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Posted: 7/17/2012 2:41:53 PM EDT
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For the purposes of reloading, how important is it to measure the chamber of a semi auto rifle?
Being that loading for various rifles is the limiting factor on one's trim length, why is it necessary to know the chamber size of a particular rifle? |
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Depends on what part of the chamber your asking about. Trim length has a standard for each caliber. SAAMI sets the standard for USA made ammo. When you let the trim length grow longer than the listed maximum, the bullet may get crimped in the chamber on loading. The strike of the firing pin can push the shoulder back, letting more of the neck enter deeper into the throat/leade of the chamber. This may raise pressure to a dangerous level. http://www.saami.org/specifications_and_information/index.cfm?page=CC why is it necessary to know the chamber size of a particular rifle? We do not need to know, if trimming when needed.
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Are you talking about measuring to the rifling like with one of the Hornady OAL gauges? If so, you can set bullet length to control the jump of the bullet into the lands and grooves. I understand that concept. I haven't gone so far as to buy that gauge yet, because I'm limited by magazine size. But was more wondering about the chamber gauges... |
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Maybe you are referring to chamber measurement as it pertains to adjusting your sizing die to minimally bump the shoulder and not overwork the brass? Or how to obtain such a measurement? Yes. I suppose I could have frame the question that way... I know that there are gauges for such measurements but fail to grasp the reason it might be necessary and wonder whether there are any limited factors to their use as it pertains to semi auto rifles... |
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You can use a case gauge to adjust your full len resize die to only resize a safe-but-minimum amount on a semi-auto gun. It may help case life or accuracy a bit. A chamber gauge is used to check if the rifles headspacing is normal and safe for factory spec ammo I'm hip to the need for a chamber gauge. On the case gauge, I would compare the barrel chamber with a fired round and then what exactly? |
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The fired case is a measure of the growth of the case in your chamber. A new case will spring back slightly, so it's slightly smaller than the chamber, a case that has been 5 times has work hardened and won't spring back as much or extract easily.
Chamber fit is measured from the case head to a datum on the shoulder. You can do this without buying special tools by simply using a bushing with ends square to the centerline and long enough to clear the case mouth. With that measurement, AR's operate reliably with the case sized to push the shoulder back 0.0015 to 0.002 inches shorter. Probably half of the reloaders here use fixed drop in gages to set up the sizer die to a generic dimension determined "best" by the gage maker. The Dillon and Lyman gages I have used all cause the sizer to be set up to set the shoulder back 0.008 inches, or produce "0.008 inches of cartridge headspace" in reloading terminology. As for trimming, trim to no more than the published maximum length, or to the "trim to" length if you want. This is a safety issue, and it is not restricted to autoloaders, it applies to all bottleneck cases in all firearms. |
| The cartridge gage tells you if your reloads will fit the chamber. If the shoulder of the brass is pushed back to far on Full length sizing a 223/5.56, a case seperation may happen. The FL die & shell holder/plate will set a usable Head Clearance 99% of the time. The 1% could have blown up my rifle, because of a defective shell plate station, setting the shoulder back more than .014" If the FL die was setup on the other 3 stations of the shell plate, ammo was OK. The bad station caused a seperation almost every time. Photo Albums |
| L. E. Wilson Case Gage Instructions > http://lewilson.com/images/CASE_GAGE.pdf |
| I've never measured a chamber, I get loose primer pockets before incipent case head seperation, and I get reliable chambering. It is slightly more difficult with a semi, but you can still drop your sized brass into the chamber, ride the bolt forward and adjust your die down in very small increments until the bolt fully closes. The measurement to the lands is useless information for the vast majority of people who enjoy mag feeding, if you are an exception you most likely know because you probably had the chamber reamed to your specs. |
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I've never measured a chamber, I get loose primer pockets before incipent case head seperation, and I get reliable chambering. It is slightly more difficult with a semi, but you can still drop your sized brass into the chamber, ride the bolt forward and adjust your die down in very small increments until the bolt fully closes. The measurement to the lands is useless information for the vast majority of people who enjoy mag feeding, if you are an exception you most likely know because you probably had the chamber reamed to your specs. Haven't special ordered my barrel's chamber, so that's not the case here. What would be the advantage of sizing the case to just barely fit into the chamber? My guess is that it would save give a person more firings / life out of their brass because it wouldn't have to be sized down as much? Which would come at the sacrifice of only being able to use them for that barrel and that barrel only. Just want to be sure I'm understanding this concept correctly. Plus I don't want to buy a piece of equipment that I won't need... |
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The average full length resizing die is designed to push the shoulder of the case back approximately .002 shorter than minimum headspace or the GO gauge. If your chambers headspace is .002 over minimum headspace when you resize the case you will push the shoulder back .004 if the resizing die is making firm hard contact with the shell holder.
