Neck tension affects bullet setback. How about accuracy?


December 10, 2013, 08:36 AM
I have been pondering about 1KPerDay's situation of inconsistent accuracy with Berry's 40S&W 180 gr plated bullets and came up with a theory to test.

For 40S&W, 180 gr bullet has produced consistently accurate loads for me, even with plated and lead bullets. Although I favored 165 gr FMJ/JHP Montana Gold bullets over 180 gr FMJ/CMJ for match shooting, when it comes to plated bullets for 40S&W, I always recommended 180 gr over 165/155 gr bullet as plated bullets never produced accuracy as good as jacketed bullets. My experience has been similar for Berry's/HSM/PowerBond/Rainier/Speer/X-Treme plated bullets compared to Montana Gold jacketed bullets.

So when 1KPerDay posted inconsistent accuracy with Berry's 180 gr plated bullets, I sent some 165 gr Montana Gold jacketed bullets so a comparison could be made. I figured if the inconsistent accuracy was due to plated bullets, jacketed bullets would produce greater accuracy. Well, the MG jacketed bullet loads did produce better shot groups but for me, range test results made me ponder some more -

In theory (without factoring mechanical inconsistencies from pistol parts), accuracy and smaller shot groups result from more consistent chamber pressures which produces more consistent muzzle velocities.

So while replying to another thread about inconsistency of published load data, I wondered whether using mixed range brass with different case wall thickness and different degrees of work hardening with varying amounts of case wall spring back would produce different amounts of neck tension? Different amounts of neck tension would result in different amounts of bullet setback when the bullet bumps the feed ramp. Different amounts of bullet setback would produce varying bullet seat depths that may result in different chamber pressures which will produce different muzzle velocities that will translate to more inconsistent shot groups and decrease in accuracy. So I offered to send verified once-fired brass with same headstamp so accuracy/consistency could be better tested using cases with similar neck tension/bullet setback -

What are your thoughts on this?

Could different amounts of neck tension/bullet setback produce enough variations in chamber pressures to affect accuracy that are measurable?

I am thinking about testing this theory by range testing mixed cases with different post-sized ID at case mouth and verified once-fired same headstamp cases with same post-sized ID at the case mouth.

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December 10, 2013, 09:00 AM
The main thing that I would think inconsistent neck tension might contribute to accuracy/inaccuracy would be a difference in velocity. Let's say that with a 0.2" radius 40 cal bullet, the typical start pressure is 1160 psi or 145 pounds of force. That seems about right with the bullets I load. 6.0g Unique on a 180g bullet gives 1036 fps out of a 4.25" barrel. Change that neck tension to a ridiculously low 50 psi (6 pounds of total neck tension force). Hand chamber the bullet, shoot it and the muzzle velocity goes down to 1032 fps. That's the equivalent of dropping the charge from 6.0g to 5.96g. That's not much.

You won't be able to measure that much difference with a chrony even with "perfectly" measured charges.

Now if the bullet sets back during an autoloading chambering cycle, you'll have a huge change in pressure and a modest change in velocity, but more likely you'll have a KB before you get a really noticeable change in velocity to where it affect the accuracy in a handgun at handgun distances.

Maybe if the neck tension is so bad that the bullet sets in at an angle, you'll have more problems with accuracy, but the bullet should still blow out of the chamber straight and go straight down the barrel so I don't think that would happen. Maybe someone else has ideas. Oh well, there's my "what if" thought for the morning.

December 10, 2013, 09:49 AM
One thing you mat nay have considered is bullet damage during loading. I ran into this with a ATD. It would shoot the copper plated very consistent but with just waxed lead bullets I was getting a flier 1 out of 10. I was breaking it down for storage and discovered the nose of the bullet had got scraped. Would not happen ever time which explained for the inconsistencies. I would say the same can happen with soft plated bullets. If you eject a live round and give the nose a good look you may find a scuff or flat spot. The profile of the bullet and chamber clearance will play a big part in this on how smooth they chamber. Does it take a bounce off the feed ramp then hits the top of the chamber, damaging the bullet. I do on my 45's but I still get good accuracy as far as I'm concern. Blind in one eye and can't see out of the other. aka Old age.

December 10, 2013, 10:27 AM
These affect accuracy - neck tension, range brass, (trim) length of brass, if head spacing on mouth, mag. primers that move the bullet on firing, causing secondary pressure spike. Thin plated soft lead bullets that lets the bullet skid on firing. Follow link to photos, more on 2nd page.

December 10, 2013, 10:29 AM
Good points.

1KPerDay used Glock 23C with factory compensated barrel. I will be testing G22/G23/G27 with regular non-compensated barrel.

December 10, 2013, 12:05 PM
I'm very new to loading and understand what setback is but I don't understand how a bullet hitting the ramp at an angle would cause that much setback unless the case was severely flared, at which point the case would not seat in the chamber. It also seems the weight of the bullet would go forward upon final chambering thereby reducing the setback. But what do I know. Just a thought.
Is setback that much of a problem in reloading?

December 10, 2013, 01:02 PM
Yes it can be.

A finished round with poor neck tension (either from too much case neck flaring or work hardened brass that lost maleability and spring back) can experience enough neck tension loss/bullet setback to compress powder charge in the case when the slide slams the case and bumps the bullet nose against the feed ramp.

But the main focus of this thread is to look at the affects of different amounts of small bullet setbacks on chamber pressures that may be expressed as shot group size change/level of accuracy.

If a batch of verified-once fired brass with none or very little bullet setback produce greater accuracy/smaller shot groups compared to mixed range brass with different amounts of bullet setback produce larger shot groups, then the holes on target would support the suspicion.

December 10, 2013, 01:22 PM
I agree that neck tension will vary somewhat when other factors change, such as case wall thickness, brass type, work hardening, etc. How much is anybody's guess. Ditto whether it has any effect on accuracy, and if so, how much.

Concerning your point on setback. If you've got good case neck tension, you shouldn't get any setback. Are you experiencing it? With all bullets you've tried? Guns? Have you measured the setback and found any correlations? Are you doing a press test on loaded rounds? If no setback after a passed press test are you still getting setback when chambering those rounds? If so, have you tried adjusting your dies to create better case neck tension? Lots of questions. Sorry. But I wouldn't just accept setback as a fact of life. It's preventable.

December 10, 2013, 02:39 PM
moxie, that's what I am trying to verify/demonstrate also with this thread - preventable bullet setback as part of the proposed accuracy testing.

I used to check neck tension by pressing the bullet hard against the bench top. When I started quantifying neck tension by measuring OAL/COL before and after the rounds were fed/chambered from the magazine, I learned that pushing on the bullet did not accurately duplicate the forces of the slides cycling.

It will be interesting what the quantified neck tension test and range test show.

December 10, 2013, 04:21 PM
Interesting. I've found that rounds that pass the press test don't experience setback, at least in my guns. Is there a particular gun (s) that are doing this? Interesting test.

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