9mm vs. 38 super comp case capacity

[Garmil]

Does anyone know if there is much or any difference between the two with the same bullet and oal?

Been playing around with shooting 9mm loaded extra long in my super.

It's working perfectly fine but the interesting thing as I develop the load it's actually behind the supercomp velocity wise with the same bullet and powder charge with a shorter oal.

[al503]

I know that 38 super has more case capacity compared to Supercomp but I'm not sure about 38 Supercomp v. 9mm, if loaded to the same OAL. I would think it would be pretty close, though.

Edited February 28, 2016 by al503

[dansedgli]

There would be more room for gases to escape shooting 9mm in a 38 super chamber. Interested to see the chrono results when you make major with 9mm in the super barrel.

[warpspeed]

I'd recommend not doing that much. It can put way too much stress on the extractor.

Edited February 28, 2016 by warpspeed

[Garmil]

There would be more room for gases to escape shooting 9mm in a 38 super chamber. Interested to see the chrono results when you make major with 9mm in the super barrel.

Yeah I'll update when I get it all figured out.

I'd recommend not doing that much. It can put way too much stress on the extractor.

Whys that?

[kneelingatlas]

9mm is wider at the base (0.007") which could account for a little more case volume/less velocity, additional the neck is a little smaller (0.004") I could imagine slightly more energy is used fire forming the case.

There is a short time between when the primer explodes, pushing the case forward in the chamber and when the charge explodes slamming the case back into the breech where the extractor is holding the case back, but as long as the time is nearly identically sized like 38SC and 9mm I can't imagine one stressing the extractor more than the other. The only time I've seen this cause an issue is when I shot a .380 case in a 9mm: the smaller rim ripped past the extractor when the primer blew, then slammed back, flattening the primer to a size larger than the primer pocket.

[Garmil]

Makes sense that width seems like a very small difference. Maybe the oal drop just makes less of a difference than I was expecting.

I think the same about the extractor will be doing the same thing. If the chamber/brass relationship was so tight in the super that it wasn't being held by the extractor that seems like a reliability problem.

[Garmil]

Did some more testing today. Definitely something going on to cause a big difference.

Super ....... 9mm

124 jhp ..... 124jhp

1.245 ....... 1.20

7.2 ac ...... 7.2 ac

169 pf ...... 159 pf

Edited February 28, 2016 by Garmil

[al503]

Did some more testing today. Definitely something going on to cause a big difference.

Super ....... 9mm

124 jhp ..... 124jhp

1.245 ....... 1.20

7.2 ac ...... 7.2 ac

169 pf ...... 159 pf

Just a wild guess but since the chamber is longer than the 9mm case, perhaps more gas is blowing by the bullet until it makes the jump to the rifling?

Edited February 28, 2016 by al503

[Bamboo]

P1V1 = P2V2

If you are shooting 9mm through a 38super chambered gun then as soon as the bullet clears the case mouth the volume of the pressure containment increases, decreasing the pressure. The 38 super containment volume remains smaller longer, and when the bullet does leave the case mouth the change in volume is less so the decrease in pressure is less. There also may be some gas escape, but the bullet is probably in front of the pressure wave.

Just spit balling here.

[Garmil]

Any potential issues with the gas escaping? Damage to chamber?

[gaijin]

I know that 38 super has more case capacity compared to Supercomp but I'm not sure about 38 Supercomp v. 9mm, if loaded to the same OAL. I would think it would be pretty close, though.

.38 Super and .38 Super Comp are identical- other than Super Comp is rimless.

​The internal case volume is the same, as is the exterior dimension-sans the "semi rim" (.38 Super).

[al503]

I know that 38 super has more case capacity compared to Supercomp but I'm not sure about 38 Supercomp v. 9mm, if loaded to the same OAL. I would think it would be pretty close, though.

.38 Super and .38 Super Comp are identical- other than Super Comp is rimless.

​The internal case volume is the same, as is the exterior dimension-sans the "semi rim" (.38 Super).

When I started shooting open division, my smith provided some load data for 38 Super and specifically mentioned that I should start about .2 grains less since my pistol was set up for Supercomp. I loaded up both 38 Super and Supercomp for the same pistol just to find out if it would shoot both reliably. The rounds in the Super case were a couple PF down compared to the Supercomp rounds with the same charge, bullet, and OAL, crimp, etc.

Here's an excerpt from Dave Re's blog http://re-gun.com/2011/05/starline-38-supercomp-brass/:

'Relative to regular .38 Super +P, Supercomp had a stronger and thicker case web, a rimless head design, and a 9×19-like extractor groove. This meant that it took less powder to get the same results (due to smaller case capacity), and better feeding properties than .38 Super...'

I'm not an expert so I can't say who's right. All I can say is that I believe Supercomp brass has less capacity based on past advice and chrono results that back it up (at least for me).