Crimp vs Velocity

[Sosoomi]

So what do you know about this relationship?

I loaded some 9mm, 115 gr XTP HP, 5.8 gr Unique, std pistol primer and got chron'd velocities around 1700 fps.... I was expecting in the 1200's.

All I can think is that I had too much crimp. Lee dies.

I am going to bet that guys don't mind having this kind of 'relationship' convo!

:-)

[Steelplate45]

I have had Lee dies yield excessive pressures

Back off the crimp and powder charge.

And check the fired primers and the case heads for pressure signs.

[Sosoomi]

You know I was at the lower end.... No pressure signs at all.... I will back off on the crimp and see how that works....

Any rule of thumb as to what is the right amount of crimp?

[J-cat]

That amount of Unique in a 9mm case under a 115gr bullet will never, ever, ever, ever, make 1700 FPS no matter what the crimp is.

[mark501w]

Gotta be a crono error unless maybe your shooting it in a carbine?

[rsrocket1]

I seriously doubt you'd be getting 1700 fps. Did you check your chrono against a 22LR or other known velocities?

Even with a 16" barrel, you'd only get about 1500 fps. You would need 84,000 psi to get 1700 fps out of a 4.5" barrel (a little high for any 9mm barrel).

Did you shoot a string of 10 shots? What was the extreme spread? With a consumer grade chrony, you can get an errant reading every now and then. If you placed the chrony too close to your gun, sometimes the muzzle blast will cause an errant reading (even one from the gun in the next lane). Your load should get you about 1200 fps just as you said. +/- 100 fps wouldn't surprise me but 500 is a little too much.

The crimp adds almost nothing to the velocity. It does hold the bullet a little better against set back and might aid in a better burn with slow starting powders, but a full load of Unique with almost no air gap in the case has no problems igniting, overcoming the crimp and starting the bullet into the barrel.

[Revoman]

Most will tell you to simply "take the bell out", which may work okay. I personally use a different method....for what it's worth.

I take the bell out and measure OAL. I then push the bullet against my wooden workbench top and remeasure to see if it has moved. If it has moved, I start crimping in small increments each time checking as above until I cannot push the bullet into the case by pressing it against the side of my wooden workbench top and detecting any movement by measuring. Sometimes only the bell needs to disappear as just bullet friction is enough to stop any movement from my pressure test. Sometimes a light crimp is needed beyond taking the bell out. And of course, sometimes a slightly heavier crimp is needed to stop the movement, however, I am very aware that too much crimp is not always a good thing and keep the crimp to a minimum.

That's my method, good or bad. I'm sure that someone else will chime in with some more information....

[mark501w]

Still , that's super fast. Crimping slow burning powders is where you usually see problems with burning &velocity.

[Sosoomi]

Well.... No.. It was a Glock 17... The carbine was the 357 load!

I started by running 10 rounds of 22 from my Ruger 22/45 to check and that was fine... The 357 and 44 mag were right were they should be... The 9 mm was the last I did and it was pretty warm and sunny...

I think I will try Revomans technique ... I had heard the same from another guy....

Rsrocket... I did about 5 rounds and didn't like what I was seeing....so stopped there.... I did visually inspect every case after the powder drop.... Weighed about half of them and they were pretty much on....

[ShootinMedic]

I'm not sure how you would get that velocity with that charge. How far did you seat the bullets? What were you shooting them out of?

[mark501w]

I don't now if you have a taper crimp but, I snug it down so the bullet doesn't move.

[Sosoomi]

OAL = 1.12 and like I said a Glock 17.......

[ShootinMedic]

sorry, I started writing my question before you said that...

[Sosoomi]

Not a problem...... ShootinMedic...

I thought about chrono error but all the other readings seemed good.... 357 and 44 ....

[GeoffLinder]

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.