If you look at the SAAMI chamber and cartridge drawing on the .223 you will see there is .010 between minimum and maximum headspace tolerances between the GO gauge and Field max. If you look at the cartridge case manufacturing specs the case can vary .007 in length when made. This means at the extreme ends of the spectrum a new case might have .017 head clearance or airspace between the bolt face and the rear of the case when it is fired. This is why they make headspace gauges and cartridge case headspace gauges, it lets you decide how much "airspace" is between YOUR bolt and the rear of the case when you full length resize the case. General thumb rule, on a bolt action rifle the shoulder of the case when full length resizing is pushed back .001 to .002 thousandths, on a semi-auto the shoulder is pushed back .002 to .004. The less "air space" or head clearance you have the longer the case will last before you start to have case head separations. |
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What would be the advantage of sizing the case to just barely fit into the chamber? My guess is that it would save give a person more firings / life out of their brass because it wouldn't have to be sized down as much? Which would come at the sacrifice of only being able to use them for that barrel and that barrel only. Just want to be sure I'm understanding this concept correctly. Plus I don't want to buy a piece of equipment that I won't need... Primarily longer case life, some argue for accuracy as well. You may or may not be limited to using the rounds in only that rifle, it just may not be optimal. However if the chamber you sized the brass for happened to be on the small end of the catridge spec and the other at the large or over it could lead to catastrophic failure with hot loads and/or brass with multiple cycles. |
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The fired case is a measure of the growth of the case in your chamber. A new case will spring back slightly, so it's slightly smaller than the chamber, a case that has been 5 times has work hardened and won't spring back as much or extract easily. Chamber fit is measured from the case head to a datum on the shoulder. You can do this without buying special tools by simply using a bushing with ends square to the centerline and long enough to clear the case mouth. With that measurement, AR's operate reliably with the case sized to push the shoulder back 0.0015 to 0.002 inches shorter. Probably half of the reloaders here use fixed drop in gages to set up the sizer die to a generic dimension determined "best" by the gage maker. The Dillon and Lyman gages I have used all cause the sizer to be set up to set the shoulder back 0.008 inches, or produce "0.008 inches of cartridge headspace" in reloading terminology. As for trimming, trim to no more than the published maximum length, or to the "trim to" length if you want. This is a safety issue, and it is not restricted to autoloaders, it applies to all bottleneck cases in all firearms. +1 Two different case gages for two different purposes: 1. Drop gage - ensures that loaded cartridges are sized to drop in any chamber for that caliber. Important for semi-automatic pistols and rifles. Not critical for bolt action rifles (because bolt guns have closing cams that can muscle SLIGHTLY oversize cases into the chamber.i.e., neck sized rounds). 2. Cartridge headspace gage - used to measure the "headspace" of a fired cartridge which infers the headspace of the rifle. Then the sizing die is adjusted until sized cases are just a little below the inferred rifle headspace. Minimizes case growth and case cold working and maximizes case life. |
Below is a animated .gif image of what happens when you fire an undersized American SAAMI .303 British cartridge case in a British Enfield military chamber. Or what happens when you fire a short skinny case in a long fat chamber, please take note of the words "head clearance" or the "air space" between the bolt face and the rear of the case and its relation to how much the case can stretch when fired..
Also note not all cases are created equal when it comes to case life and reloading. 
When full length resizing you are decreasing the fired cases diameter and also decreasing cartridge headspace by pushing the shoulder back to "YOUR" desired point for your individual rifle. (what works best for you and nothing is written in stone) 
And not all resizing dies are created equal.  These dies are used on my bull barrel .223 bolt action and the bushing die would not be used on a AR 15.
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Below is a animated .gif image of what happens when you fire an undersized American SAAMI .303 British cartridge case in a British Enfield military chamber. Or what happens when you fire a short skinny case in a long fat chamber, please take note of the words "head clearance" or the "air space" between the bolt face and the rear of the case and its relation to how much the case can stretch when fired.. http://i122.photobucket.com/albums/o254/bigedp51/headspacestretch-1.gif Also note not all cases are created equal when it comes to case life and reloading. http://i122.photobucket.com/albums/o254/bigedp51/308fail.jpg http://i122.photobucket.com/albums/o254/bigedp51/308fail-2.jpg When full length resizing you are decreasing the fired cases diameter and also decreasing cartridge headspace by pushing the shoulder back to "YOUR" desired point for your individual rifle. (what works best for you and nothing is written in stone) http://i122.photobucket.com/albums/o254/bigedp51/IMGP7212-b.jpg And not all resizing dies are created equal. These dies are used on my bull barrel .223 bolt action and the bushing die would not be used on a AR 15.
http://i122.photobucket.com/albums/o254/bigedp51/IMGP7650.jpg http://i122.photobucket.com/albums/o254/bigedp51/Headspace_2_lg.jpg http://i122.photobucket.com/albums/o254/bigedp51/Headspace_1_lg.jpg This is the stuff I was needing to understand better. Thank tremendously? When someone neck sizes only, or shoulder bumps, (I assume they are the same) (I know that's not appropriate for semi auto's) what process is taking place? I assume it doesn't stretch the brass as in the above gif... |
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Neck sizing means the shoulder is not touched by the die. Bumping or setting the shoulder back means the case shoulder is pushed back to increase the clearance between the case and chamber.
