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Lets assume, factory ammo is assembled with their specific components to be within a pressure that will safely function through all guns chambered for the cartridge. Given a gun with a tight chamber, and min headspace is still safe with this factory ammo, how close would the pressure of the cartridge be to what would start causing signs of excessive pressure, such as sticky cases and backed out primers?
~Bazoo
hard to say due to way too many variables, but by the time you visually see signs, you are way way over max "book" pressures.
Minimum headspace rifles don't show backed out primers. Flat primers, yes. Backed-out, no. Generally, backed-out primers means LOW pressure, and a case that doesn't fill the chamber. The case has failed to stretch to fill the chamber, and recover the primer that backs out to fill the gap. If it doesn't stretch enough due to insufficient pressure, backed-out primer.Quote:
Originally Posted by Bazoo
In general, headspace has zero affect on pressure IN AND OF ITSELF. Some headspace related factors such as bullet jump and bullet jam do affect pressure, and if headspace affects that, yes there can be a change.
But there is more to a "tight chamber" than headspace. A short neck can pinch a case end, disallowing bullet release, sending pressures sky-high. The case might be headspaced on the shoulder properly when this happens (on rim, or whatever).
A tight chamber neck and a thick case can cause insufficient room for bullet release, which also sends pressures sky high. Again nothing to do with headspace.
The biggest factor of a tight rifle that affects pressures is the leade, or throat. But the throat is outside the chamber, and so falls outside the scope of your question.
A fairly reliable indicator I have when developing loads is to start with a new or once-fired case, and using a hand priming tool, reload and fire that one case sequentially at the range, while carefully assessing whether primers seat easier and if the primer pocket becomes "loose" in subsequent reloads.
If primers seat noticeably "easier" than a new or once-fired case, needing only with minimal pressure to fully seat the primer, in less than FIVE reloads, the load is "too hot."
If the case can be sequentially reloaded TEN times without the primer pockets becoming noticeably "loose" the load is OK.
This method is dependable in cartridges of the .30-'06 class which operate at an MAP not exceeding about 50,000 psi.
Higher pressure rounds like the .270 Win., or 5.56mm you may get not loosening of primer pockets in five reloads, but almost certainly will in ten.
This all carries the caveat that different makes of brass vary. My observations are based upon use of .30-'06 USGI Lake City Match or Ball M2 cases swaged on the Dillon Autoswager. I have found it also generally true using LC M118 brass in the 7.62mm.
In 5.56 brass loaded in full charges in published loads often loosens in 5 reloads.
Loose primer pockets in brass used for semi-auto rifles like the M1, M14/M1A and the ARs is a SAFETY HAZARD because it sets you up for a SLAMFIRE either from the primer backing out, or from the primer being "dropped", leaving loose debris in the boltface, which sets off the next round.
For this reason I DO NOT USE FEDERAL .30-'06 BRASS IN GARANDS BECAUSE IT IS SOFT! I smash it with a hammer to prevent reuse and put it in the recycling barrel at the range.
Bazoo
"Lets assume, factory ammo is assembled with their specific components to be within a pressure that will safely function through all guns chambered for the cartridge."
That is a safe assumption for the most part. US factory ammunition adheres to SAAMI guidelines (there are exceptions) with ammunition developed in specially made test barrels having minimum bore, groove, chamber and headspace specifications. Pressure problems can arise if someone uses a barrel with less than those specifications. Also understand that most all US factory ammunition is not loaded to the SAAMI MAP, most is several thousand psi under the SAAMI MAP when pressure is measured in most commercial barrels but there are exceptions as we will see.
"Given a gun with a tight chamber, and min headspace is still safe with this factory ammo, how close would the pressure of the cartridge be to what would start causing signs of excessive pressure, such as sticky cases and backed out primers?"
As mentioned, in a previous post, there are too many variables to give an answer to that question. Also as previously mentioned, "backed out primers" is not a sign of excessive pressure. Having measured the psi of thousands of reloads and factory loads the last 9 years I would not hazard a guess at answering your question.
