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Thread: Cast bullet size

  1. #1
    Boolit Mold
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    Cast bullet size

    Why is it that I reload with a .308 size jacketed bullet and yet the cast boolits are .309 and the sizer die is also .309? Why are cast boolits .001" larger than the bore of the firearm?

  2. #2
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    Cast bullet size

    Copper jacketed bullets can catch the rifling with less width and still do their thing. Cast bullets need a bit more size in relation to the bore to work properly.
    They will skid on the rifling at high velocity otherwise, and cause leading in the barrel.


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  3. #3
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    That's a large debate. Size to what will work. Size your bullets to what your gun likes. Some size to bore diameter and it works for them, but not me. Some size .001-.002 over and it works great and others size to throat. It all has to do with gas cutting and obturation. A jacket bullet is protected, so gasses can cut and not really mess up your accuracy by deforming the bullet before it has a chance to obturate and seal the bore. Sizing .001 to .002 ensures quicker obturation when the bullets passes into to the riffling. Down side to .001 to .002 is the thought that gasses can still cut around the bullet when it passes into the throat, through the leade and into the riffling. The thought behind sizing to throat is that the throat supports the bullet during ignition and helps keep the throat sealed to prevent gas cutting thereby increasing accuracy. So, try out all the methods, research them and see what works for you. I personally prefer to size to throat.
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  4. #4
    Boolit Master GhostHawk's Avatar
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    rcmaveric is giving you the straight dope.

    In the end let your rifle tell you what it needs.

    But for cast 1-3 thousandths bigger is a good place to start.

  5. #5
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    Sizing to groove diameter has worked well for me, but even 1/10,000th under and one can get gas blowby and leading from flame cutting. So the rule of the thumb has been .001" over just to make sure. Some guns have further requirements, especially those with large/long throats. Revolvers are in a category all to themselves when it comes to sizing and alloy requirements.

    Be wary of anyone here who starts out with a post with "have to". Undersized boolits can work quite well, if soft and/or driven hard enough to get them to "slug up" or expand to the point they obturate (fully block the bore). Also a copper gas check can cure a multitude of ills. I personally shot a bunch of .309" sized cast in my ".308 bore" FN 30/06... zero leading, good accuracy... until I slugged the bore and found out it was really a .310" bore. Since then I've switched to .310" (most of my molds just won't drop at .311") and continue to make advancements in accuracy.

    But having said that, I don't recommend starting off with undersize boolits, if it can be avoided at all.
    Last edited by HangFireW8; 03-23-2018 at 07:55 PM.
    I give loading advice based on my actual results in factory rifles with standard chambers, twist rates and basic accurizing.
    My goals for using cast boolits are lots of good, cheap, and reasonably accurate shooting, while avoiding overly tedious loading processes.
    The BHN Deformation Formula, and why I don't use it.
    How to find and fix sizing die eccentricity problems.
    Do you trust your casting thermometer?
    A few musings.

  6. #6
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    some time it works out best to size larger than bore size
    not always though like others have said a reference not a rule
    only your rifle,gun knows for sure
    Hit em'hard
    hit em'often

  7. #7
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    Don't you guys mean to size larger than groove diameter?

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    Quote Originally Posted by Snagman View Post
    Why is it that I reload with a .308 size jacketed bullet and yet the cast boolits are .309 and the sizer die is also .309? Why are cast boolits .001" larger than the bore of the firearm?
    The "why" is that ideally, you want the cast boolit to fill the grooves. If your boolit is .001 over barrel groove diameter, that is more than likely gonna happen.

  9. #9
    Boolit Bub Catpop's Avatar
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    Success of shooting cast boolits is fit of boolit to actual groove diameter plus .0005 (or more.) This has nothing to do with what caliber the gun is marked, but rather what the groove actually measures.
    Once that is established other factors come onto play.
    “Fit is King”

  10. #10
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    the initial 'fit' is in the throat not the barrel.
    the most and the hottest gas is concentrated right there.
    stop the gas cutting before it starts and you won't have to try and figure out why you have leading and/or poor accuracy.

  11. #11
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    Quote Originally Posted by Dusty Bannister View Post
    Don't you guys mean to size larger than groove diameter?
    Yes. At least .001 larger than groove diameter. I just slug my barrels and go from there. Works for me. Never done the throat thing.


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  12. #12
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    Quote Originally Posted by runfiverun View Post
    the initial 'fit' is in the throat not the barrel.
    the most and the hottest gas is concentrated right there.
    stop the gas cutting before it starts and you won't have to try and figure out why you have leading and/or poor accuracy.
    This again.

    The gas is not concentrated there in all cases.

    Sizing to groove instead of throat often works very well.

