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Thread: Chamber and throat relationship.

  1. #61
    Boolit Master
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    Quote Originally Posted by badgeredd View Post
    I am guessing you've misunderstood part of this. Your throat will be the area immediately in front of where the case ends. The .342" dimension has to be in error. With a .313" grove diameter, the minimum I'd size my bullets to would be .314". As for a design...Lee makes a 125ish pointed bullet that casts .314" in my mold. I wouldn't hesitate to lap one out to get the minimum of .314" diameter. Lee also make a 150ish grain bullet mold that works quite well in AK/SKS design rifles. You SHOULD be able to get groups with cast bullets equal to jacketed groups, but I seriously doubt you can get better groups with cast. LUBE for gas operated semi auto needs to be like baby bear porridge...just right. Too soft or hard of a lube will foul up the works. Too much capacity will add unnecessary lube to the barrel (and therefore the gas tube).

    Overall length of a loaded cartridge will be determined nicely with the tool mentioned above, BUT it will do nothing for filling the throat and for a load that is as accurate in your gun as possible.

    I hope this is helpful.

    Edd
    This is what I have, maybe it will make some sense. The inner diameter of a fired case neck is .315-.317. The outer diameter of the same neck is .340-.343. The latter number is maintained forward of the case where the bullet resides. I can confirm this by measuring the lead slug, which gives .342 +/- .001. It then tapers down in size until it forms the lands and grooves of the rifling, which results in .313 groove and .300 land. I might need to take a pic of the slug. It will be easier to explain. Finding the right bullet to fit the groove diameter and provide sufficient weight has been a chore. What I am seeing (though possibly in error)with large throat numbers only makes the bullet hunt more interesting. Thanks.

  2. #62
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    Quote Originally Posted by Animal View Post
    This is what I have, maybe it will make some sense. The inner diameter of a fired case neck is .315-.317. The outer diameter of the same neck is .340-.343. The latter number is maintained forward of the case where the bullet resides. I can confirm this by measuring the lead slug, which gives .342 +/- .001. It then tapers down in size until it forms the lands and grooves of the rifling, which results in .313 groove and .300 land. I might need to take a pic of the slug. It will be easier to explain. Finding the right bullet to fit the groove diameter and provide sufficient weight has been a chore. What I am seeing (though possibly in error)with large throat numbers only makes the bullet hunt more interesting. Thanks.

    You desrcibe your "throat" as basically being a funnel instead of a proper parallel-sided cut (not really unusual). Use your minimum inside fired case neck diameter and subtract .001 for clearance.
    In this case your fired neck ID is .315, your groove diameter is .313. A cast measuring .314 would be a good place to start.

  3. #63
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    It is helpful to actually see the bullet, case and barrel as they fit together. And I thank Badgeredd for taking the time to educate us/me on this. I have a better understanding of the rifle now, but I was wondering if anyone has done this with a revolver. I am still not sure where the throat on revolver is. Is it in the cylinder, or past the forcing cone?

  4. #64
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    Quote Originally Posted by cainttype View Post
    You desrcibe your "throat" as basically being a funnel instead of a proper parallel-sided cut (not really unusual).
    Yes, this is exactly how I see it. I would imagine these loose fitting tolerances would be very helpful to a rifle that is designed to run reliably in the most harsh of environments. I'm glad to know .313 bullets are still worth pursuing... That is already a chore enough.

  5. #65
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    Quote Originally Posted by Animal View Post
    This is what I have, maybe it will make some sense. The inner diameter of a fired case neck is .315-.317. The outer diameter of the same neck is .340-.343. The latter number is maintained forward of the case where the bullet resides. I can confirm this by measuring the lead slug, which gives .342 +/- .001. It then tapers down in size until it forms the lands and grooves of the rifling, which results in .313 groove and .300 land. I might need to take a pic of the slug. It will be easier to explain. Finding the right bullet to fit the groove diameter and provide sufficient weight has been a chore. What I am seeing (though possibly in error)with large throat numbers only makes the bullet hunt more interesting. Thanks.
    When folks measure the ID of the fired case they are are not gaining useful information about the throat of their rifle barrel. The neck of the fired case is not in the throat but the chamber. Chambering reamers are cut to a variety of specifications with military chambers being the largest and match chambers being the smallest. Also there is a wide variety of brass thickness and the necks tend to increase in thickness if fired with high pressured loads.

    Somewhere down the road of cast bullet shooting in recent years, somebody came up with the notion of measuring the ID of a fired case to determine cast bullet size. Such a measurement may or may not provide a guide for bullet diameter, but it is not a reliable method.

    A true measurement of the throat is the only numbers that matter. A bullets that is .0005 or so under that number will introduce the bullet into the throat straight and that is what it is all about.

