RiverRider
Member
I've seen commentary here that suggests no one has promoted the [real] proper way to set up sizing dies for bottlenecked cartridges. If you follow the manufacturers' instructions, you'll end up with a cartridge that will chamber and fire reliably 99.95% of the time in most rifles, and that is the intent of the instructions which is fine and dandy. If that's good enough for you, you are a "reloader" and your goal is simply to assemble usable ammo and that's fine as long as it suits your purposes. There's no shame in that if it works for you and makes you happy...but if you're a handloader your purposes and goals may be little different and you'll look for ways to do a better job of it, and here's a hint.
In a nutshell, there are potential benefits to be had by paying more attention to detail in the case sizing step.
One of them is better accuracy. As an illustration, I once had a Remington 700 Classic in .223 Remington. That was the Y2K offering from Remington. It was a handsome little devil and it was very accurate, easily capable of 1/2-inch five-shot groups shooting 50-grain V-Max. I could look up the powder and charge but that's not what I am writing about but I think I was using Benchmark. What happened was that one day I found that the rifle would not shoot those 1/2-inch groups anymore, but was shooting about double that figure. I checked all kinds of things, including replacing of the J-lock bolt shroud and the firing pin assembly with parts from GreTan. After struggling for weeks I gave up and sold the rifle and I bought another one---a beautiful little Savage Model 14 American Classic (maybe the only Savage I ever saw or owned that I would describe as beautiful) and it would not shoot any better. After several more weeks of struggling, I got rid of it as well. I don't remember how many rifles I went through searching for one that would shoot my *proven* load like I knew it should. Then one day after many weeks of frustration (and the departure of two rifles I'd dearly love to have back), I decided to look closer at my loading process.
To my horror, I found that the lock ring on my sizing die had moved, and I was now sizing .223 cases as much as my Redding die would allow (and which is how the instructions would have you do it). Then I remembered that I had reset the die to load up a bunch of 5.56 intended for use in my AR15 months before all this began. Measurements showed I was pushing the shoulder back about 0.008 inch further than I needed to for my bolt guns. I normally set up to bump shoulders back around 0.002 inch to 0.0025 inch for a bolt action. Bottom line---YES, the amount of shoulder bump matters for accuracy. Further, I was shooting ammo that presented excessive headspace (all due to a "headspace" error bewteen my own ears).
Another benefit to proper setup is case life. Some cartridges are more prone to lengthening than others, but in my experience they will all grow at least a bit with repeated firings. The reason for that is that when the firing pin hits the primer, the cartridge is pushed forward into the chamber as deeply as it will go as the primer ignites. The case itself is confined to the chamber's dimensions under pressure and the brass grips the chamber surfaces---especially near the shoulder. Those areas up and down the length of the case are mutually supporting while the prsessure remains high---except near the head of the case. Near the case head, around where the web of the case head is there is slippage in the chamber and the case head will move to the rear bumping the breech or bolt face. Depending on the elasticity of the case near the web (and it is intended to be quite hard) it will permanently distort. That is how and why a bottlenecked case will develop thinning just above the web. If you're bumping the shoulder back more than necessary for reliable chambering, then your cases will thin at the web much sooner.
Those are the reasons I set my sizing dies to bump the shoulder minimally for a bolt gun.
In case you wonder, I use a Forster Co-Ax press and once I have set my die for optimal sizing, I tighten the lock ring so that it will not move. Since the tragic fiasco with my 700 Classic, I purchased another FL die for my AR and set the shoulder bump for about 0.005" or so for reliable feeding. I still mourn the 700 Classic. And I do NOT screw with the lock rings!
So, that takes care of the WHY. Now for the HOW.
If you've ever wondered about the infamous "datum line" on a bottlenecked case, you are not alone. You can't really measure it and verify that it's within specs but you CAN detect the movement or positional change of the case's shoulder, and it's not hard to do. If you're a handloader you should have a comparator such as the old Stoney Point bullet comparator set (now marketed under the Hornady name). The set includes comparator inserts for bullets up to 0.375" in diameter. I do use the comparator for setting bullet seating depth (a whole nuther subject for another day), but they can also be used to detect relative movement of the shoulder. All you need is *a* datum line, not *the* datum line. The .30-cal insert works well for a .223 case, for example. Hornady also offers comparators specifically for measuring "the" datum line and I am sure they work fine but I do not find them to be necessary. The RCBS Precision Mic will do the same thing for you but they are generally much more caliber-specific and not so versatile.
