What type crimp? As others have said, I believe a Lee FCD will change neck tension and I use it for that purpose. I do not anneal and I'm convinced the FCD makes my neck tension more consistent in rifle loads.
Crimping and neck tension
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The above chart is when powder was not working correctly in 308. Using the correct powder would not normally need a crimp, but did improve results in the Gov. test.
Hugnot
Well-Known Member
As brass necks become more work hardened or annealing uniformity/quality uncertain, brass spring back occurs & increases & neck tension becomes less uniform. The best remedy is to re-anneal but a little squeeze with a Lee FCD (no flames or heat) helps with neck tension uniformity. A bushing die will provide the desired amount of neck compression (.246 for my .224 calibers), but work hardening is always a factor. Upon seating bullets into necks that have not been annealed for many loadings differences in seating force can be easily noted - neck tension will vary.
Slam & bang ammo used in gas guns is always crimped.
Slam & bang ammo used in gas guns is always crimped.
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Mikecr
Well-Known Member
They got their numbers fudged up.
No way THEY got highest velocity from lowest pressure, and no way YOU or I will either.
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Mikecr
Well-Known Member
If I had peer reviewed their data here, I would have kicked it back.
I get what they were trying to show, but aside from obvious pressure error, the data still seems deceptive to conclusion.
With Std. Dev. so high, they clearly had an extreme problem load. Perhaps tailored to show a crimping affect improvement.
Why would they do that?
Who thinks that's normal enough to apply to normal?
I could picture crimping 'fixing' an existing problem (like extreme tension variances).
Tension could be removed, with only crimping relied on for bullet release. Maybe that would alleviate the issue.
But then, the extreme tension variance could just be fixed, for no crimping needed. Back to normal.
If someone was using a very wrong powder, it could be that a higher than normal starting pressure could get it cooking.
I don't know.
I get what they were trying to show, but aside from obvious pressure error, the data still seems deceptive to conclusion.
With Std. Dev. so high, they clearly had an extreme problem load. Perhaps tailored to show a crimping affect improvement.
Why would they do that?
Who thinks that's normal enough to apply to normal?
I could picture crimping 'fixing' an existing problem (like extreme tension variances).
Tension could be removed, with only crimping relied on for bullet release. Maybe that would alleviate the issue.
But then, the extreme tension variance could just be fixed, for no crimping needed. Back to normal.
If someone was using a very wrong powder, it could be that a higher than normal starting pressure could get it cooking.
I don't know.
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It holds pressure longer, so it gives fake higher pressure readings. Fill 2 balloons half full of air. Pop 1 as is, then compress the other before popping it. Both have the same volume, but the compressed 1 will give off more boom. Issue explained!
I crimp every cartridge from pistols to rifles regardless of platform. Most are AR platforms including pcc. The only few that aren't in an AR form are 270wim, 264wm, 300wm, 338lm, 45-70, 44mag, 50ae, 5.7x28 and 357sig. The rest are AR forms, 5.56, 243win, 6.5gren, 6.5 creed, 260rem, 6.8spc, 300bo, 308win, 9mm, 40s&w, 10mm, 45acp.
Hugnot
Well-Known Member
With a progressive burning powder as pressure increases the burn rate will increase. Loads that are not crimped will exhibit a more gradual pressure increase occurring in an increased volume as the bullet is pushed into the bore by expanding gasses. Boyles law states that pressure is inversely proportional to volume, and this would normally result in lower pressures as the volume increases and uncrimped loads should have less velocity.
But
"Initial testing centered on preparations for the factorial test. Since this study centered on the lack of consistency of the M80 round, it was crucial to minimize the introduction of additional inconsistencies during the testing of the factorial test. However, due to the added manpower necessary to crimp the cartridge cases around the bullets, it was decided to conduct a small test prior to the factorial test to assess whether crimping of the cases was a necessary step. The testing centered on velocity and case mouth pressures. It was presumed that if this test yielded consistent results between the crimped and the non-crimped rounds, crimping was an unnecessary step"
"This test used M80 ammunition only: reference M80; M80 cartridges that had the bullets pulled, replaced, then crimped; and M80 cartridges that had the bullets pulled and replaced with no crimp. The reference rounds were fired at ambient temperature only. The test rounds fired at ambient, hot (125°F), and cold (-65°F) temperatures. As can be seen from table 2, the differences between the non-crimped and the crimped rounds were obvious. By not crimping the case around the bullet, the differences in the standard deviations of the velocity at ambient temperature was greater than 500 ft/s, which is over 20% of the total velocity measurement. The differences between the pressure standard deviations were also large, with the non-crimped rounds showing a standard deviation of nearly five times greater than that of the crimped rounds. At the hot and cold temperatures, differences were again seen, but not as drastic as at ambient. Again, the differences here were greater for the non-crimped rounds than the crimped rounds. It was thus decided that the rounds used in the factorial test must be crimped in order to achieve good results"
However, the non-crimped loads were shown to give lower velocities with higher pressures.
"Also of note is that the peak pressures measured were mid-case not case mouth, so the peak pressures appear higher than the allowable limit of 52,925 psi but are not comparable. Also, due to limitations in the test equipment at LCAAP, the time measured to partial peak pressure is at 25% of peak pressure rather than the 17,000 psi measurement from ARDEC testing. This 25% of peak pressure corresponded to an average value of 12,885 psi."
Then
"From the factorial tests, it appears that the only factor that positively influences the time-topeak pressure with the M80 was that of the intrusion depth. Changing the primer increased the time to peak pressure and changing both intrusion depth and the primer resulted in essentially no change. This leads us to believe that the intrusion depth is the most important factor in the time-topeak pressure"
My guess is crimping maintained the .57 intrusion depth longer making the crimped M80 round a smaller capacity cartridge & provided a shorter time-to-peak pressure & higher velocities. Uncrimped rounds allowed more gas expansion to a longer time-to-peak pressure - slower velocities but higher and slower time-to peak pressures.