    Sizing to throat is often not necessary. Of sometimes, even possible.
    Last edited by HangFireW8; 03-23-2018 at 07:56 PM.
    I give loading advice based on my actual results in factory rifles with standard chambers, twist rates and basic accurizing.
    My goals for using cast boolits are lots of good, cheap, and reasonably accurate shooting, while avoiding overly tedious loading processes.
    The BHN Deformation Formula, and why I don't use it.
    How to find and fix sizing die eccentricity problems.
    Do you trust your casting thermometer?
    A few musings.

  13. #13
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    .............Snagman, as being new to the board, and possibly cast lead boolits some clarification might come in handy, maybe not, but here goes: The Bore, this is the ID from the top of one land to the top of the opposing land. In a 30 cal rifle we'll figure the BORE is .300". However, the term 'Bore' can also take in the entire interior surface of the barrel encompassing the lands AND the grooves. Grooves are descriptive of what they are. They are the grooves cut into the BORE to create rifling. If this is not new then please forgive me for assuming



    Not having a sectioned barrel, I'll just use this drawing I did for showing the chambering on a M1879 Argentine contract Remington Rolling Block rifle, vs the more common chambering we're used to. In the lower drawing is the type we're most accustomed to. The throat is the unrifled portion of the barrel directly in front of the chamber. Obviously the drawing is exaggerated. By fitting the cast lead slug to 'Just' fit the throat can help marginalize the afore mentioned 'Gas Cutting' of the lead slug which will majorly have a detrimental affect on the lead slug.

    The most perfect example of gas cutting I ever saw was a recovered 58 cal Minie' boolit. I wished I'd saved it. Cast of pure lead probably in a cold mould. It looked like a tiny acetylene torch had gotten to it. There was a cut place through the bottom band. The gas then followed a grease groove about half way around the Minie' where it encountered another wrinkled drive band, and passed through into the middle lube groove. Then ditto for the top drive band where the gas found release to the atmosphere. It had the appearance like where a small narrow stream of water had wondered around on a mud flat. It's obvious path was very smooth. It's usually a good idea to size to the throat if possible.

    However sometimes the firearm may not allow for sizing to the throat. This usually happens in older rifles, or military surplus rifles. One of the best examples of this I can think of would be the Austro-Hungarian M1895 rifles/carbines chambered to 8x56R.

    Another way to defeat gas cutting, is if the cast lead bullet is long enough to engrave in the leade while the base is still in the neck. Upon initial ignition the boolit can seal the barrel, and there is no place for the gas to go, except to remain behind the slug and push If the gas can find a place to escape to the low pressure atmosphere, it WILL go there

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  14. #14
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    Thanks for the terminology correction, Buckshot. Yes, technically bore sized boolits would ride the lands. What I am saying is that filling the grooves is often enough. If it is not, then sure, fill the throat.

    Also what I am saying is that there is more than one way to get sizing to work. I am getting more than a little tired of some who think there is only one way, or that beginners have to do things there way.

    Some older BP cartridges rely on upset to seal. Sizing to throat will result in a cartridge that will not chamber. But good results can be had with proper alloy and start force.

    Also hard, long boolits can seal the bore quickly even if sized to groove. Good results can be had without filling the throat with an oversized boolit.

    There is no need to force every question into the fill the throat answer.
    I give loading advice based on my actual results in factory rifles with standard chambers, twist rates and basic accurizing.
    My goals for using cast boolits are lots of good, cheap, and reasonably accurate shooting, while avoiding overly tedious loading processes.
    The BHN Deformation Formula, and why I don't use it.
    How to find and fix sizing die eccentricity problems.
    Do you trust your casting thermometer?
    A few musings.

  15. #15
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    To determine the correct bullet diameter for a rifle, the groove diameter of the barrel is NOT the determinant.

    INSTEAD you want to measure the throat, or the unrifled portion of the barrel forcing cone or "ball seat" ahead of the case mouth, before the rifling starts. The best way to do this is from a chamber cast or upset throat slug.

    Most accurate for measurement purposes and easiest is to upset a throat slug, or as some people call it a "pound cast."

    Start with a sized case with DEAD primer in its pocket. The way I do this is to heat the lead pot, then fill the sized case with DEAD primer plugging the flash hole, and generously overflowing the case.

    After the lead cools, clean all spilled lead off the case exterior, then file the exposed lead FLUSH to the case mouth.

    Take a piece of PURE lead buckshot or short chunk of pure lead wire and drop it into the EMPTY chamber, letting it fall into the throat of its own weight. (With very long throats you can use a longer piece of wire or a SOFT bullet with long bore-riding nose and not a long grooved section).

    Insert your lead-filled dummy case and GENTLY tap it into the chamber using a piece of brass rod until you can close the breech. You are using the lead filled dummy case to force the lead slug into the ORIGIN of rifling. In short throated barrels it helps to drive the slug first into the origin of rifling, far enough to chamber the lead dummy behind it, then close the bolt and upset the slug against the lead dummy using a Brownell Squibb Rod threaded onto the end of your cleaning rod.