Let me give you an "example" of the variables which are out there of which you will not know of unless you can measure the pressure in your own rifle. Earlier this year I conducted a test two different new .308W factory loads with different lot numbers. Both tests were done in the same test rifle consecutively on the same day. Test procedure followed SAAMI guidelines. The pressure/velocity was measured using an Oehler M43 PBL. The barrel (match chamber with minimum SAAMI headspace) was clean at the beginning of the each test string and two foulers were fired prior to the ten shot test string.
The first test was with Federal Premium which was loaded with the 165 gr Sierra Game King bullet as listed on the box. Under the 165 GK bullet was 42 gr of a ball powder. The 10 shot average velocity was 2702 fps (muzzle from 24" barrel) and the average pressure (M43) was 55,800 psi.
The second new factory Federal load tested was the Premium 165 gr. There was no mention of what bullet was used but it was externally very similar to the Sierra bullet. That load contained 47.3 gr of a ball powder. The average velocity was 2778 fps (muzzle) and the average pressure(M43) was 63,700 psi.
That result is with back to back tests with Federal Factory ammunition having the same weight bullet. However it was two different loads with two different lot numbers. The SAAMI MAP for the .308W is 62,000 psi (transducer/strain gauge) and in this test we had one factory load 6,200 psi LES than the SAAMI MAP and the second factory load was 1,700 psi over the SAAMI MAP????
See why there is no hard and fast answer to your question?
I do not have access to a lot of sophisticated instruments for measuring the pressure of new loads so I try to run all new stuff through a chronograph (both reloads and factory). If the velocity is way higher than expected the pressures are also likely higher than they should be. I also pay attention to the primer and extraction, but honestly have never noticed anything concerning. The FPS on the chronograph seems to be a much more reliable indicator of pressure.
To answer the OPs question, you are unlikely to have an issue with modern factory ammo in a modern gun unless one or the other are way out of spec.
If a certain brand of ammo in a certain rifle is causing you some concern I would highly recommend running a few shots over a chronograph. If the velocity is significantly higher than advertised I would have the rifle examined by a competent smith. Calling the ammo maker and checking to see if there were any recalls on your particular lot might also be prudent.
Ammo and guns are both made by men, and men make mistakes. It’s not inconceivable that a gun manufactured at the minimum end of chamber tolerances might meet up with factory ammo made at the upper end of the spectrum. Still I believe most modern guns are engineered with enough extra strength to compensate for any pressure differences generated by a minimum spec chamber combined with a maximum spec cartridge.
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Thanks for the relies. I know there are many variables involved and you cant quantify an exact answer.
I suppose mostly im curious as to how ammo manufacturers set the pressure limits to ammo that will have to function in a variety of guns. And by extension, learn more about my own load development.
I've had federal 357 mag that flattened primers where as other did not in the same gun. Also recoil was sharper. They still ejected fine, and I was shooting them out of a NM blackhawk, So I shot them up. On the other end I had an AE winchester 94 with headspace on the max side, and it backed primers out.
~Bazoo
If the gun is within spec. and the ammo is within spec. i dont think you would see any problems. I assume this is how they do it. So a gun with a large bullet jump and big chamber is just not going to preform the same velocity all things equal to another gun with tighter chamber.
The government converted entirely to piezoeletric pressure measurement in the 1980s.Quote:
Originally Posted by gunlover
SAAMI has mostly converted everything in new production to piezoelectric, but existing radial copper equipment still in use for low-volume, low-demand applications can remain in use as long as it still produces results with reference ammunition which comply with the accepted calibration standards.
The powder and bullet companies may still use radial copper test methods until the test barrels wear out.
Test barrels in the lower-pressure handgun calibers like .38 Special last a VERY long time, whereas higher pressure rifle rounds like the .270 Winchester lose calibration due to throat wear in about 3-4,000 rounds.