Other concerns would be bullet damage by excess crimping & head-space maintenance for rounds like the .45 Auto that headspace on cartridge mouth (slight taper crimp).
The Lee FCD only allows so much crimping & looking down into the die the degree of crimping may be seen.
But
"Initial testing centered on preparations for the factorial test. Since this study centered on the lack of consistency of the M80 round, it was crucial to minimize the introduction of additional inconsistencies during the testing of the factorial test. However, due to the added manpower necessary to crimp the cartridge cases around the bullets, it was decided to conduct a small test prior to the factorial test to assess whether crimping of the cases was a necessary step. The testing centered on velocity and case mouth pressures. It was presumed that if this test yielded consistent results between the crimped and the non-crimped rounds, crimping was an unnecessary step"
"This test used M80 ammunition only: reference M80; M80 cartridges that had the bullets pulled, replaced, then crimped; and M80 cartridges that had the bullets pulled and replaced with no crimp. The reference rounds were fired at ambient temperature only. The test rounds fired at ambient, hot (125°F), and cold (-65°F) temperatures. As can be seen from table 2, the differences between the non-crimped and the crimped rounds were obvious. By not crimping the case around the bullet, the differences in the standard deviations of the velocity at ambient temperature was greater than 500 ft/s, which is over 20% of the total velocity measurement. The differences between the pressure standard deviations were also large, with the non-crimped rounds showing a standard deviation of nearly five times greater than that of the crimped rounds. At the hot and cold temperatures, differences were again seen, but not as drastic as at ambient. Again, the differences here were greater for the non-crimped rounds than the crimped rounds. It was thus decided that the rounds used in the factorial test must be crimped in order to achieve good results"
However, the non-crimped loads were shown to give lower velocities with higher pressures.
"Also of note is that the peak pressures measured were mid-case not case mouth, so the peak pressures appear higher than the allowable limit of 52,925 psi but are not comparable. Also, due to limitations in the test equipment at LCAAP, the time measured to partial peak pressure is at 25% of peak pressure rather than the 17,000 psi measurement from ARDEC testing. This 25% of peak pressure corresponded to an average value of 12,885 psi."
Then
"From the factorial tests, it appears that the only factor that positively influences the time-topeak pressure with the M80 was that of the intrusion depth. Changing the primer increased the time to peak pressure and changing both intrusion depth and the primer resulted in essentially no change. This leads us to believe that the intrusion depth is the most important factor in the time-topeak pressure"
My guess is crimping maintained the .57 intrusion depth longer making the crimped M80 round a smaller capacity cartridge & provided a shorter time-to-peak pressure & higher velocities. Uncrimped rounds allowed more gas expansion to a longer time-to-peak pressure - slower velocities but higher and slower time-to peak pressures.
Other concerns would be bullet damage by excess crimping & head-space maintenance for rounds like the .45 Auto that headspace on cartridge mouth (slight taper crimp).
The Lee FCD only allows so much crimping & looking down into the die the degree of crimping may be seen.
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So, now we're back to Greenhill's formula. Old and mostly outdated, but partially relevant compared to updated twist calculations. 1 can get close, but not enough to be exact. Like Newton's 3rd law.....a bit outdated and yet not that close. Boyle has me trying to get others to understand the difference between CNG and propane. CNG operates off volume at low pressure and propane is high pressure with lower volume. Even though paths will cross like 0ing a gun. Take 2 cartridges that use the same powder, charge and bullet weight. 2 are different calibers, but the smaller bore takes longer to lower in pressure. Bearing surface can have an effect on outcome, so trying to make both equal could be the for testing not to mention twist rate.
Hugnot
Well-Known Member
When the Lee FCD became available some bullet manufactures were negative as to crimping, like, no cannelure no crimp.
Giving case necks & bullets a little squeeze with a Lee FCD makes me believe my neck tensions are more uniform & powder burn rates & subsequent pressure peaks are more uniform. My crude annealing procedures having uncertain temperatures and unknown degrees of brass crystal modification/hardness need improvement to provide uniform neck tensions. My evaluations of crimping vs. no crimp was made with same brass, powder, primer, barrel (chamber & twist) & bullet.
https://apps.dtic.mil/docs/citations/ADA341390 seems valid.
I use CCI 34's in .22-.250, 6mm CM, 6.5 CM, 6mm Rem, 6mm AI & 6.5-06. No Lee FCD for 6.5-06 but with some adjustment & modification a .264 WM die might work.
Giving case necks & bullets a little squeeze with a Lee FCD makes me believe my neck tensions are more uniform & powder burn rates & subsequent pressure peaks are more uniform. My crude annealing procedures having uncertain temperatures and unknown degrees of brass crystal modification/hardness need improvement to provide uniform neck tensions. My evaluations of crimping vs. no crimp was made with same brass, powder, primer, barrel (chamber & twist) & bullet.
https://apps.dtic.mil/docs/citations/ADA341390 seems valid.
I use CCI 34's in .22-.250, 6mm CM, 6.5 CM, 6mm Rem, 6mm AI & 6.5-06. No Lee FCD for 6.5-06 but with some adjustment & modification a .264 WM die might work.
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34's for BA or you just like the harder cups? Not judging, just wondering as I have 34's also. All I've ever used are 200, 250, 400 and 450's in AR's, but none are full or burst autos. I only bought 34's (SRP) and 41's due to hard to find LRP's.
The pulling & reseating of the bullets may have changed neck tension/bullet hold?
