Bullet Setback Overpressure Calculation

[Hartkopf]

Does anyone have a program to calculate the pressure increase of a hypothetical bullet setback?

I’m curious about a 45ACP with 185gr XTP, 7 grains Longshot and OAL of 1.230.

Then the pressure increase with the same load .100 shorter at 1.130.

I'm looking for examples a kaboom worthy setbacks. There's lots of free space in a 45ACP so the bullet could sink substantially in the case before contacting powder. I've read that slower powders increase pressure more than fast powders for any given reduction in COAL so I chose Longshot as an example.

 

[mcb]

QuickLoads or GRT should be able to do that calculation reasonable accurately. I will try tonight with QuickLoads after work assuming one of the retired members does not beat me to it.

 

[film495]

figure out the % volume decrease from the setback, and add that percentage to the pressure. my guess is this question would be very powder specific though, one powder might go kaboom, and another might not burn correctly in that condition and just leave a bunch of unburned powder. per powder I bet the manufacturors have done some version of research that answers this question, but if that is available? no idea.

 

[MEHavey]

45ACP/185 XTP/Longshot/7.0gr/OAL= . . .

1.230" - 12,154psi
1.130" - 17,965 psi

 

[Hartkopf]

45ACP/185 XTP/Longshot/7.0gr/OAL= . . .

1.230" - 12,154psi
1.130" - 17,965 psi

Thanks!. That’s a substantial increase. I guess my hypothetical load was too far from max to matter. From data for a different bullet I should have started closer to 9.3 grains of Longshot.

 

[N555]

Longshot is pretty forgiving.
You loading up some 45acp thunder rounds?

 

[Hartkopf]

Longshot is pretty forgiving.
You loading up some 45acp thunder rounds?

According to a THR member a while back, Longshot is not forgiving when it comes to seating depth. It actually increases pressure much more than titegroup.


I’m just trying to put a real number to the “bullet set back blew up my gun” issue. Looks like my hypothetical is almost 6k psi.

 

[Double_J]

I remember looking at a chart showing the pressure curve based on charge size and seating depth. It was NOT a linear curve, i.e. it does not go from one to two to three. Rather it goes frome one to three to eleven to kaboom very quickly. Do not play with setback without a considerable safety margin in terms of powder (minimum starting charge).

 

[Hartkopf]

I remember looking at a chart showing the pressure curve based on charge size and seating depth. It was NOT a linear curve, i.e. it does not go from one to two to three. Rather it goes frome one to three to eleven to kaboom very quickly. Do not play with setback without a considerable safety margin in terms of powder (minimum starting charge).

I gathered it was a steep curve. I’ve read about certain cartridges being more sensitive than other’s. Like in certain combinations, the 40S&W doubling the pressure after a setback.

 

[Leon tP]

...
I’m curious about a 45ACP with 185gr XTP, 7 grains Longshot and OAL of 1.230.

Then the pressure increase with the same load .100 shorter at 1.130.

I'm looking for examples a kaboom worthy setbacks....

According to my Hodgdon Reloading Annual Manual using a 185 grain HDY (Hornady) JSWC .451 Dia. Col: 1.135" and 7.2 gr Longshot @ 919 fps results in 11,300 CUP. Using 8.2 gr at the same depth results in 1044 fps and 17,000 CUP.

Using your 7.0 gr of Longshot propelling a 185 grain XTP at an AOL of 1.230" seems like it might be on the mild side.

Someone with QL should be around shortly.

My major concern with setback isn't the whole gun blowing up, it's the unsupported area of the chamber that can result in a case head separation.

Aside from an accidental double charge while reloading, if you don't have a fully supported barrel then a case head separation seems a more likely scenario with setback or using mystery high pressure ammo.

Jmho

 

[mcb]

45ACP/185 XTP/Longshot/7.0gr/OAL= . . .

1.230" - 12,154psi
1.130" - 17,965 psi

Thanks for beating me to the data.

For everyone reading this thread. I realize that even with the set back condition in this specific case it is still safe in the intended cartridge/gun but look at the percent change in pressure for no change in components. That is nearly a 50% increase in pressure for just .1 inch setback. If the starting load was near max pressure for the cartridge a 50% increase puts it at the high end of proof pressures. Every powder is unique in its properties but OAL changes will always make a difference to pressure, sometimes small, sometimes large. A cartridge shorter than the original almost always results in increased pressure. As with all of our reloading OAL is one more thing we can ignore at our own peril.

 

[AJC1]

Your using slow powders in 45 acp. Check the numbers with a powder like competition or Clays. Powders like Longshot will likely be compression limited before a boom. Tightgroup will give you the powder density with setback your looking for.

 

[Palladan44]

Longshot is pretty forgiving.
You loading up some 45acp thunder rounds?

It's also slightly cleaner, meters better, has less flash than ...well, Bluedot.
Just as loud though Double up on your ears.

 

[CQB45ACP]

Thanks for beating me to the data.

For everyone reading this thread. I realize that even with the set back condition in this specific case it is still safe in the intended cartridge/gun but look at the percent change in pressure for no change in components. That is nearly a 50% increase in pressure for just .1 inch setback. If the starting load was near max pressure for the cartridge a 50% increase puts it at the high end of proof pressures. Every powder is unique in its properties but OAL changes will always make a difference to pressure, sometimes small, sometimes large. A cartridge shorter than the original almost always results in increased pressure. As with all of our reloading OAL is one more thing we can ignore at our own peril.

Lee’s second edition has the better part of 2 1/2 chapters on pressure including the discussion here. It’s a good read if one wants more.

 

[Hartkopf]

Your using slow powders in 45 acp. Check the numbers with a powder like competition or Clays. Powders like Longshot will likely be compression limited before a boom. Tightgroup will give you the powder density with setback your looking for.