A little more background information is probably in order here. Early on in my learning, I tried "neck sizing using a FL die." I didn't know I was attempting partial full-length sizing; I'd never heard of it. So the idea was to run the fired case up into the die only far enough to squeeze the neck down to hold a bullet firmly but without resizing the body at all. It didn't take a whole lot of fiddling with it to make an intersting discovery, and that was that if I ran the case deep enough into the die BUT without contacting the shoulder I could actually make it more difficult to chamber that case in my rifle. So the discovery was this: during FL sizing, the shoulder begins to flow forward before it makes the contact in the die which pushes it back. That was a watershed moment because I then realized I could set the die up to bump the shoulder a minimal and optimal amount.
If you do not believe the shoulder flows forward during resizing, set up a sizing die with about a 0.010" gap between the top of the shell holder and the mouth of the die and run a case up into it. You can then see how it feels to close the bolt of your rifle on it before and after, or measure it before and after sizing with the die adjusted like that. If you don't measure a lengthening or feel more resistance when closing the bolt then screw the die in JUST A TIC more and try it again.
To get this setup done, measure a datum for three or four cases fired in your rifle using a comparator and record the average. They should all be VERY close to the same measurement. You might want to chamber them in your rifle also just to get an approximate feel of the resistance when closing the bolt.
Set up your sizing die with about 0.010" clearance between the shell holder and the mouth of the die, then lube a case and run it up in there. Measure the datum length again. It may not begin to lengthen right away or you may notice it right off the bat---different case gemoetries will respond differently. You should or probably will see a lengthening first, and then as you adjust the die a little deeper between each try you will see the datum length beginning to decrease---which means you're getting very close. Clean and chamber the case each time to monitor the feel of closing your bolt handle, and when you see the datum length begin to decrease you should feel the bolt closing just a little bit more easily. Where you stop is up to you, but I like to feel a little bit of resistance, and I mean *a little bit* when I close the bolt. The idea of taking all the measurements during this process is to guage your progress in a way that you can see and quantify. Just keep in mind, you don't want a chambered round to be causing undue strain in your action.
Take heed when adjusting the die. Dies are threaded 14 turns per inch and that means a 360-degree turn into the press represents 1/14 of an inch. If you do the arithmetic, you'll find that a 5-degree turn of the die deeper represents about 0.001" of shoulder bump. One single tick of the second hand of an analog watch or clock is 6 degrees of movement, so it takes only a tiny bit of turn of the die to change shoulder bump by one thousandth of an inch. One caveat: do not expect to see exactly precise changes in datum length each time you size a case because the ductility of each case shoulder comes into play (annealing can address that to some extent if you want to be really anal about it). Variability is always lurking.
So...that is a long read, I know, but once you grasp the WHYs and the HOWs of it, it's not really a hard technique to employ. The effort may or may not improve the accuracy of your rifle---that depends on many [lurking] variables. But, if you just want to do things as precisely as you can *just because* or if you want to extend the life of your brass, give it a try.
Happy shooting.
In a nutshell, there are potential benefits to be had by paying more attention to detail in the case sizing step.
One of them is better accuracy. As an illustration, I once had a Remington 700 Classic in .223 Remington. That was the Y2K offering from Remington. It was a handsome little devil and it was very accurate, easily capable of 1/2-inch five-shot groups shooting 50-grain V-Max. I could look up the powder and charge but that's not what I am writing about but I think I was using Benchmark. What happened was that one day I found that the rifle would not shoot those 1/2-inch groups anymore, but was shooting about double that figure. I checked all kinds of things, including replacing of the J-lock bolt shroud and the firing pin assembly with parts from GreTan. After struggling for weeks I gave up and sold the rifle and I bought another one---a beautiful little Savage Model 14 American Classic (maybe the only Savage I ever saw or owned that I would describe as beautiful) and it would not shoot any better. After several more weeks of struggling, I got rid of it as well. I don't remember how many rifles I went through searching for one that would shoot my *proven* load like I knew it should. Then one day after many weeks of frustration (and the departure of two rifles I'd dearly love to have back), I decided to look closer at my loading process.
To my horror, I found that the lock ring on my sizing die had moved, and I was now sizing .223 cases as much as my Redding die would allow (and which is how the instructions would have you do it). Then I remembered that I had reset the die to load up a bunch of 5.56 intended for use in my AR15 months before all this began. Measurements showed I was pushing the shoulder back about 0.008 inch further than I needed to for my bolt guns. I normally set up to bump shoulders back around 0.002 inch to 0.0025 inch for a bolt action. Bottom line---YES, the amount of shoulder bump matters for accuracy. Further, I was shooting ammo that presented excessive headspace (all due to a "headspace" error bewteen my own ears).