    I don't know what kind of rifle you have, but some rifles, such as some Marlin and have almost no throat, just a chamfer at the end of the barrel in front of the chamber. These can be an issue for the cast bullet shooter. There are ways around it and ways to fix it, but it can be an issue unless dealt with.

    If the outer diameter of fired case is the same as place where the bullet is, you are not measuring the throat, but a long chamber neck or a throat that has been severely eroded by the firing of many thousands of rounds.
    Disclaimer: The above is not holy writ. It is just my opinion based on my experience and knowledge. Your mileage may vary.

  6. #66
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    "long chamber neck" seems to be a better description than a throat. Char, the rifle is a Romanian made WASR-10 AK47 variant. Tolerances are pretty tall with this beast. Seems to me "the looser the better" was probably Kalashkinov's motto when designing the rifle. This approach has probably paid dividends in reliability.
    Aside from that, I think the accuracy problems with this rifle are going do be obvious by design, but with my previous handloads I have found that the ammunition available for his rifle probably make up 50% of it's accuracy short comings. A good handload can make up for half of its problems. At least, this is what I have learned so far with my rifle.
    This is why I found interest in this thread. Measuring the chamber and throat space will be very helpful when selecting bullets... Jacketed or Cast.
    Where I live, a 100yrd shot is rare. But a good handload will make the difference between 3in group and 6in group. Factory steel cased ammo is plenty good for 75 and less, which is probably about 90% of the shooting that is likely to happen.

  7. #67
    Boolit Master Char-Gar's Avatar
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    Quote Originally Posted by Animal View Post
    "long chamber neck" seems to be a better description than a throat. Char, the rifle is a Romanian made WASR-10 AK47 variant. Tolerances are pretty tall with this beast. Seems to me "the looser the better" was probably Kalashkinov's motto when designing the rifle. This approach has probably paid dividends in reliability.
    Aside from that, I think the accuracy problems with this rifle are going do be obvious by design, but with my previous handloads I have found that the ammunition available for his rifle probably make up 50% of it's accuracy short comings. A good handload can make up for half of its problems. At least, this is what I have learned so far with my rifle.
    This is why I found interest in this thread. Measuring the chamber and throat space will be very helpful when selecting bullets... Jacketed or Cast.
    Where I live, a 100yrd shot is rare. But a good handload will make the difference between 3in group and 6in group. Factory steel cased ammo is plenty good for 75 and less, which is probably about 90% of the shooting that is likely to happen.
    OK, that explains allot. The AK47 is a battle rifle deluxe and was designed to run full auto with it guts full of sand, mud and blood. Keep plugging away, victory will surly be thine!
    Disclaimer: The above is not holy writ. It is just my opinion based on my experience and knowledge. Your mileage may vary.

  8. #68
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    If you google "freebore" and click on "images for freebore" you will see a lot of photos and drawings which illustrate that person's view or concept of what freebore is, and also what part of the chamber/bore/whatever is called the "throat." Many of these drawings are from our own member blammer, who designs a few boolits and molds here and there. His drawings explain perfectly the difference between "freebore" and "throat" and they describe freebore as a section of parallel smooth bore, equal to or .0005" larger than the boolit diameter or the "bore rider" section of the boolit, and throat as the tapered part that contains the leade in to the rifling.

    So, going by blammer's description, (which is as good an explanation as *I* have seen thus far) freebore, is not really free bore, as in the unsupported space you would have when firing a .38 Special load in a .357 Magnum cylinder, rather it is a smooth, parallel, section of bore devoid of rifling or taper, which by it's diameter supports the boolit in flight, aligns and guides it straight to the throat and into the rifling.

    This explanation would be sufficient to describe freebore and throat as it is used in a rifle chamber.

    For a revolver, the word "throat" is generally used to describe the smooth parallel section of cylinder in front of the chamfer at the end of the chamber, and since this part is located in the cylinder and not the barrel, we generally accept the term "cylinder throats" and associate this terminology with the cylinder. By dimension, since this is a smooth parallel section that the boolit travels, shouldn't it be called freebore? If you apply the terminology used to describe a rifle chamber then it would be correctly called freebore. However since this is a revolver (which has separate firing chambers detached from the leade in to the rifling and detached from the bore), freebore would apply more to unsupported space that the boolits must travel through to reach the throats, or the distance from the case mouth to the cylinder throat in a caliber where you could shoot companion cartridges. .38 Special in a .357 Magnum cylinder, .44 Special in a .44 Magnum cylinder, .45 Schofield in a .45 Colt cylinder, etc..

    For an autoloader, which does not have a separate firing chamber like a revolver, it should also be described using the same guidelines one would use with a rifle chamber. IF there is smooth parallel bore in front of the chamber, then this should be known as freebore. IF there is taper in the area in front of the chamber, it should be known as throat. Yes you can have BOTH freebore and throat in an autoloader barrel, also in a TC barrel, or any firearm that does not have a separate firing chamber such as a revolver.