That is true. Longshot might allow for .150 setback (guessing)but Titegroup might allow for .400 setback(guessing). I picked Longshot since it is more sensitive to a given depth reduction but at least it has the built in safety margin of bulk.

 

[Hartkopf]

Thanks for beating me to the data.

For everyone reading this thread. I realize that even with the set back condition in this specific case it is still safe in the intended cartridge/gun but look at the percent change in pressure for no change in components. That is nearly a 50% increase in pressure for just .1 inch setback. If the starting load was near max pressure for the cartridge a 50% increase puts it at the high end of proof pressures. Every powder is unique in its properties but OAL changes will always make a difference to pressure, sometimes small, sometimes large. A cartridge shorter than the original almost always results in increased pressure. As with all of our reloading OAL is one more thing we can ignore at our own peril.

If any of yall get the chance to re run the numbers with 9.3 grains of Longshot we could see how far and above 21,000psi a 45ACP would go with a .100 setback.
(I'm guessing at the 9.3 grains as a solid max load. I haven't actually found data for this hypothetical)

 

[GeoDudeFlorida]

Thanks for beating me to the data.

For everyone reading this thread. I realize that even with the set back condition in this specific case it is still safe in the intended cartridge/gun but look at the percent change in pressure for no change in components. That is nearly a 50% increase in pressure for just .1 inch setback. If the starting load was near max pressure for the cartridge a 50% increase puts it at the high end of proof pressures. Every powder is unique in its properties but OAL changes will always make a difference to pressure, sometimes small, sometimes large. A cartridge shorter than the original almost always results in increased pressure. As with all of our reloading OAL is one more thing we can ignore at our own peril.

And, it can’t be stated enough times that OAL - cartridge overall length, from bullet tip to rim face - is just a convenient approximation of seating depth - from bullet base to powder cup bottom. What we hope to get a sense of is the powder column volume and the expansion chamber volume.

A word of caution here: this is an area of ballistics where “precision” becomes a potentially dangerous illusion. These pressure changes are not linear; nor are they predictable in any meaningful way using regressive methods.

Be careful and avoid setbacks. If you suspect such a thing can happen, change your methods to avoid it.

 

[AJC1]

It's also slightly cleaner, meters better, has less flash than ...well, Bluedot.
Just as loud though Double up on your ears.

When you say loud, the only powder I've used that was just loud was power pistol. Is Longshot kinda like that? I don't find blue dot blasty...

 

[MEHavey]

True OAL comparisons apply only to the same bullet/same case.
(The real issue is comparative seating depth against available powder case volume)

The lesson is that details matter when approaching max, and a chronograph is your friend.

 

[CQB45ACP]

And, it can’t be stated enough times that OAL - cartridge overall length, from bullet tip to rim face - is just a convenient approximation of seating depth - from bullet base to powder cup bottom. What we hope to get a sense of is the powder column volume and the expansion chamber volume.

A word of caution here: this is an area of ballistics where “precision” becomes a potentially dangerous illusion. These pressure changes are not linear; nor are they predictable in any meaningful way using regressive methods.

Be careful and avoid setbacks. If you suspect such a thing can happen, change your methods to avoid it.

Guess this is why there’s no table or chart showing simple combinations and permutations. To many variables?

 

[Palladan44]

When you say loud, the only powder I've used that was just loud was power pistol. Is Longshot kinda like that? I don't find blue dot blasty...

I'm thinking in handguns, muzzle blast is simply just a product of velocity more than anything.
Power pistol is just about the best for velocity in standard service pistols. In say 10mm, Power Pistol definitely gets eclipsed by BD or LS in which case theyres definitely more noise, recoil, etc.

 

[fxvr5]

Here is a OAL & Pressure chart from an old Ramshot manual (see the link below). It shows a linear relationship between OAL and pressure using Zip in the 9mm and 40 S&W.

http://www.castpics.net/LoadData/Freebies/RM/Ramshot/Ramshot_3.pdf

 

[CQB45ACP]

Here is a OAL & Pressure chart from an old Ramshot manual (see the link below). It shows a linear relationship between OAL and pressure using Zip in the 9mm and 40 S&W.


View attachment 1187547

http://www.castpics.net/LoadData/Freebies/RM/Ramshot/Ramshot_3.pdf

Well I’ll be.

So wouldn’t it be interesting if there were a similar graph plotting the pressure changes resulting from each .x gr of powder increase?

Then you could overlay the two to identify how to maintain (or change) a certain pressure by adjusting different things—up two grains of powder equals the same increase as a reduction in one hundredth of an inch, for example.

 

[film495]

Well I’ll be.

So wouldn’t it be interesting if there were a similar graph plotting the pressure changes resulting from each .x gr of powder increase?

Then you could overlay the two to identify how to maintain (or change) a certain pressure by adjusting different things—up two grains of powder equals the same increase as a reduction in one hundredth of an inch, for example.

this would be an interesting way to test OAL to accuracy, if the pressure was maintaned the same.

 

[rhino465]

I remember looking at a chart showing the pressure curve based on charge size and seating depth. It was NOT a linear curve, i.e. it does not go from one to two to three. Rather it goes frome one to three to eleven to kaboom very quickly. Do not play with setback without a considerable safety margin in terms of powder (minimum starting charge).

This is good advice. I'm not in the ammunition business, but my area of research in grad school was combustion. There are too many variables to accurately predict pressure changes based solely on an estimate of the change in the volume of the region in which the powder burns and expands. It's not linear as you mentioned and I would be highly skeptical of any graphs showing a linear relationship between change in case volume and maximum pressure. Even if you developed a pressure vs. volume curve for a powder in a specifically shaped chamber, it's unlikely to remain the same for different lots of the same powder or for the same powder in a differently shaped chamber/case.