Home Forums Hunting & Shooting Ask The Gunwriters Question about pressure signs when working up a new load?

Steelhead got to the point in an economy of words and is spot on

With some rifles/cartridges you'll notice a trend. As you go up in powder (say .4gr) you'll see the velocity climb. It won't be linear, but you'll see say something in the vicinity of 17~25fps. All of a sudden you'll see the velicity climb by a low number, or even loose velocity.

Here's your sign...

In addition ... this point appears to be about 1.5-2gr (with large 308sized cases ... not a 22Hornet) of powder below where you'll notice sticky extraction on opening the bolt.
Cheers...
Con

I agree 100% with your posts John. I also agree that a chronograph is nothing more than another aid, its not the "where with all" or the "2nd coming", but it is an indicator. My post was a guide only based on how we did it back when there was no other way, but these steps worked even then if you were flexible and used the prime component of handloading and that is a 1000 grs. of common since..Any one componient by itself tells you little but when you start putting them together, it starts to make since.

BTW whats a push feed?

I do load for some lever guns you know, I just praised you in another thread for your contribution to my useing Varget, and your advise to me turned my 99F, .308 into a world class bench rest rifle! May Allah raise you amongst the exhaulted for that, and may the good Lord take a lik'en to you.:)

If your chamber is the least bit oily, you will have scuffed case heads and flattened primers. I use a 12ga shotgun patch with lighter fluid on it to clean the chamber, removing any oil or bore solvent that could be in there.

Another indication is the bolt hard to lift? Do you have to full length size the brass after each firing?

Brass will vary in hardness by up to 12% in the same lot, some softer, some harder.

Another issue, Weatherby brass is SOFT!

While you are getting pressure signs in soft brass, brass made from 264 Winchester (winchester brand may not give you pressure signs at all).

Good luck!

Keith,

Actually, I have had pretty good results from Weatherby brass in recent years. Just bought yet another .270 Weatherby this spring, along with new brass, and am getting 3400 fps with 130 E-Tips without the slightest sign of softness.

Have also used quite a bit of .240, .257 and .300 Weatherby brass in the past 3-4 years with no problems getting top velocities.

Ditto, I've had no issues with 'soft' Bee brass in the 257 and 300.

It's soft...I had it Rockwell'd along with Fed, Win, and Rem.

Federal is neck and neck with the bee brass.

Keith,

So what?

If Weatherby brass doesn't show expanded primer pockets (or any other sign of "excessive pressure") when shot at maximum/velocity/pressure loads, what difference does it make if it's softer than X brand brass?

Actually the case comes back with the same force with an oiled
chamber. What happens is that it comes back at a lower pressure
then stretches forward. This avoids the "stress ring".
The old timers used an oiled case as a proof because otherwise
with low pressure cartridges the locking mechanisms wern't being
tested. That is why Ackley could fire a 94 winchester without a
locking bolt. It wasn't the improved case that allowed it.
Why an oiled case would cause a flatened primer is beyond me,
I've never seen it, anyone else seen such a thing?.
Good luck!

i've read about some guys getting pressure signs up to and including blown primers from wet ammo (such as rain on exterior of cartridge). i've shot wet ammo less issue before, so i was curious and emailed Hornady, Federal, and two guys posting on the boards that work in ballistics labs asking why. the replies just acknowleged the issure and cautioned against using wet ammo, but i did not get a satisfactory explanation as to why.

Oil or grease or water if thick enough can be pushed around the neck and prevent the neck from expanding. A real "maybe".
Good Luck!

toad,

I've asked a couple of ballistic techs about the wet ammo thing myself. Basically what they said was that water on the bullet can end up inside the barrel, which creates excessive pressure due to extra friction--much like we sometimes feel when driving a pickup into a puddle.

Water can end up in the bore without ammo being wet, of course, but the problem is particularly acute when there's water right in front of the chamber, where pressure peaks. Water further down the bore doesn't affect pressure as much, both because pressures are lower and because some of it may be blown out of the muzzle by the air being pushed by the bullet.

I too believe this to be true...

In my 338WM the vel I get for the middle of the road Nolser 200g loads are much faster than the book, near Max... so I believe the pressure is Max too

Velocity ain't everything. If you get hard bolt lift, ejector marks, CHE, blown primers (black rings), OR velocity in excess of maximums listed in manuals where labratory pressure testing was used during development, it is time to back off. Velocity indicated by a chronograph alone is not a valid reason to keep adding powder if the other pressure signs are present.

The only objective measurement is a pressure transducer bonded to a test barrel and associated software to utilize it. Chronograph interpolation is only an approximation, and can exceed maximum pressure without warning.

Otherwise, just stick to published data, follow the recipe exactly - especially primers used for any given load. Do not reduce charges of slow powders below the recommended, or exceed the maximum listed. Do not swap flat based bullets of a given weight for loads published for boat tail bullets. In other words, follow all instructions in any CURRENT manual.

