Tapered vs straight walled cases

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Tapered vs straight walled cases Empty Tapered vs straight walled cases

Post by inthebeech on 10/14/2019, 7:09 am

Apparently it does not significantly affect accuracy that the 9mm case is tapered?  If the bullet swages out the case in the region that is inside the case, because the bullet is a constant diameter, then whatever the industry specs are for the chamber, they apparently can accommodate this increased need for space but this added space does not impact accuracy?  If there is an influence, then all other things being equal, and assuming the spec chambers of straight walled cases are tighter (because they can be), then a 38 Special/Super or 45 barrel by itself in a barrel test fixture has the potential for greater accuracy?  Is the SAMMI chamber for the 9mm cut tapered or straight?  I see no benefit to cutting it tapered if it is just going to take the shape of the straight-sided bullet?
Anyone actually talk to or read from an actual SAMMI representative/engineer? 
Thanks,
Ed
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Post by DA/SA on 10/14/2019, 7:37 am

The chamber is tapered as well.

I would assume that it is tapered to aid in chambering and ejecting the round.

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Post by WesG on 10/14/2019, 11:03 am

The original Luger chamber was stepped. I'd guess as a manufacturing expedient.

My little bits of playing with a 929 revolver, and powder puff cast bullet loads, has me wondering if the taper inside and out, along with the short straight section at the mouth, makes the case not expand easily and seal the chamber. Theres a lot of crud that appears to be blowing back past the case and collecting on the breech face.

Makes me wonder if this relates to the comments I see about the 9mm needing to be loaded relatively hot for best accuracy. You need the case to expand and seal for consistency.

Pure speculation on my part.

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Post by Bigtrout on 10/14/2019, 11:15 am

When using powder puff loads in my 986 the result is a much dirtier cylinder frame and immediate area aligned with the cylinder gap.  Mid-range and semi-hot loads not so much.
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Post by WesG on 10/14/2019, 12:09 pm

My initial tests with .356 sized bullets resulted in a lot of leading around the breech end of the barrel. It basically filled the muzzle brake as well. The barrel measures 357. I've only done limited testing with 358 bullets. Using somewhere in the neighborhood of 2.5 Titegroup. Took some fiddling with dies to expand them properly (shortened 38 sp expander), and not squeeze them back down in crimping (lightly mod'd 38 super crimp die).

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Post by Wobbley on 10/14/2019, 12:16 pm

The reason the 9mm needs to be hot loaded to seal, is as a result of relatively thick walls. The 38 Special has case wall of around .010. The 9mm is much thicker .012 and tapers very quickly due to the high pressures of the 9mm.
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Post by Slartybartfast on 10/15/2019, 9:55 am

WesG wrote:The original Luger chamber was stepped. I'd guess as a manufacturing expedient.
Walther PPQ chamber is stepped. As it seems are the P99 & PPS, and the H&K VP9.
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Post by inthebeech on 11/23/2019, 2:14 am

Well thanks DA/SA and Wobbley for, together, solving the mystery.

Tapered chamber + Tapered case walls + ID of case is straight.
Bullet diameter is constant.
All is right in the world again.

I think that Wobbley's subtle little statement there about why 9mm loads need to be hot, deserves more than a brief passing.  A good seal is critical to small variation in pressures and this is probably pretty important to accuracy.  Isn't this the answer to the interminable discussions we all see about why the nine can't be accurate at 38/45 velocities?  Well, I think it's a big deal anyway.  Thanks Wobbley.
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