    I think I learned something by this thread, the difference in freebore and throat and how to tell one from the other...

    I also learned that freebore in a rifle means smooth parallel section of bore, and freebore in a revolver means totally unsupported space between the case mouth and the rear of the cylinder throat. Two totally different things both described correctly I might add, by the same word, freebore.

    Edit: And yes, a revolver's barrel DOES have a throat. The throat would be the length of bore that contains the leade in to the rifling, which would begin where the machined angled surface of the forcing cone meets the barrel's groove diameter, and ends where the rifling gains it's full height.

    Edit #2: While we are discussing this, let's not overlook the word "freejump." Freejump, would be a description of how far the boolit can move when it pulls crimp and begins to travel, until it encounters either the taper in the throat OR the leade in to the rifling, OR the chamfer leading into the cylinder throat, if it is a revolver cylinder.

    For example, my Ruger M77 in .308 caliber has approximately .240" of freebore in front of the case mouth when a round is chambered. When I seat my boolit out long, where it touches the rifling when chambered, I have 0.00" of freejump. If I screw my seating stem in about half a turn, this gives my boolit approximately .025" of freejump, this is the distance that the boolit can travel before it begins to encounter the leade in to the rifling. If I seat the boolit in the cannelure, it has .165" of freejump, which for the sake of discussion, opens up groups to the size of a tennis ball @ 100yds. With .025" of freejump, groups shrink to the point that I can cover a 3 shot group fired @ 200yds with a guitar pick. THIS is why I felt it important not to leave out an explanation of the word "freejump" when discussing freebore and throats, because these three things all work together.
    Last edited by DougGuy; 03-11-2015 at 08:11 PM.
    Got a .22 .32 .357 .38 .40 .41 .44 .45 .480 or .500 S&W cylinder that needs throat reaming? 9mm, 10mm/40S&W, 45 ACP pistol barrel that won't "plunk" your handloads? Shoot me a PM! Also on Facebook: https://www.facebook.com/Cylinderhone-756429174391912/

  9. #69
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    DougGuy, nice description. Thanks

  10. #70
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    The large funnel shaped section you're dealing with, as you've surmized, is often associated with reliable functioning under possibly adverse conditions. The AK47 "battle rifle" certainly is a good candidate for that line of reasoning, but even SAAMI chamber specs for the 458 Winchester Magnum have a similar design for the same purpose.
    My 458 WM has a taper from .470 to .458 that stretches over 1 inch before reaching the lands. For that reason, I generally ignore the normal fit-to-throat wisdom and try to find a projectile by starting .001 above groove and work up in diameter to see if I have improvements. The casts that are long enough to start the nose into the lands while the base is still in the neck seem to work really well, but in the 458 that makes them very heavy.
    You stated earlier that your minimum neck clearance from fired casings was .315 and your groove diameter was 313. I would start at .314 because of those measurements, ignore the big taper, and try to find a design that was long (and wide-nosed) enough to provide some fit at it's nose. Hopefully, your case neck can help give some alignment help at the rear as your cast's nose reaches the barrel's groove diameter origin.
    If culling casings for fired neck IDs with a minimum of .316 is no problem, I'd consider trying casts up to .315 to see if I could see any improvement.

    That funnel shaped end-of-chamber/start-of-throat is also obvious in a Sako 375 H&H I have, but it isn't very long so I just ignore it and use throat measurements taken about 1/4" from the end of the chamber's neck.
    There are just starting points that work for me in rifles with similar issues. YMMV

  11. #71
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    Cainttype, that is the logic I'm following. It seems like the bullet should have a long enough profile that it can engage the lands and grooves while the base is still supported by the mouth of the case-neck. Also, the forward portion of the bullet should be broad enough that the bullet can index more efficiently as it starts its trip into the rifling. A .315dia should engrave in the rifling very nicely. The challenge at this point is to find a bullet that doesn't push the weight limit of standard cast 7.62x39 rounds. Most bullet molds I see for sale that meet this requirement are 180gr and up. I've considered the prospect of trying one of these molds, but load data for such a combination in this caliber is elusive at best. A mold like this looks promising if I can size it down efficiently to .314, but it's flat-point concerns me that reliable chambering might be an issue. Mind you, these loads would be treated like a fine wine--Seldom fired, and never rapid fired. It wouldn't be much heavier than the Lee 160gr bullet that is made for this caliber.
    http://www.midwayusa.com/product/383...ProductFinding
    Last edited by Animal; 03-12-2015 at 10:31 PM.

  12. #72
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    thank you for this post
    the pictures helped me understand it more

  13. #73
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    I just happened upon this thread it answers my questions beutifuly visual aids are great when trying to explain something like this, kudos to the originator.

  14. #74
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    Yep a picture is worth a 1,000 words.

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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