Pressure vs. Velocity

[wpcexpert]

Another thread got me to thinking. Instead of hijacking another thread here is this one. In regerds to bullet seating depth, how/what effect does pressure have on bullet velocity?

We know that the deeper you seat a bullet can cause a rise in pressure. On the same line, lengthing the COL can lessen pressure. Will that pressure flux equate to velocity differences, and how bad?

The reason I ask is that I usually load my bolt action riffle round to almost or touching the lans(lands?). I read and was told that better accuracy can be had that away so that is where I start. But how much am I losing in velocity by doing that?

[dmazur]

The simplest answer I can give is that the velocity that is developed during the process of firing a bullet is a function of the pressure curve rather than the peak pressure.

Shooters are concerned about peak pressure because of problems like damaging the case (premature case failure), or even damaging the rifle.

The pressure change that occurs is also a function of total case volume. So, bullet setback of 0.020" in a 9mm Parabellum can be a lot more serious than having a bullet seated 0.020" deeper in a .308 Winchester case.

Also, as the bullet approaches the lands, pressure can begin to rise again. The bullet can't accelerate instantly, and peak pressure is higher with the bullet touching the lands than with the typical 0.010" jump.

However, the burn time of the powder and the barrel length is what develops the "push" on the bullet, not the peak pressure affected by seating depth.

In other words, barrel length and powder type are the determining factors.

[brickeyee]

Pressure is what drives the bullet.

The average pressure applied to the bullet in the barrel determines the velocity.

The problem is keeping that average pressure high while making sure the peak pressure stays within limits.

Average pressure gives velocity, peak pressure blows things up.

[wogpotter]

As most modern powders burn progressively (never "explode") there is a correlation that is almost linear over any reasonable loads pressure/velocity relationship. That means you can calculate that x increase in pressure will produce y change in velocity. This holds true till you approach dangerous levels (maximum pressure & so charge weight).
This is in fact one of several warnings, the fact that x increase in pressure will no longer produce y change in velocity, to the reloader that he/she has started hitting the practical maximum for a given load.

Given that, as long as you aren't in dangerously high pressure situations this will be the same for other things like seat depth.

[Unclenick]

For a given charge of powder, increasing peak pressure either by seating the bullet deep enough to take up enough powder space to increase rate of pressure rise, or by seating the bullet out far enough to reduce gas bypass before it obturates the bore, or even by just going to a hotter primer, will result in increased muzzle velocity. This is because the peak pressure increase also increases ballistic efficiency (BE), which is the percent of the chemical energy stored in the powder that is converted to kinetic energy in the bullet by the time its leaves the muzzle.

The above BE increase is caused by a couple of mechanisms. The minor one is just that any influence that raises the peak pressure of a given powder charge weight has done so by raising the rate of combustion, so the gas evolves in a tighter space. The space is tighter because the bullet hasn't moved as far when the peak occurs. In turn, that means the bullet has a fraction of an inch more post-peak barrel length to accelerate in before getting to the muzzle.

By far the more significant mechanism is that because higher pressure peaks earlier in the bullet's trip down the tube, the early rate of acceleration is higher, providing higher early velocity. The action of later post-peak pressure, declining due to expansion, is to further accelerate an already higher early velocity. Thus, even if the powder is fast enough that it all burns inside the barrel in both the higher and lower pressure cases and makes the same total gas volume, the higher peak burn produces greater muzzle velocity and shorter barrel time.

So, how much higher? I've seen 4% velocity increase documented in the .223 just by changing primers (which increased peak pressure). Seating depth effect varies with the size of the case and the bullet diameter. The rate of acceleration a given pressure produces depends on bullet sectional density. So the answers there are chambering and load specific. There is a natural limit of 100% to BE. You seldom see guns achieve 50% BE, though some handgun loads with very fast powders do. You seldom see numbers much below the mid-teens for extreme overbore rifle cartridges. That's more or less the range. Hatcher opined that 30% was optimal for .30-06. So, most chamberings have some wiggle room and can be changed several percent pretty easily.

Seating out near the lands often improves accuracy, but not always. Long secant ogive VLD shapes like to be further back than shorter tangent or eliptical ogive tip shapes. Berger says their VLD's are as much as 0.165" back of the lands at peak accuracy, though this varies with chamber geometry and may be a good bit closer. Some short tangent ogive and eliptical ogive rifle bullets, like round nose bullets, seem to want to be up very close or even touching or jammed into the lands a little.

What works best in your gun with your bullet is something you have to discover by taking a load at least 10% below maximum and starting with the bullet in the lands and seating deeper until you get best accuracy. Steps as small as 0.010" can affect it, though most bullets I've seen are like Berger's in that they have a seating depth band over which they seem to be happiest. Usually around 0.020", IME, though you get a few that are picky to 0.010" and some as loose as 0.030" (Berger's VLD seating depth band number).

Once you have the best seating depth established, work the powder charge up to best accuracy.

Below is a plot for a round nose bullet that is derived from data in the old Frank Brownell 1965 study at Michigan. This is for a round nose bullet and shows peak pressure change with distance from touching the lands. Note that there is a minimum pressure seating depth. Seating deeper and reducing powder space raises pressure. Seating further out and reducing gas bypass around the bullet before it obturates the bore also increases pressure.

[wncchester]

"We know that the deeper you seat a bullet can cause a rise in pressure. On the same line, lengthing the COL can lessen pressure."

We don't quite know that. Seating deeper in small, high intensity cases with very fast buring powders will increase peak pressures but that is largely limited to pistol rounds thst would blow apart at standard rifle pressures. But, as Nick's excellant rifle pressure:seating depth graph well illustrates, seating deeper actually lowers the peak pressure, at least until the bullet gets pushed WAY back to a rediculus degree. Seating out close to lands contact certainly DOES increase rifle pressures tho.

[wpcexpert]

discover by taking a load at least 10% below maximum and starting with the bullet in the lands and seating deeper until you get best accuracy.

So, after I find the best accuracy at the 10% below max, that should/will be the position the bullet likes best no matter what the powder charge?

I guess all in all I'm more worried with accuracy over velocity. The primary cartrige in question is 7mm WSM. The only two bullets I've tried are the Nosler Accubond and the Hornady SST. The Accubond performed a slight better, but neither one is consistently under MOA. Mostly 1.25-1.5 MOA.

I'll start over with the bullet seating depth.

Thank you all for the info.

[Unclenick]

Yes, the best seating depth for any particular bullet seems to be universal. Except for catsneeze loads, most high power rifle cartridges tend to let any particular bullet go and start to move at about the same pressure, regardless of the powder employed. That pressure is what is present during the bullet's jump into the lands. The reason you find seating depth first, then tune the powder charge afterward is shown on that graph I put up. You really don't want to find a best accuracy load that is near maximum pressure, then do something that changes the pressure, as seating depth does. You're better off to use a load on the light side that minimizes barrel and action deflections and vibration while you isolate the seating depth, then work the charge back up for best accuracy.

Once you have seating depth and best charge established, there is nothing to stop you from slightly tweaking the depth (say, a hundredth of an inch either side of the established) just to check that the load dynamics haven't made the exact bullet position a little more particular. You just don't want to make major position changes with an accuracy load as it will change the pressure and barrel time and probably make you tweak the powder charge over again.