RotoMetals2WidenersInline FabricationLee Precision
ADvertise hereTitan ReloadingMidSouth Shooters SupplyRepackbox

Page 1 of 2 12 LastLast
Results 1 to 20 of 26

Thread: Obturation

  1. #1
    Boolit Master



    Join Date
    Feb 2018
    Posts
    1,373

    Obturation

    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?

  2. #2
    Boolit Bub
    Join Date
    Mar 2012
    Location
    Bartlesville, OK
    Posts
    61
    I will wait for the responses to this also.

  3. #3
    Boolit Grand Master
    Join Date
    Aug 2005
    Location
    South Jersey
    Posts
    5,422
    A good read to answer your questions ... http://castboolits.gunloads.com/show...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
    Regards
    John

  4. #4
    Boolit Master



    Join Date
    Feb 2018
    Posts
    1,373
    [QUOTE=John Boy;4827562]A good read to answer your questions ... http://castboolits.gunloads.com/show...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

  5. #5
    Boolit Grand Master


    Larry Gibson's Avatar
    Join Date
    Jul 2005
    Location
    Lake Havasu City, Arizona
    Posts
    17,879
    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".
    Larry Gibson

    “Deficient observation is merely a form of ignorance and responsible for the many morbid notions and foolish ideas prevailing.”
    ― Nikola Tesla

  6. #6
    Boolit Master 243winxb's Avatar
    Join Date
    Aug 2005
    Location
    USA
    Posts
    1,725
    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.
    NRA LIFE MEMBER

  7. #7
    Boolit Grand Master
    Join Date
    Aug 2005
    Location
    South Jersey
    Posts
    5,422
    Greg, instead of me rooting through my BPCR news ... read this thread. http://castboolits.gunloads.com/show...and-obturation
    Regards
    John

  8. #8
    Boolit Master
    Join Date
    Feb 2007
    Location
    South Western NC
    Posts
    2,063
    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.

  9. #9
    Boolit Grand Master
    Join Date
    Mar 2005
    Location
    Lincoln, Nebraska
    Posts
    5,627
    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.

  10. #10
    Boolit Buddy
    Join Date
    Jul 2012
    Location
    Central Texas
    Posts
    431
    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?

  11. #11
    Boolit Grand Master Outpost75's Avatar
    Join Date
    May 2013
    Location
    over the hill, out in the woods and far away
    Posts
    7,843
    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.
    The ENEMY is listening.
    HE wants to know what YOU know.
    Keep it to yourself.

  12. #12
    Boolit Master



    Join Date
    Feb 2018
    Posts
    1,373
    Elmer Keith used 1:16 alloy. Anyone know if he oversized the boolits? If so how much?

  13. #13
    Boolit Mold
    Join Date
    Sep 2019
    Posts
    29
    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.

  14. #14
    Boolit Master
    Join Date
    Sep 2007
    Location
    Oklahoma
    Posts
    570
    Quote Originally Posted by lightload View Post
    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.
    Last edited by Gohon; 02-14-2020 at 07:39 PM.

  15. #15
    Boolit Grand Master Outpost75's Avatar
    Join Date
    May 2013
    Location
    over the hill, out in the woods and far away
    Posts
    7,843
    Quote Originally Posted by GregLaROCHE View Post
    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.
    The ENEMY is listening.
    HE wants to know what YOU know.
    Keep it to yourself.

  16. #16
    Boolit Bub
    Join Date
    Nov 2019
    Location
    East Texas
    Posts
    68
    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."

  17. #17
    Boolit Grand Master

    gwpercle's Avatar
    Join Date
    Sep 2011
    Location
    Baton Rouge, Louisiana
    Posts
    5,185
    Quote Originally Posted by cupajoe View Post
    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
    Certified Cajun
    Proud Member of The Basket of Deplorables

  18. #18
    Boolit Buddy
    Join Date
    May 2012
    Posts
    414
    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 .
    We go through life trying to make the best decisions we can based on the best infomation we can find, that turns out to be wrong.

  19. #19
    Boolit Master



    Join Date
    Feb 2018
    Posts
    1,373
    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.

  20. #20
    Boolit Grand Master






    Lloyd Smale's Avatar
    Join Date
    Mar 2005
    Location
    munising Michigan
    Posts
    15,470
    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.
    Quote Originally Posted by 35remington View Post
    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.
    Soldier of God, sixgun junky, Retired electrical lineman. My office was a 100 feet in the air, closer to God the better

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check