low pressure stuff like 45acps and 38s.
What is your definition of hard?