Inquiry: Bullet Setback Increases Case Pressure

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SuperNaut

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Part two in my ongoing series of gun myths, actually just me wondering aloud (in print?).

So I've read it and heard it everywhere: Be careful with rounds that have been (re)chambered but unfired, the bullet setback causes the case pressure to rise to dangerous levels. Usually told in this fashion: "My cousin's nephew's uncle's grandad had a KB due to bullet setback, he almost lost an eye." In researching this myth I found many of the same anecdotes but no documented cases of bullet setback causing a gun to KB, let alone an injury. Anecdotally I've personally fired many cartridges that have been setback to various degrees with no noticeable effect. But I admittedly don't know how to measure case pressures, so I don't really know.

What say you?
 
Increased pressure from bullet setback is NOT a myth, with enough setback in a case full of powder that reaches peak pressure quickly the bullet will not clear the barrel fast enough and rapid disassembly may result. What IS a myth is the idea that all ammunition suffers from bullet setback from multiple rechambering and there's nothing you can do about it.
 
It can happen. Proper crimp helps prevent it. I've never seen a KB from it, but I don't doubt that it people operate it, it can happen.

RMD
 
Yes, the same qualifiers I've read everywhere else:

"May"

"Can"

I have no doubt that bullet setback increases case pressure somewhat, but enough to be dangerous? Enough to cause a KB? I can't find a single documented instance.

Edit: Just found some info stated by Rauch that it is a problem confined to a single caliber and single manufacturer; Glock .40. This was accompanied by the usual "cans" and "mays" however...
 
Enough to be dangerous. If you don't want to accept this idea (or any of the other basic laws of physics) then you don't have to. It's a free country.
 
I'm looking for proof Drail. So far I've found one semi-documented instance involving a G22, that is far from ubiquitous and far from definitive.
 
Well I don't think you're going to find PROOF asking people's opinions on an internet forum. Why do you think this is a MYTH?
 
Okay here you go from experience. I loaded up some 230 grain winchester FMJ bullets with the start load of HP 38 I set the bullets to 1.26 OAL. Went out to the range shot the bullets and they did not have enough power to cycle the ol 1911 properly. So I went home sunk the bullets to 1.22 without adding more powder. Went to the range to see if that had fixed the problem and it did. I don't know the science of it but when I shrunk that combustion chamber inside the case by sinking the lead deeper it increased the power of the cartridge's small explosion enough to cycle the slide when before the cartridge did not have enough power to do so. There you go so in my experience bullet setback increases case pressure.
 
I don't reload, this got moved here from General.

So I guess the question is, how much setback in factory rounds before case pressure could cause a KB?
 
Well if you have a gun you don't care about destroying, a caliper and a remote way of firing the gun we can certainly find out.
 
Inquiry: Bullet Setback Increases Case Pressure

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Part two in my ongoing series of gun myths, actually just me wondering aloud (in print?).

So I've read it and heard it everywhere: Be careful with rounds that have been (re)chambered but unfired, the bullet setback causes the case pressure to rise to dangerous levels. Usually told in this fashion: "My cousin's nephew's uncle's grandad had a KB due to bullet setback, he almost lost an eye." In researching this myth I found many of the same anecdotes but no documented cases of bullet setback causing a gun to KB, let alone an injury. Anecdotally I've personally fired many cartridges that have been setback to various degrees with no noticeable effect. But I admittedly don't know how to measure case pressures, so I don't really know.
Click to expand...

While it may or may not prove the point, you can see the +/- pressure effect seating depth changes make with reasonable accuracy using QUICKLOAD. It ain't cheap but it's very good. It's really pretty easy to get many pistol loads to pressures equaling or exceeding that of centerfire rifles and certainly more than sufficient to destroy a handgun.

I doubt the proof you're looking for is to be found since once fired, you have a blown case, busted gun or other and are left to try and piece together what caused the problem. A number of LE agencies traced KB issues to repeated chambering of duty ammo and imposed limits on the number of chamberings as a preventative measure...AFAIK, it worked in those particular cases.
 
For semi auto rifles and pistols commercial LE ammo from makers like Winchester, Remmington and ATK is normally rated for no more than three rechamberings before bullet setback could be a problem.

In my reloads: 45ACP with plated bullets I noted about .010 setback per chambering in several 1911s (properly crimped to .472). A tighter crimp to .469 reduced setback to about .005 but can crack the plating.

200gr hard cast lead 45ACP SWC crimped to .471 didn't set back at all until 3-4 rechamberings.

45ACP, 38SPL and other calibers with relatively large case volumes compared to charge volume are less sensitive to bullet setback than small volume cases like 40SW and 9MM.

38SPL in a well made revolver and 9MM in a properly chambered barrel seems to be more resistant to the effects of over pressure than 40SW.
 
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That sounds like a good start for the experiment DBR. Three rechamberings = roughly .030 setback = ?
 
Yes, bullet setback does increase pressure. Depending on how much powder, what type, and the net reduction of the combustion capacity will vary. Large cavernous cases like the .45 ACP and .45 Colt will take better to bullet setback than a short case such as the 32 ACP.
 
SuperNaut:
That was with 230gr Ranier RN in once fired WW brass using Dillion dies and giving a "coke bottle " profile to the loaded round. I have found copper plated bullets to be "more slippery" in the case and more sticky on the feed ramp than jacketed or lead bullets. To me this makes the plated bullets a worst case for setback (depending on the gun).
 
I would think .015 might be a good idea of setback from chambering a round if that.
 
Maybe .005 increments? I don't really know how to do that. Maybe once I find a suitable (i.e. very inexpensive) pistol I can get one of you guys to make a set of rounds to test with? Who is willing?
 
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