Outpost75 I understand that part you explained very well. What I am talking about is Larry Gibson mentioned SAAMI and a strain gauge. I couldn't find anything at all about the strain gauge on their website. They only had, and explained, what you just posted. Am I incorrect in assuming that no major pressure testing ballistic labs will publish reloading data for the general public based off strain gauge readings?
no one of any importance in the industry has used a strain gauge for a very very long time.
gunloverQuote:
Originally Posted by gunlover
The "transducer/strain gauge" was listed that way because the strain gauge measurement with an Oehler M43 and M83 are similar to the psi measurements of the transducer. Transducer psi measurements are different than the older CUP psi measurements. The older CUP and newer measurements are now referred to as "CUPs" and the newer transducer/strain gauge measurements are referred to as "psi". It was put that way to avoid confusion.
Perhaps a better Google search of strain gauge measurements, particularly those done with the M43 and M83s, might have shown you that. However, I have posted in the past the comparative test of strain gauges, CUP and transducers to measure chamber pressure conducted by Dr. Oehler. That test confirms the equivalency of strain gauge pressure measurements to transducer pressure measurements.
Perhaps you weren't aware the pressure and velocity development of the Winchester short magnums was done using an Oehler M43?
lefty o
Since most ammunition manufacturers have or are phasing out the use of a CUP test fixture in ammunition load development and quality control I'm wondering if you might be confusing the CUP measurement as "strain gauge" measurement?
FYI; while the transducer in a test fixture is the industry standard (SAAMI membership is severely limited, voluntary and extremely expensive) mostly supplanting the older CUP method the Oehler M83 (a more sophisticated and much more expensive version of the M43) is the industry standard to test production ammunition pressure/velocity in commercial production rifles. SAAMI has no actual control over what any ammunition company actual does, they only issue "guidelines". SAAMI also does not have guidelines for every cartridge produced commercially. If you read SAAMI guidelines carefully you will also see they have several alternative guidelines that ammunition production members may use.....loopholes to the guidelines if you will. The M83 is used with production rifles because it is much faster and easier to use than a transducer system. And besides quality control it also gives the manufacturer a much more realistic picture of the actual pressures and velocities the end user will get. The use of strain gauge measurement of pressure in production firearms, in particular, is why, in the last 20 years, we have seen more ammunition recalls. The use of strain gauge measurement of pressure and velocity is why we also see much more realistic published figures in advertisements and manuals. The M83 is also used by some smaller ammunition producers for actual load development and production quality control.
When I did more reloading I picked up lots of brass at a public range. Most of the centerfire factory cases had flattened primers.
not confusing anything, i know what a strain gauge is. as for more recalls, that has more to do with high volume, lack of QC (mostly in smaller companies), and poorly trained people, not what kind of pressure testing equipment was used.Quote:
Originally Posted by Larry Gibson
Guess you know more about it than the factory techs I talked to.
The fact that Larry talks to factory techs ,begs the question /who"s splaining to who /think Larry was on the splaining end :bigsmyl2:Quote:
Originally Posted by Larry Gibson
guess me working in the factory and having buddies in the ballistics labs isnt worth anything! LOL
I have argued in more than one forum that there is no way to accurately measure chamber pressure mainly because there is no way to calibrate them agianst known primary standards. Can you get a bottle of 50,000 pounds per square inch that is traceable to the National Bureau of Standards? Length, weight, time, chemicals etc are available. Now using length and weight standards one can calibrate lower pressures such as gauges.
lefty oQuote:
Originally Posted by lefty o
I've noticed over the last 30 +/- years most all the recalls are from the larger well known companies. Lack of quality control is certainly one of the major causes but unless it is a visual defect how do they "control" the internal ballistics, especially on large runs? The point to using regular firearms with strain gauge measuring is 10 rounds can easily be pulled "off the line" and quickly tested. Testing can be easily and quickly done periodically during production. Thus if a problem arises they know the whole run isn't bad but only from the last QC check.
Testing a 10 shot string with a C.U.P. test fixture is very slow and tedious. Even with a transducer it is still quite slow. With a test rifle having a strain gauge the 10 shot test can be done in less than 15 minutes with instant feed back.
So what do you do at the factory?Quote:
Originally Posted by lefty o