Crimp vs Velocity

[Bill Steele]

You don't crimp straight-wall pistol cases used in semi-auto's, you just use the crimp die (must be a taper crimp type) to remove the case mouth belling done to allow for bullet seating, If the existing neck tension interference fit doesn't hold the bullet securely in place then there is an issue with what the size die is doing versus the bullet diameter. ID of case mouth needs to be .002 under bullet diameter. Straight-wall case semi-auto pistols headspace the case in the chamber against the case mouth rim, you DO NOT want the case mouth rim turned in to ANY measurable degree no matter what you may hear from others about doing this to hold the bullet in place. Measure the case mouth OD versus the case OD further down on some factory 9mm rounds to see what I am talking about here.

Absolutely right.

The 1700fps was an error.

[rsrocket1]

I would try again doing an A/B test between your "hypersonic" loads and some factory loads. Same gun, same exact set up. Something is wrong here. Maybe you were misreading the chrony or the sun was getting into the sensors or something else. No sort of crimp will boost a 1200 fps round by 500 fps.

I would certainly back off on the powder in any case. You are near the peak of +P pressures (37,500 psi).

I have had problems with my Shooting Chrony Beta near sunset when the sun peeks directly into the sensor. If you were shooting late in the day and the 9mm was the last to be shot, the sun may have moved enough to start wacky readings.

[jimmykan]

Seating depth of the bullet affects how much remaining volume there is in the case for the powder to combust. I think that less case volume results in a steeper pressure curve (peak pressure is greater and it is reached more quickly). This usually translates into higher velocities.

[GeoffLinder]

BTW, crimp will have a very small effect on velocity, less than a few percent at most, no way it creates hundreds of FPS difference. You will never realize 1700 fps with a 115gr in a typical 9mm pistol without blowing it up. I load major 9mm to primer flattening levels and in a 6" bbl I can get maybe 1450fps with a 124gr and just a tad over 1525fps with a 115gr. This just ain't gonna happen. Definitely a chrono reading error.

[Sosoomi]

GeoffLinder and Jmmykan....

Luckily I had a dozen rounds left over....

Two of them... And I think the last unfired in the row measured an OAL of 1.07 ... Rather than 1.12-1.13... The crimp seemed excessive as well...

Tomorrow I will run a few dummy rounds and see if my setup was compromised for the last rounds...

It would seem that that much undersized and over crimped might be part of the issue....

Chrono error could come into play as it was later but only 2'ish...and all readings were more or less the same...

Thanks all.... Excellent input.. I am obliged!

[GeoffLinder]

GeoffLinder and Jmmykan....

Luckily I had a dozen rounds left over....

Two of them... And I think the last unfired in the row measured an OAL of 1.07 ... Rather than 1.12-1.13... The crimp seemed excessive as well...

Tomorrow I will run a few dummy rounds and see if my setup was compromised for the last rounds...

It would seem that that much undersized and over crimped might be part of the issue....

Chrono error could come into play as it was later but only 2'ish...and all readings were more or less the same...

Thanks all.... Excellent input.. I am obliged!

The ONLY way you will EVER get anywhere near 1700fps with 9mm is in a long barreled 9mm carbine using a hefty load of slow burning powder. A charge level that will provide 1700fps in a standard pistol using Unique (which is a fast powder) will blow out the primer at best and blow up the gun at worst. You did not actually realize 1700fps IMO, there is no way you got that actual velocity with that powder in a standard pistol without damaging something, no way! This is a chrono reading error OR you are an incredibly lucky person.

BTW, 5.7gr of Unique under a 115gr is not enough of a charge to get that velocity.

Read the numbers here from Alliant on 6.3 gr Unique under a 115gr FMJ and then tell me how you actually got 1700fps with 5.8gr.

http://www.alliantpowder.com/reloaders/RecipePrint.aspx?page=/reloaders/RecipePrint.aspx&gtypeid=1&weight=115&shellid=23&bulletid=25&bdid=28

[rsrocket1]

If the crimp is excessive, don't shoot those loads. The cartridge headspaces on the case mouth and if the crimped portion gets pinched between the bullet and the barrel wall,

it say BLOOM!

[JagerDog]

For sure something else is going on. Crimp is rather insignificant re: velocity/pressure.

1700fps isn't on the same planet.

[Sosoomi]

Well I agree with you but when the chrono read that I said stop.... I read the tables and loads before loading..that's how I decided to start at 5.8 gr.

[GeoffLinder]

Well I agree with you but when the chrono read that I said stop.... I read the tables and loads before loading..that's how I decided to start at 5.8 gr.

5.8 gr Unique is at best an 1150 fps range recipe under a 115gr no matter what OAL and crimp you have set PERIOD. That 1700fps reading was a chrono error, end of story here!

Again, look up the data on Unique in 9mm, OK!