Do not rely on data published in obsolete manuals, powders have changed over the years, even those of the same name. You should find cartridge data on Hodgdon's (IMR-Winchester) web site, or the various powder manufacturer's web sites, or at www.loaddata.com, otherwise use extreme caution.

If more velocity is required, move to a larger cartridge, don't try to horse a few more fps out of a given cartridge. In my 50 years of reloading I have never blown a case or primer (I have had primers pin hole due to weak cups) stuck a case or seized a bolt. Accidents happen too fast to get out of the way, you'll be blinded, lacerated and bleeding before the pain reaches your brain. As the guy in the welding tank supply said, "If one of those oxygen cylinders blows, you'll never know it happened." Be careful with high pressure gas, especially when it's a few inches in front of your face.

WranglerJohn,

Actually, chronograph data is a VERY good indication of pressure. Like a strain gauge or a piezo transducer, it IS another measure of pressure.

I have visited professional pressure labs, and even helped in one. It is astonishing how closely pressure and velocity are tied together, the reason many professional ballisticians also suggest that a chronograph is the best affordable home test of pressure.

Now, you can run into trouble with a chronograph if it's giving false readings, and some do, especially under certain conditions such as bright sunlight. Which is why we should always remain cautious--and as Big Redhead pointed out, we still have to be observant of traditional signs of excessive pressure.

But I have been using a chronograph as my primary method of pressure measurement for several decades now. While I've run into signs of high pressure now and then, most were also accompanied by too-high velocities.

The exceptions that I can remember were rare instances of too-soft brass, and a few bolt-faces that had little machining ridges around the ejector hole that left a mark on the case heads even with mild loads.

As the last pressure-lab ballistician I visited (just a few months ago at the Western Powders lab) said during our afternoon: "The chronograph is the handloader's best friend!"

I've wrestled with the idea of using a chronograph to indicate pressure, and think the following is correct:

For a given bullet and powder combination, in a given rifle, peak pressure and MV are highly correlated most of the time. As long as you're in this situation, MV is an excellent indicator of relative pressure.

The problem is that for reasons I don't yet understand, some cartridges are finicky. By finicky, I mean that straightforward orderly progression of MV and pressure are not to be depended on. 7mm Mag, 243, and 338 Lapua have this reputation. Some cartridge and bullet combinations hit a point where additional powder does not add MV, but apparently does add pressure. Although I didn't measure pressure, I did find a point with my 308 and 2520 powder that more powder did not increase the MV.

So I would say that if you are working up a "book" load for a known combination of powder and bullet, and are observing orderly increases in MV as you increase powder, you're in the "well behaved" region and can use MV measurements as an indicator of pressure.

It also increases bolt thrust. Oil, resizing lube, residual cleaning solvent in the chamber, or even water prevents the brass case from "sticking" against the microscopic (and sometimes not so microscopic) tooling marks inside the chamber left over from the chambering reamer. The case is confined in the chamber at peak pressure as if it were a brake shoe expanded against a brake drum. This reduces bolt thrust and locking lug loading during the firing cycle.

As a test Ackley removed the locking bar from a Model 94 and fired it (remotely), the bolt did not fly out or the case burst because it was held in place by case "stiction."

Fire forming an improved cartridge with too light a load will cause primers to back out appearing as if there was excessive head space. Because the case swells to fill the chamber, the primer backs out as the case shortens, and there is insufficient residual pressure to force the case backward to pick up the primer. All caused by the relationship between case stiction and pressure.

Yes, I use a chronograph during every load development session. However, during my career in the water and wastewater treatment field, I installed and operated various electronic and mechanical pressure, flow, temperature, turbidity and analytical instruments. Each different type of device has a range of accuracy related to percent of scale. Many accuracy standards range between 2% to 10% of full scale value, 2% across middle half of scale, etc. Operational instruments such as bourdon tube pressure gauges are less accurate than precision calibrated electronic transducer gauges, but are sufficient for the purpose. Online real time analytical instruments are usually less accurate than laboratory instruments calibrated at time of use.

In the case of chronograph data, there are too many variables to use for other than determining trends in pressure based on velocity. While there is a interrelationship between charge density increase, percent of velocity gain and pressure, the accuracy of all instruments involved determines the result, as you point out.

My main concern is that to make the test requires discharge of a cartridge that may or may not go to failure. If this test is conducted by live firing, it exposes the shooter to a hazardous condition.

While the advanced loader can find the chronograph a useful instrument in recognizing velocity and pressure trends relative to increases in charge density, he or she is still not measuring actual pressure or pressure/time curves, but determining the point where additional powder is not producing velocity gain. Regardless of the velocity he or she is able to say that load is near maximum based on efficiency, but not the actual pressure.

So I believe the novice or disinterested loader is better served by using tested data from reliable sources.