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Gunsmithing Pressure Sensitivity?
- Thread starter CharlieNC
- Start date
Its the barrel. Each one is different to an extent. Brass has a little to do with it also. You will probably pick up some fps as the barrel gets broke in. Id say after 100-200 rounds your going to find your fps has reached its peak. I dont think you can build to comp for this, Its basically run what you got
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I know barrels make a difference; but which parameter(s) have the most influence? For example the guys I know who shoot Savage 308's run into pressure problems at moderate loads; same as mine. Other makes successfully run much hotter loads. So if/when I get a new barrel, how do you choose a chamber spec for this? Or is there something besides the barrel to address?
There is a lot more than "the barrel" that ads to things.
First thing is first, what is "pressure"?
IF, and this is a BIG ASSumption, IF a company ONLY built brass by the metals handbook formula. Cartridge brass has a tensile strength of about 70,000psi, so when you notice expansion, guess where your pressures are?!
So what formula DID they use to make the brass? They aren't saying. That is a piece of what people see for " pressures".
Next is the primer. Again there is no "spec" for primers. So how "tough" they are can vary wildly. There is part #2.
Now the chamber, the bore. How much your brass has to stretch to conform; and how far your dies push things back also contributes. If you over work your brass, you will begin the work-hardening process. This will further skew things.
Now powder. Marketers like Hodgdon try and sell things like "extreme" miracle powders. MOST people won't even read Hodgy's info about it. If they did, they would see that there is very little info speaking to the science of it. Furthermore MANY of their examples aren't statistically valid differences.
The theory does work, but only in a very specific setting. As an example. Varget is, relatively speaking, temp insensitive in the 308, when using 150gr bullets. But it's a streaming pile of crap when used in the 223. Doesn't mean it won't group in the 223, just means if you think things are "extreme" in that setting, you are out to lunch.
Finally you have chamber time. The biggest challenge for any powder to be "insensitive" is brought on by a hot chamber. Letting cartridges sit in a hot chamber is bad juju.
More on the point of powder: remember that we, the canister-grade using reloader, is a waste market.
WC 846 is a non-canister powder. The original patented recipe is from about the 1930's, IIRC. It allows for a max amount of CaCO of 1%, anything under that is kosher. When Olin began segregating and anything that had less than 0.25% was produced, they called it WC844. Ask a random reloader if BL-c(2) and H355 is the same powder, they will look at you like you are on the short bus. When the truth is, they are the same. Ask them how long this miracle copper cleaner in CFE223 has been around, they'll tell you about 2012. The Tin/Bismuth compounds that clean copper were discovered by the French about 1900. WC842( which is where CFE223 comes from) has been produced by General Dynamics for over a Decade. Those compounds have also been in Win748 & Win760/H414 for MANY decades.
How many people use a volume device, for bloody weight?! Extruded powder has its burning rate controlled by geometry. Weight and volume are NOT the same damn thing. Extruded powders have a very tight relationship between Bulk Density and Burning Rate. IF you want consistent pressures with extruded, STOP weighing powder and throw a volume. Cutting grains will not affect anything. A consistent VOLUME will limit burning rate fluctuation MUCH more than a weight of powder.... Unless you think your powder "company" who won't give you nominal burning rate variance and Bulk densities can ONLY build A thing....
Ball powder has burning rates controlled by Coatings, with no tie to BD, or amount of Nitro Glycerin. Not as awesome to load that by volume, again, unless you don't load by volume; you use a volume device to get a weight....
Seek out work done by Dr. Denton Bramwell. Has done a bunch of good work on this, and the "extreme" properties of powder.
If you want to know what pressures are really doing, buy a testing system. The Pressure Trace strain gauge system is only about $4-500. Know for yourself.
First thing is first, what is "pressure"?
IF, and this is a BIG ASSumption, IF a company ONLY built brass by the metals handbook formula. Cartridge brass has a tensile strength of about 70,000psi, so when you notice expansion, guess where your pressures are?!
So what formula DID they use to make the brass? They aren't saying. That is a piece of what people see for " pressures".
