Internal ballistics theory

[mbarasing]

***Disclaimer*** I have never reloaded and am trying to wrap my head around concepts of internal ballistics. Here, I will post my ignorant understanding and hope to learn from your experience; i.e. correct me when wrong.

Mods: if this thread belongs somewhere else please move it, I didn't know where to post this.

I'd like to explore the internal ballistics of straight cased cartridges and the effects therein of powder type, freebore, cartridge length, and seating depth. Examples could be 357 and 38 spc, or 458 lott and 458 WM.

My current understanding: Faster powders are better for straight cased rounds because as the bullet starts moving the area behind the bullet increases in volume more than it would in a necked-down small bore caliber. Longer freebore gives the bullet a running start and a pressure spike will occur once the lands are contacted. No freebore can cause excessive pressures because it takes more pressure to start moving the bullet.

Questions:
1. If a 357 was deep seated to the same COAL as a 38 and charged the same as a 38, would there be difference between the deep seated 38 and the down-loaded 357?
2. Does a lot of freebore and long throats (like weatherbys and the 458 WM) allow the bullet to expand in diameter before squeezing into the rifling and thereby decrease lead fouling OR do the escaping gases during the "jump" cause more lead fouling?

[Wayne Smith]

There are entire books written on internal ballistics, and the mathematics of such are well beyond my limited understanding of that subject, much less the specific application of it. That is to say, specific answers to your questions on the basis of internal ballistics will not be coming from me!

For your first question, define the difference you would be looking at? Pressure, length of the pressure wave, or other?

For your second, very few people are applying lead boolits to the overbore Weatherby cartridges! There are other variables involved in these cases, and looking at a case where lead is commonly used - i.e. the 45-70, and long vs short throats may be a better question. The speed of the powder, ie how fast the boolit starts moving, may be as important as the freebore. Will a very slow powder i.e. 5010, cause as much boolit expansion as a faster one?

I hope you are beginning to see that this is a complex subject with many variables at play all at once.

[mbarasing]

I hope you are beginning to see that this is a complex subject with many variables at play all at once.

Indeed I am and thank you for giving me food for thought. I guess I was hoping for some slow-motion descriptions of what is occurring internally and how different powders can be used to control the internal behavior of ballistics.

In my first question I was just wondering what would change internally: if a 38sp bullet was loaded to 38sp COAL inside of a 357 case and charged the same as a 38sp, would it shoot the same as a 38sp in a 357 chamber? There will be less chamber volume because of the longer brass (not much). Perhaps there would also be less gas escape before engaging the rifling as the bullet would be inside the case longer. Maybe the deep-seated bullet gets a slick, fast running start and them slams into the landings and causes a pressure spike, while the standard 38sp allows pressure to bleed-off during the "jump" to the landing (but then still creates a spike but not as much of one.) And if those theories actually happened (because I'm only guessing), how can one use different powders to correct problems or maximize efficiencies?

In my second question I was wondering about gas escape during the jump as noted in the preceding paragraph.

[Bmi48219]

Can’t answer your internal ballistics questions but if you did load a round using the 38 special bullet, powder charge and overall length in a 357 case and fire it in a 357 chamber you shouldn’t get the ring of built up carbon fouling you get when shooting 38 special in a 357 chamber. That would be a big plus.
If 357 cases are like standard 38 special cases the ID decreases to some extent the deeper you go into the case. Seating to 38 special cartridge overall length may produce a slightly bulged case increasing grip of bullet by the case and requiring more pressure to get bullet moving. Or it may swage bullet diameter slightly or both. Either change would skew the results of your experiment.