Question for those who are proficient with QuickLOAD.

[milton]

Just out of curiosity I ran two sets of numbers for 38 special and it appeared that QuickLOAD does not take into account possible velocity differences between jacketed and cast bullets.
Experience has shown that cast bullets have for the most part shot to a higher velocity than jacketed with the same load data,especially in cartridges like the 38 special.
In the calculations I chose a jacketed and cast 158gr. bullet set to the same depth in the case with the same powder charge in each test load.The calculation revealed pressure and velocity numbers very close to each other.
I would guess that pressures would actually be close but would the real velocities be the same?

[earlmck]

I dunno milton -- I used a 7.5" bbl, put in the 158 grain Lee and the 158 grain Hornady XTP with COAL of 1.55" and 5 grains Unique and got minutely less velocity (1007 vs 1003 fps) and a somewhat less pressure (15,700 vs 14,850) with the Lee, not very surprising numbers to me. So QL seems to think you get about the same velocity with less pressure going with the cast bullet. Are you sure this is wrong?

I never load jacketed bullets in the pistols so don't have any real-life test results to look at.

[milton]

Well,it may be right but I have seen chrono numbers that show a higher velocity with cast bullets and a lower velocity with the same load and jacketed bullets.There may be something else to consider because the loads that were measured were not evaluated(seating depth) as one can experimentally do with QuickLOAD.With higher pressure cartridges the difference is less but it is still there in the real world.
I am just thinking out loud and always full of questions and this program lets one play around with the possibilities.

[KWK]

... the loads that were measured were not evaluated (seating depth)...

You'll want to do so. In the somewhat simple math modeling QuickLOAD does of powder burning, peak pressure and fps are influenced by the bullet's weight, it's engraving force, and the seating depth. In my old copy, one could specify a bullet barrel drag (probably in later copies, too), but the effect is usually small and is by default off; if on, it will also affect pressure and speed some.

Powder does not burn in an instant. The bullet moves before the burn completes, and this motion creates volume behind the bullet. This volume must in turn be pressurized by the gases generated from the powder. A lighter bullet will move out quicker, and this lowers the pressure by creating more volume. Lower pressure in turn slows down the burn and thus gas generation. There is, then, a balancing act between pressure and bullet acceleration, and this plays on the powder's burning rate characteristics.

In QL, the burn rate is correctly modeled as varying with both pressure and amount of the powder burned so far. This latter effect is due to kernel geometry and depth of deterrent penetration into the kernel. The combined effect is measured experimentally and stored in burn curves for each powder QL has. Burn rate is far more than just a position in a chart!

How fast all this gets going is determined by the initial net case capacity and the bullet engraving force. In the .38, small changes in seating depth --perhaps the effect you're seeing on the chrono-- make larger percentage changes in the volume available for the gases generated. Less volume under the seated bullet, more pressure created at the start of the burn, thus faster burning and pressure. Combined with this is the bullet engraving force, which QL models as "no motion until a certain pressure is reached." For lead, this start pressure is about half that for a jacketed pistol bullet. Lead bullets, then, move out sooner in the burn and start relieving pressure, which in turn lowers peak pressures and thus acceleration.

QL predicts a lead bullet will develop less pressure than a jacketed bullet, and this is a generally accepted fact. Lower pressure in turn means lower acceleration and thus lower speeds in it's predictions. I don't know how correct this in fact. I think it is true. Yes, lead bullets can safely be driven faster than jacketed in the .38 with a given powder, but load books show you usually have to increase the charge weight to get this. However, since QL's modeling of the initial bullet motion is somewhat crude, there may be an effect it is missing. The author notes the program's accuracy falls off at lower pressures. This may be part of that, although he mentions the burn model is off at lower pressures.

I ran a .38 Spl in QL. For a 158 gn bullet .3" deep, it predicted (for the charge I used) a loss of 2 fps in switching to lead's start pressure, accompanied by a lower peak pressure as well, of course. Looking at it another way: Since fast pistols powders (I used Red Dot in the runs) burn completely in short order, the final fps is primarily a function of the expansion ratio, and this doesn't change in switching from jacketed to lead. Jacketed make higher peak pressures (for a set charge), so they will extract a bit more energy from the charge's gases, but the difference isn't great.

Karl

[milton]

Thank you very much for your reply!If quickload does what you describe it is indeed a very powerful program.Please understand I am not implying that the program is not capable of what you describe.
You present a very understandable description of internal ballistics,a nice read.
I also found your web site very interesting.
As I stated,just thinking out loud.I am very interested in internal ballistics and various powder properties.
Now to digest all this and relate it to chronograph results.
Thank you!

[KWK]

... it is indeed a very powerful program.

It's not really all that sophisticated. Military software is far more detailed, especially with regards to initial bullet motion and engraving. QL's big contribution is the large number of burn tests which had to be instrumented to determine a suitable burning rate curve for all the scores of powders available on the market. Along with that effort, there was the development of a reasonably simple math model of the burn and then correlating all those tests to it. Then there was the drudgery of measuring all the bullet lengths, case capacities, etc. QL is surprisingly accurate, and it's a great tool to learn about internal ballistics. It's accurate enough to be used to estimate starting loads, which is handy for obscure chambers found in old rifles. I highly recommend it. Herr Broemel has done the reloading community a great service.