Next is the primer. Again there is no "spec" for primers. So how "tough" they are can vary wildly. There is part #2.
Now the chamber, the bore. How much your brass has to stretch to conform; and how far your dies push things back also contributes. If you over work your brass, you will begin the work-hardening process. This will further skew things.
Now powder. Marketers like Hodgdon try and sell things like "extreme" miracle powders. MOST people won't even read Hodgy's info about it. If they did, they would see that there is very little info speaking to the science of it. Furthermore MANY of their examples aren't statistically valid differences.
The theory does work, but only in a very specific setting. As an example. Varget is, relatively speaking, temp insensitive in the 308, when using 150gr bullets. But it's a streaming pile of crap when used in the 223. Doesn't mean it won't group in the 223, just means if you think things are "extreme" in that setting, you are out to lunch.
Finally you have chamber time. The biggest challenge for any powder to be "insensitive" is brought on by a hot chamber. Letting cartridges sit in a hot chamber is bad juju.
More on the point of powder: remember that we, the canister-grade using reloader, is a waste market.
WC 846 is a non-canister powder. The original patented recipe is from about the 1930's, IIRC. It allows for a max amount of CaCO of 1%, anything under that is kosher. When Olin began segregating and anything that had less than 0.25% was produced, they called it WC844. Ask a random reloader if BL-c(2) and H355 is the same powder, they will look at you like you are on the short bus. When the truth is, they are the same. Ask them how long this miracle copper cleaner in CFE223 has been around, they'll tell you about 2012. The Tin/Bismuth compounds that clean copper were discovered by the French about 1900. WC842( which is where CFE223 comes from) has been produced by General Dynamics for over a Decade. Those compounds have also been in Win748 & Win760/H414 for MANY decades.
How many people use a volume device, for bloody weight?! Extruded powder has its burning rate controlled by geometry. Weight and volume are NOT the same damn thing. Extruded powders have a very tight relationship between Bulk Density and Burning Rate. IF you want consistent pressures with extruded, STOP weighing powder and throw a volume. Cutting grains will not affect anything. A consistent VOLUME will limit burning rate fluctuation MUCH more than a weight of powder.... Unless you think your powder "company" who won't give you nominal burning rate variance and Bulk densities can ONLY build A thing....
Ball powder has burning rates controlled by Coatings, with no tie to BD, or amount of Nitro Glycerin. Not as awesome to load that by volume, again, unless you don't load by volume; you use a volume device to get a weight....
Seek out work done by Dr. Denton Bramwell. Has done a bunch of good work on this, and the "extreme" properties of powder.
If you want to know what pressures are really doing, buy a testing system. The Pressure Trace strain gauge system is only about $4-500. Know for yourself.
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Its still mostly the barrel. Would you try and shoot one of your buddies "hot" loaded rounds in your own gun? I dont think so
There are many things that contribute to high pressure signs and I would agree the barrel plays a significant part in this. The twist of the rifling is a major contributor to actual pressure, the chamber being tight tolerance or more on sloppy side, the amount of free-bore and how much the bullet jumps or how tight its jammed into lands. All things being the same as far as powder, case capacity, bullet weight the barrel plays probably the most significant part of how much actual pressure is developed and how much signs of it we see. There is a difference in how hard primers are so one may flatten where another does not but the actual pressure can still be the same. These are just some of the reasons one rifle shows signs and another doesn't with same load, but make no mistake if all is equal the actual pressures are the same weather we can see there signs or not.
Sorry, let me clarify my question. Not a reloading inquiry, but a gunsmithing question. If building a new rifle, or modifying one, what can be done that will allow it to run higher pressure loads without problems? Are there specific chamber designs or dimensions, headspace, how the bolt locks up, etc to safely allow this? So when I spec a new barrel I would like to know what to ask for in general, or if getting an action trued what to focus on.
Extend the free bore, open the neck. The things you could do to lower pressures are the things you don't want to do for accuracy.