Another benefit to proper setup is case life. Some cartridges are more prone to lengthening than others, but in my experience they will all grow at least a bit with repeated firings. The reason for that is that when the firing pin hits the primer, the cartridge is pushed forward into the chamber as deeply as it will go as the primer ignites. The case itself is confined to the chamber's dimensions under pressure and the brass grips the chamber surfaces---especially near the shoulder. Those areas up and down the length of the case are mutually supporting while the prsessure remains high---except near the head of the case. Near the case head, around where the web of the case head is there is slippage in the chamber and the case head will move to the rear bumping the breech or bolt face. Depending on the elasticity of the case near the web (and it is intended to be quite hard) it will permanently distort. That is how and why a bottlenecked case will develop thinning just above the web. If you're bumping the shoulder back more than necessary for reliable chambering, then your cases will thin at the web much sooner.
Those are the reasons I set my sizing dies to bump the shoulder minimally for a bolt gun.
In case you wonder, I use a Forster Co-Ax press and once I have set my die for optimal sizing, I tighten the lock ring so that it will not move. Since the tragic fiasco with my 700 Classic, I purchased another FL die for my AR and set the shoulder bump for about 0.005" or so for reliable feeding. I still mourn the 700 Classic. And I do NOT screw with the lock rings!
So, that takes care of the WHY. Now for the HOW.
If you've ever wondered about the infamous "datum line" on a bottlenecked case, you are not alone. You can't really measure it and verify that it's within specs but you CAN detect the movement or positional change of the case's shoulder, and it's not hard to do. If you're a handloader you should have a comparator such as the old Stoney Point bullet comparator set (now marketed under the Hornady name). The set includes comparator inserts for bullets up to 0.375" in diameter. I do use the comparator for setting bullet seating depth (a whole nuther subject for another day), but they can also be used to detect relative movement of the shoulder. All you need is *a* datum line, not *the* datum line. The .30-cal insert works well for a .223 case, for example. Hornady also offers comparators specifically for measuring "the" datum line and I am sure they work fine but I do not find them to be necessary. The RCBS Precision Mic will do the same thing for you but they are generally much more caliber-specific and not so versatile.
A little more background information is probably in order here. Early on in my learning, I tried "neck sizing using a FL die." I didn't know I was attempting partial full-length sizing; I'd never heard of it. So the idea was to run the fired case up into the die only far enough to squeeze the neck down to hold a bullet firmly but without resizing the body at all. It didn't take a whole lot of fiddling with it to make an intersting discovery, and that was that if I ran the case deep enough into the die BUT without contacting the shoulder I could actually make it more difficult to chamber that case in my rifle. So the discovery was this: during FL sizing, the shoulder begins to flow forward before it makes the contact in the die which pushes it back. That was a watershed moment because I then realized I could set the die up to bump the shoulder a minimal and optimal amount.
If you do not believe the shoulder flows forward during resizing, set up a sizing die with about a 0.010" gap between the top of the shell holder and the mouth of the die and run a case up into it. You can then see how it feels to close the bolt of your rifle on it before and after, or measure it before and after sizing with the die adjusted like that. If you don't measure a lengthening or feel more resistance when closing the bolt then screw the die in JUST A TIC more and try it again.
To get this setup done, measure a datum for three or four cases fired in your rifle using a comparator and record the average. They should all be VERY close to the same measurement. You might want to chamber them in your rifle also just to get an approximate feel of the resistance when closing the bolt.
Set up your sizing die with about 0.010" clearance between the shell holder and the mouth of the die, then lube a case and run it up in there. Measure the datum length again. It may not begin to lengthen right away or you may notice it right off the bat---different case gemoetries will respond differently. You should or probably will see a lengthening first, and then as you adjust the die a little deeper between each try you will see the datum length beginning to decrease---which means you're getting very close. Clean and chamber the case each time to monitor the feel of closing your bolt handle, and when you see the datum length begin to decrease you should feel the bolt closing just a little bit more easily. Where you stop is up to you, but I like to feel a little bit of resistance, and I mean *a little bit* when I close the bolt. The idea of taking all the measurements during this process is to guage your progress in a way that you can see and quantify. Just keep in mind, you don't want a chambered round to be causing undue strain in your action.
Take heed when adjusting the die. Dies are threaded 14 turns per inch and that means a 360-degree turn into the press represents 1/14 of an inch. If you do the arithmetic, you'll find that a 5-degree turn of the die deeper represents about 0.001" of shoulder bump. One single tick of the second hand of an analog watch or clock is 6 degrees of movement, so it takes only a tiny bit of turn of the die to change shoulder bump by one thousandth of an inch. One caveat: do not expect to see exactly precise changes in datum length each time you size a case because the ductility of each case shoulder comes into play (annealing can address that to some extent if you want to be really anal about it). Variability is always lurking.
So...that is a long read, I know, but once you grasp the WHYs and the HOWs of it, it's not really a hard technique to employ. The effort may or may not improve the accuracy of your rifle---that depends on many [lurking] variables. But, if you just want to do things as precisely as you can *just because* or if you want to extend the life of your brass, give it a try.
Happy shooting.
