Bullet setback experiment


December 28, 2010, 01:40 PM
I have been curious for a while about how much a bullet gets pushed back upon chambering. I figure others have had the same question.

So, I quick whipped up an experiment to shed some light on the question.

The pistol: Gen 3 Glock 17 Serial #HWN2XX (no modifications)
The ammo: 9mm Remington 115 JHP (factory ammo)

The test:
1.) I measured the OAL of the cartridge.
2.) I pulled the slide back until it locked on the empty magazine.
3.) I placed one cartridge in the magazine as far back as possible.
4.) I inserted the magazine.
5.) I pushed the slide release with my thumb.
6.) I racked the slide back until it locked and ejected the cartridge.
7.) I repeated steps 1-6 15 times with same cartridge, recording results.

I did this entire test on three different cartridges from the same lot number. Here are the results: (The first number in each row is the chambering number.)

# Ctg#1 Ctg #2 Ctg #3
0 1.067 1.070 1.071
1 1.067 1.070 1.071
2 1.067 1.070 1.071
3 1.067 1.070 1.071
4 1.067 1.070 1.070
5 1.066 1.069 1.070
6 1.063 1.067 1.070
7 1.062 1.066 1.070
8 1.060 1.065 1.069
9 1.058 1.065 1.068
10 1.055 1.065 1.068
11 1.055 1.063 1.068
12 1.053 1.063 1.068
13 1.052 1.062 1.068
14 1.050 1.062 1.067
15 1.048 1.060 1.067

Conclusions: Every cartridge was safe even after 5 chamberings. One cartridge was still safe after 15 chamberings. All the cartridges looked really beat up at the end of the test.

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December 28, 2010, 01:44 PM
It is usually the first round in a full mag that gets re-chambered over & over again.

A full mag will provide much more force against the slide, slowing it down some.

Probably get less set-back with a full mag test.


December 28, 2010, 01:48 PM
I think I will try that for my next experiment and see what happens.


December 29, 2010, 07:58 PM
This is how I avoid bullet setback on all my Glocks:


I personally don't feel this causes any undue wear on the extractor or the extra long Glock extractor spring, but I anticipate that others will disagree.

This doesn't work on my other locked-breech semiautos, though. So that data is nice to know. It looks like the bullet stays put for several chamberings before moving. Perhaps getting hit at multiple angles finally loosens it up. I know some of the name brand defensive ammo has been advertised as guaranteed for only 4 chamberings. Your findings seem to suggest that's your magic number, here, as well.

December 29, 2010, 08:08 PM
I certainly won't disagree.
Been rechambering carry rounds that way for years.
Yet to have a problem.

December 29, 2010, 08:41 PM
Nice video, GLOOB.

I like how you lifted the extractor to help it clear the rim instead of letting it slam forward. Great idea!

December 29, 2010, 11:42 PM
Good idea for trying this.

One thing this test doesn't do though, is while it measures the total setback and that can determine whether it is within the allowable cartridge specs, it doesn't measure the excessive pressure the setback can cause, and whether that will result in a kaboom.

Of course that also varies with ammo brand, and make/model of pistol.

Jim K
December 29, 2010, 11:51 PM
I ran tests in which I forced the bullets back into the cases .10" at a time until the bullets either compressed the powder or expanded the case so it wouldn't chamber. I fired those cartridges, with no signs of any high pressures, even with compressed powder. I don't claim my tests were either extensive or exhaustive, but I am convinced that whatever may be the cause of occasional kabooms, bullet setback is not it. I think it is a red herring to cover up some other problem, either in the loads or in the gun design. (Or some other problem, like having a 9mm round in the magazine of a .40 pistol and chambering it ahead of a .40 round.)


December 30, 2010, 11:39 AM
I think it depends on the specific cartridge, bullet, and powder whether setback would be dangerous or not.

The old Lyman example comes to mind. It states that .3" of setback CAN cause a doubling of peak pressure in 9mm luger, with the right powder. Of course, even with this powder, double the peak pressure still doesn't necessarily mean a kaboom. Presumably, the peak would occur sooner and be of shorter duration than normal. Barrels can typically stand that kind of pressure peak. So in this case, I'm more interested what the pressure is at the time the action opens.

December 30, 2010, 11:58 AM
If you are reloading, you can use powders that fill the case. Blue Dot and Accurate #9 are what I use in 9mm, 40SW, 10mm, and 357Sig. These slower-burning powders give top velocities at reasonable pressures and both powders almost completely fill the cases. Makes for a compressed load, but each is safe and minimizes or eliminates any bullet set-back. Combine that with a good taper crimp, and you should not have any problems.


December 30, 2010, 12:57 PM
Kel-Tec P-3AT, P-32 and Ruger LCP pistols have a design flaw that causes the feed ramp to slam into the top round in the magazine during recoil.

The impact of the feed ramp on the next round is severe enough to cause deformation in it's bullet as you can see in the picture below and surely this causes some bullet setback, but still I have never heard of a problem from this.
http://img.skitch.com/20101230-g5mgn7q2egtu2e7f4634wftqis.preview.jpg (http://skitch.com/jerryjerry-w/r8rhn/smilies)Click for large view (http://skitch.com/jerryjerry-w/r8rhn/smilies) - Uploaded with Skitch (http://skitch.com)

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