Home Forums Hunting & Shooting Ask The Gunwriters Question about velocity/pressure

In a separate post on barrel cleaning, Mr. Charlie Sisk mentioned the above quote. I'm curious what is happening in this scenario, and how it is expressed in pressure testing. Also, sort of percentage changes in velocity and pressure are testers seeing from the first shot from a clean barrel to stabilization?

Assuming the combustion and the flame front is consistent between shots is consistent and thats a big if, the physical velocity at any given point can be measured, (cutting a barrel at different lengths and chronograph). The bullet velocity would be effected by factors that would change less from one test to the next. The pressure at any point in the barrel is usually a log fitting curve based on data points, either by piezoelectric or extrapolation using differential equations based on flame front velocities. These inputs could have wide swings at the first measurements, between each test, but the extremes would "iron out" after repeated firings.

As I recall there was quite a study done using T/Cs when they were popular and there was an ongoing argument whether longer barrels were more accurate. The single shots were used because different barrel length availability.

Here is something for those who don't use pressure testing equipment: After you get a barrel clean, velocity will usually stabilize by 5 firings. It takes 12-15 for pressure to stabilize.
Yes...velocity can be stable and pressure still having some significant swings.
Charlie

If this is the case than when shooting for groups it would be best to shoot 12-15 fouling rounds because pressure causes velocity swings, and after 12-15 fouling rounds the rifle would settle down, or am I missing something?

What I have experienced is this: lets say you are running a cartridge/powder/primer/bullet at an even 60,000 psi and 3000 fps.
A very good load will have less than 600 psi swings and less than 20 fps extreme spread on velocity. That's with a barrel with that has not been cleaned. Then clean it very good, all the carbon and copper removed. Usually velocity will stabilize in 5 or so shots. But it takes about 12 to 15 for pressure to stabilize, usually 2000 psi or so swings.
But....some loads and barrels may take longer. I have experienced some factory barrels (not match grade) that from a clean barrel to a fouled barrel could change 10,000 psi. Depends on the barrel. And how in particular it was fouled.
And here again I will include this part that pisses everyone off: pressure is described in PSI or CUP, not with adjectives or adverbs.
Don't think that you can measure pressure with eyesight, it wont work. And looking at primers (even if its blown), doesn't absolutely mean anything. Or measuring case heads. And sometimes just because the bolt is hard to lift, because sometimes other things can cause that. Like a front action screw that's too long and hitting the bolt lug.
Now to be fair, I have done very little testing with factory barrels. And that experience has shown me that factory barrels are not as good as match grade barrels. Its that "kinda get what you pay for" thing.
And this: I don't have much experience with cleaning factory barrels. I usually am cleaning match grade barrels with properly cut chambers. That's a whole different ballgame.
Charlie

I think Mr. Sisk was referring to peak pressure. To get the same velocity the average pressure would need to be nearly the same assuming the average drag on/friction between the bullet and the barrel was close.
I’m pretty sure that Mr Sisk is more knowledgable on this subject than me, especially the friction on clean vs fouled barrel.
But acceleration is directly related to pressure pushing minus the drag. With area and mass being part of the equation.

Edit: Oops, I see that Charlie answered before I got my two cents in.

Some loads can have 5000 psi swings and less than 10 fps spreads. When you look at the pressure curves, you will see the area under the curve is the same, just shaped differently.

I truly wish several here on the Fire would purchase some pressure testing equipment. And spend a fair amount of time and money learning how to use it. And I promise, it will be more time and money than you think. One of the most eye opening things I learned first, was that the electric grid in the US has far too swings to be able to use for this type testing. At least in Kentucky and Texas.
And test a lot of different cartridges and powders and bullets. Then, start trying to test primers. That's a real head scratcher.
I used to think I knew a lot about handloading and such until I went down this path.
Charlie

Mr Sisk,
For a given bore diameter do smaller cases yield more consistent pressures or velocities?

Are there specific BG cartridges that you like? I use the 270W a lot and will welcome your comments.

Do consistent pressure or velocity or both predict group sizes?

Do specific powders produce predictable results?

Thank you
R

RinB
I'll need to answer this in pieces:
Usually, smaller cases are better. The PPC line, the BR's and those usually run the most consistent . 221 Fireball runs like that too. The larger cases not so much so. Seems the longer the case, and the more over bored it is, the more difficult to get pressure and velocity you want. But.....and there is always a "but" in this....I have tested the 300 Win mag with WLRM, Magnum, 180 grain Bergers. You can get pressure swings less than 300 psi, and thats about as accurate as the equipment is.
7.62x39 with WLR and Exterminator and match bullets does very well to. Maybe a little better than the PPC cartridges, its hard to tell. But these are in the same family so its to be expected.
On the other end, I built a 257 STW for a customer. That had such big swings I wouldn't let him have it, gave him his money back. For me it was too much liability.

Consistent pressure and velocity is certainly a step in the right direction. But..(damm these buts), it aint a guarantee. But usually if you have both, small changes in seating depth will produce good accuracy.

I bet the 300 Savage would check out as pretty consistent. Short powder column, 30 degree shoulder, modest powder capacity relative to bore size.

About powders...thats a hard one.
Usually, faster powders are used in smaller cases, PPC, BR,etc. Single base extruded powders (IMR in particular) are easy to ignite compared to a double base ball powder. So most of the time, if a cartridge will show good pressure curves, it will with a single base extruded powder. Right up until it doesn't. I have never been able to get any powder in 300 Win mag to work as well as Ramshot Magnum. But a kinda rule of thumb, a faster powder will show a better curve. Most of the time. Usually. Until it doesn't.

And something else that wasn't in your post: BULLETS
Some cockeyed bullets will really throw $h!t in the soup. Hunting bullets wont ever give curves as good as match bullets. Unless you get some junk match bullets, and that happens. But..(another one of these damm buts)...I used my Juenke machine to test bullets when I was pressure testing. If you test and get a batch of say Partitions that were really good, compared to some Berger or Sierra or Hornady that were not, the Partitions would produce a more consistent curve. But usually , the match bullets were better.

Thank you, Charlie. This now makes sense to me. Internal ballistics can be different and muzzle velocity relatively consistent. Friction would be one factor. Maybe, tool marks in throats/barrels and the bullet inconsistencies you mentioned. Alot of mysterious things going on in there. Jordan chasing upswing nodes in barrel movement with seating depth kind of blew my mind. I just wasn't thinking in the super slo-mo of interior ballistics regarding pressure curves while the bullet is still in the barrel. But, that one sentence turned on the light bulb. Thanks.
Andy

Charlie, what equipment are you using to generate pressure curves?

Pressure Trace

Yes.....then there is neck tension.....a huge variable....I learned the hard way.

Yeah, both of us did!

I spent considerable time "experimenting" with Charlie and his Pressure Trace when he was in Texas. Was also lucky enough to spend some time with Western Powders' piezo setup in Miles City--and have visited other pressure labs of bullet companies and one major American ammunition company.

They all provided lots of information, including things not described in manuals.....

Right, makes total sense. I think, the big thing standing in the way of my thinking about this is that MV is one measurable number where pressure is a curve over the time with an infinite amount of numbers along the curve and different shaped curves based on all kinds of factors. And, a curve with a spike might wind up with the same MV at the end of the curve as a differently shaped curve with no spike. I had it in my simple head that pressure is one number like MV. Super interesting.