Some dies are manufactured to only size the case without touching the neck, those are body sizing dies. Dies that are made to use a bushing to size the neck might be made as neck sizers or as full length dies when the bushing is installed. Leaving the bushing out allows the die to be used for sizing the case body without touching the neck. What is not shown in the animation above is the reason trimming is important. Notice how the end of the case mouth clears the transition at the end of the chamber where the bullet enters the leade. If a case with the neck left too long extends into that transition, then the case mouth can't expand to release the bullet. |
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Neck sizing means the shoulder is not touched by the die. Bumping or setting the shoulder back means the case shoulder is pushed back to increase the clearance between the case and chamber. Some dies are manufactured to only size the case without touching the neck, those are body sizing dies. Dies that are made to use a bushing to size the neck might be made as neck sizers or as full length dies when the bushing is installed. Leaving the bushing out allows the die to be used for sizing the case body without touching the neck. What is not shown in the animation above is the reason trimming is important. Notice how the end of the case mouth clears the transition at the end of the chamber where the bullet enters the leade. If a case with the neck left too long extends into that transition, then the case mouth can't expand to release the bullet. I see that. Not trimming would be a big problem if stuffed into the chamber. A bolt action would suffer from that more as operating the bolt could force the neck into that transition while a semi would more likely not go into battery. Though I suppose both could be forced against that part of the chamber. But when a cartridge (new or resized) goes into the chamber, is it's shoulder / datum point touching the chamber? Or does it only touch when its fired (as the animation shows)? Isn't the headspace determined by the datum point to the bolt face? How could there be space between the head and the bolt face? Maybe I'm mixing too many subjects now but I need a better grasp of these issues... |
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I'm not trying to be a smart ass when I say this BUT, if you do not have a reloading manual its time to buy one. If you do have a reloading manual you need to sit down and open the reloading manual to the first page and start reading.
All the knowledge in the world is written books and all you have to do is read. At the age of 24 in 1974 I had an epiphany on how little I knew about reloading when I sat down and read this reloading manual below from cover to cover. 
In this manual was a section on "Modern benchrest reloading techniques", in this article it stated that more reloaded ammunition is loaded having "MORE" case neck run out than factory ammunition because the decapping-expander button spindle was locked down "OFF CENTER" and pulling the case necks off center. 
I now use rubber o-rings under my lock rings that allow the stem to float and be self centering and reduce neck runout. Read your manual and you will have a much better understanding of reloading principles. And then as you advance you can get better equipment and reload more accurate ammunition. 
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I think I've answered my own question. The datum point touches the chamber of the case gage to get a measurement, not the chamber or the barrel, Or does it? A headspace gauges gives you the datum point of the chamber and a case headspace gauge gives the datum point of the cartridge case. The further you push the case shoulder back when resizing the more "air space" you will have inside the chamber when the cartridge is chambered. (head clearance) 
Headspace 101 Lesson number one The worst type of headspace does not happen in firearms. It happens in dimly lit bars and is caused by unscrupulous bartenders. 
The foam is "head clearance or air space"
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I think I've answered my own question. The datum point touches the chamber of the case gage to get a measurement, not the chamber or the barrel, Or does it? A headspace gauges gives you the datum point of the chamber and a case headspace gauge gives the datum point of the cartridge case. The further you push the case shoulder back when resizing the more "air space" you will have inside the chamber when the cartridge is chambered. (head clearance) http://i122.photobucket.com/albums/o254/bigedp51/rimless.jpg Headspace 101 Lesson number one The worst type of headspace does not happen in firearms. It happens in dimly lit bars and is caused by unscrupulous bartenders. http://i122.photobucket.com/albums/o254/bigedp51/10_foambeer_lgl.jpg The foam is "head clearance or air space"
I kept looking for this page in my sierra manual but couldn't find it anywhere. Who knew you find so much good info in a bar!
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The beer glass is your rifles chamber
The beer is the cartridge case When neck sizing only you want a little bit of beer to spill over the rim when the glass is full. When full length resizing you want the beer .002 to .004 below the rim of the glass and have very, very little foam. When you can get 32 reloadings from a British .303 case you become a brewmaster and only drink the best of beers.
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The beer glass is your rifles chamber The beer is the cartridge case When neck sizing only you want a little bit of beer to spill over the rim when the glass is full. When full length resizing you want the beer .002 to .004 below the rim of the glass and have very, very little foam. When you can get 32 reloadings from a British .303 case you become a brewmaster and only drink the best of beers.
http://i122.photobucket.com/albums/o254/bigedp51/IMGP2800.jpg Its becoming more clear to me the fuzzier my vision gets. After a few more of those lager experiments I'll have it down pat, or down flat. Which ever comes first. 
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