I think nxs hit it, there is a difference between pressure and pressure signs. A sloppy loose chamber may not hold onto the brass as much and let more of its force go into the bolt showing pressure signs. But then you may have the same pressure in a tight chamber that grips the brass a tad more before slamming into the bolt and it won't show pressure signs. But as he said, the pressure is still there (probably, we still have the different barrel dilemma which in unpredictable for the most part).
I think nxs hit it, there is a difference between pressure and pressure signs. A sloppy loose chamber may not hold onto the brass as much and let more of its force go into the bolt showing pressure signs. But then you may have the same pressure in a tight chamber that grips the brass a tad more before slamming into the bolt and it won't show pressure signs. But as he said, the pressure is still there (probably, we still have the different barrel dilemma which in unpredictable for the most part).
There are things which I believe can be done to allow for more pressures, and yes as Jonaddis84 said some are detrimental to accuracy. I believe I can safely say there are quite a few of us who have surpassed the loading books as far as max loads go and I will go on to say these specs given are so a loose chambered maximum headspace rifle can somewhat shoot these loads without catastrophic failure. ( I do not advise nor suggest you ever go over max loads in the books!) but I have done it and I know many who have. Just a few suggestions you may think about:
Do not spec an over rifled twist barrel, pick the slowest rifling twist to stabilize whatever bullet you are going to shoot. This is hard because most people want a barrel that will shoot all different weight bullets. Your not making a leatherman tool here just a simple flathead screwdriver meant to do one job...lol. The reason you notice savages show pressure signs quicker than say remmy's is because they usually have faster twist barrels as a rule, this makes a higher pressure spike when fired with same load and it shows.
Make sure the chamber is NOT polished you want it to have texture, smooth but not polished slick as glass. The brass has to grip the chamber to not allow the stretch among other things, which is a whole other topic. If you have the rifle chambered tight and prep the brass with the utmost of care it will go along way to not show signs of pressure. Remember though the pressure is still there but it just doesnt show as easy.
Ultimately the basic Idea works like this, we use faster burning powders to push the bullet faster because the slower powder simply will not fit in the case which spikes the pressure. If you want to push a bullet faster increasing the case volume and making more room for more of the slower powders is the way to do it and this will usually need an increase in barrel length to burn that amount of powder. The other side to this is most cartridges in my limited experience seem to shoot best at the higher pressures as the powder seems to burn more consistent, this can be seen with a simple chronograph. This thread can probably go in a million directions but the things I put out there are probably your best bet to accomplish your goal IMHO.
Do not spec an over rifled twist barrel, pick the slowest rifling twist to stabilize whatever bullet you are going to shoot. This is hard because most people want a barrel that will shoot all different weight bullets. Your not making a leatherman tool here just a simple flathead screwdriver meant to do one job...lol. The reason you notice savages show pressure signs quicker than say remmy's is because they usually have faster twist barrels as a rule, this makes a higher pressure spike when fired with same load and it shows.
Make sure the chamber is NOT polished you want it to have texture, smooth but not polished slick as glass. The brass has to grip the chamber to not allow the stretch among other things, which is a whole other topic. If you have the rifle chambered tight and prep the brass with the utmost of care it will go along way to not show signs of pressure. Remember though the pressure is still there but it just doesnt show as easy.
Ultimately the basic Idea works like this, we use faster burning powders to push the bullet faster because the slower powder simply will not fit in the case which spikes the pressure. If you want to push a bullet faster increasing the case volume and making more room for more of the slower powders is the way to do it and this will usually need an increase in barrel length to burn that amount of powder. The other side to this is most cartridges in my limited experience seem to shoot best at the higher pressures as the powder seems to burn more consistent, this can be seen with a simple chronograph. This thread can probably go in a million directions but the things I put out there are probably your best bet to accomplish your goal IMHO.
Some chambers will allow higher speeds, mostly from longer freebore accompanied by proper hand loading for the longer throat. One of the bigger variables has to do with barrel bore and groove dimensions (and land/grove ratios), not going to swear to it, but going a little over on groove dimension could offer the benefit you're looking for
