Inquiry: Bullet Setback Increases Case Pressure

[Steve C]

There are few universal truths in reloading and bullet set back MAY cause an increase in pressure, even a significant amount but it depends upon several factors like the powder being used, the load density, the case size, the bullet and crimp.

Some argue that reduction in volume due to setback must increase pressures, however ammunition pressure is a dynamic of many factors such as powder burn rate. The volume is not a simple constant since the bullet is moving out of the case and into the barrel while the powder is still generating combustion gas, thus the volume the gas expands into to create pressure is increasing over time.

Many powders function perfectly well as a compressed load and the amount of compression doesn't seem to make any significant differece in pressure or velocity.

I've personally shot many pistol rounds that had the bullet set back from being chambered many times and have never observed any high pressure signs in the fired cases. For commercial ammo the main issue of bullet set back is that it can reduce the feed reliability of the ammo, additional pressure is not a real problem that presents itself.

 

[SuperNaut]

You doin' the testin' SuperNaut? Please do...Let us know what you find out...

Heck yeah! This sounds like a cool experiment, I just need to track down a pistol for cheap.

 

[1858rem]

hipoint

 

[Riss]

Simply look at 38 and 357 load data. The 38 is a shorter case and for the same bullet weight at the 357 the charges listed are NOT the same. Going to a 40 S&W and a KB is not that difficult. The 40 is a much higher pressure case, than say a 45 acp, and the 40 case was not designed for the pressure. Yes, to answer the question directly, bullet set back is bad.

 

[SuperNaut]

Yeah 1858rem, I'm thinking Hi-Point too.

 

[Ol` Joe]

You guys are looking for answers to questions that have already been answered, and experiments that have been done.

VihtaVuori in their reloading manual has a small warning on overly deep seating bullets in small cartridges. They show a load they tried in the lab useing 3N37 in the 9MM where a bullet seated at the book depth gave 230 MPa or 33,358 psi. They seated it 2MM (0.078") deeper and the pressures jumped to 310 MPa or 44,961 psi. This is a increase of over 12, 000 psi or about a 30% increase in pressure.
The test gun likely didn`t blow up but the possibility of real damage to one of ours from a deep seated bullet is very real from what I got from this.


BTW the velocities they recorded during this rose from 1130 to 1180 fps. Most of that would be hidden in extreem spread if we tried to find it without a large sample to work with. Pressure increased by 30% and velocity only 0.9%.

 

[1858rem]

so the setback increases pressure substantially but hardly increases velocity at all, how about felt recoil?


makes me wonder about .308 rifle loads, not setback, but seating OUT closer to the lands, would seating out .05"-.1"(if attainable in your chamber) farther towards the lands (while still being at least .015 off the lands) reduce pressure while loosing little to no velocity, all the while gaining accuracy?

 

[bayboy]

There are several well established and validated interior ballistic models that can address this question. It seems that the sensitivity to changes in seating depth is somewhat cartridge dependent. I suspect that the propellant loading density plays a part. Here are some results (sorry that the columns don't line up right):


9mm, 124gr, 6.0gr Unique powder
Seating Pressure Velocity
Depth (ksi) (ft/s)
-.15" 114.5 1289.
-.10" 52.9 1212.
-.05" 36.9 1172.
Nom. 29.5 1147.
+.05" 25.4 1129.



38 Spec., 125gr JSP, 5.3gr Unique Powder
Seating Pressure Velocity
Depth (ksi) (ft/s)
-.15" 18.0 1040.
-.10" 17.5 1030.
-.05" 17.0 1021.
Nom. 16.5 1012.
+.05" 16.1 1073.

 

[NoName0815]

I think what you´re looking for is this:

http://le.atk.com/pdf/357_SIG_Setback_Length-vs-Pressure.pdf

Also Broemels´Quick Load can give you a very realistic idea, what happens, when bullet is set back too much.

And last but not least: Even in very old Speer Relaoding Manuals it was well documented, that bullet Setback can cause pressure-doubling in some cases.
9mm Luger for Example is a candidate with very small room for errors....

NoName0815

 

[rcmodel]

farther towards the lands (while still being at least .015 off the lands) reduce pressure while loosing little to no velocity,

No.
It is likely the pressure reduction from seating longer (increased case capacity) would be more then off-set by the reduction in free-bore length. (Distance the bullet has to get moving before encountering the rifling friction).

rc