    You don't need to use a hammer, just let the weight of the rod drop onto the slug, making many light taps of the squibb rod against the slug until you get a clear "ringing" sound. It need go no farther!

    What you want to measure is the diameter of the UNRIFLED portion of the chamber forward of the case neck BEFORE the rifling starts! Extract the dummy and GENTLY tap the lead slug out and measure it. THAT is the diameter you want to size your bullets to!

    Using Cerrosafe, etc. is lots more trouble and you then need to compensate for shrinkage, etc.

    The upset pure, dead-lead slug is exact and straight forward!

    If you forget EVERYTHING you ever read about slugging barrels and simply cast chambers from now on, and get bullets to FIT THE THROAT you will be far happier in the long run.

    The limiting factor in safe cast bullet diameter is neck clearance. You MUST also measure the neck diameter of the chamber on the cast. Most chambers have enough clearance ahead of a fired case mouth that a properly upset throat slug will get you a portion of the case mouth and its transition angle to the throat or ball seat, so that you can measure neck diameter at the mouth and throat diameter of the ball seat.

    The loaded cartridge neck diameter must not be larger than 0.0015" SMALLER than the chamber cast at that point, to ensure safe expansion for bullet release. This is absolutely essential for custom target barrels which often have tight-necked chambers which require neck-turned cases.

    As a general rule the largest diameter of cast bullet which chambers and extracts freely, without resistance, will shoot best.

    For instance in a .308 Winchester target rifle with .339" tight-necked chamber and using case necks turned to 0.012," maximum bullet diameter is determined by"

    [neck (.339")-2(neck wall thickness 0.012)] - 0.0015 = 0.3135" for a "fitted neck" in which fired cases do not require sizing, but bullets will be held by case springback only. For necked sized fixed ammo, subtract another 0.0015" or .312" IF the chamber ball seat is that large. In a new barrel chambered for jacketed bullets, probably not. Min. SAAMI throat as on the .308 Winchester pressure test barrel is 0.3105".

    Unless you know that your particular throat is cut smaller with a non-standard, custom reamer, try .310", which works in most.
    If the barrel has been fired more than 1500 rounds with full power jacketed loads .311" will probably fit better. If you shot a couple seasons season of NRA highpower with it, with lots of rapid fire and barrel heating, then .312" will fit just fine.

    John Ardito set all of his CBA benchrest records shooting .312" bullets in his .308 Win. and wildcat .30 cal. rifles.

    Bullet fit is seldom a problem in milsurp because military chambers tend to be sloppy!
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  16. #16
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    One of the most important things to remember is that no matter what you size the boolit to. You will need a properly sized "M" die to expand the case mouth!
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  17. #17
    Boolit Grand Master Outpost75's Avatar
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    Quote Originally Posted by stubshaft View Post
    One of the most important things to remember is that no matter what you size the boolit to. You will need a properly sized "M" die to expand the case mouth!
    Good point. Another thing is that you don't want the expander plug to be more than 0.002" smaller than the bullet, otherwise when a larger cast bullet is jammed into a tight neck, it will "size" a soft bullet and may deform and crumple a soft, thin case neck as in the .32-20, .38-40 or .44-40 cartridges.

    The RCBS cast bullet neck expander is available with plugs in 0.001" increments and Buckshot and NOE also make custom plugs.

    I always inside deburr cast bullet cases well to break the sharp inside corner of the case mouth, which reduces the tendency to shave lead if the case is perhaps not belled quite enough, which can be common with mixed range brass which people seldom trim to uniform length.
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  18. #18
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    Buckshot is not making expander plugs anymore...much to my regret.
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  20. #20
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    Quote Originally Posted by Outpost75 View Post
    John Ardito set all of his CBA benchrest records shooting .312" bullets in his .308 Win. and wildcat .30 cal. rifles.
    Hmm I have custom barreled rifles, some of them chambered in .308 caliber. Guess what the throat size is cut to? (The answer is in there).

    Quote Originally Posted by stubshaft View Post
    One of the most important things to remember is that no matter what you size the boolit to. You will need a properly sized "M" die to expand the case mouth!
    I've found the sharp step of the "M" expander knocks cases eccentric. But then most here don't ever measure that, so I guess they wouldn't know.
    I give loading advice based on my actual results in factory rifles with standard chambers, twist rates and basic accurizing.
    My goals for using cast boolits are lots of good, cheap, and reasonably accurate shooting, while avoiding overly tedious loading processes.
    The BHN Deformation Formula, and why I don't use it.
    How to find and fix sizing die eccentricity problems.
    Do you trust your casting thermometer?
    A few musings.

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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