Why the difference in velocity?

[XDman]

OK, here is my experience at the range with the crono.

I loaded 9mm with 147 gr MG bullets, 3.6 gr N320, sm pistol primers, mixed brass.

I am using a Dillon 550, with no adjustments made during the reloading session.

So I shoot 5 loads out of my 6 in sti limited:

893

920

1348

940

935.

Then I shoot 5 loads out of my 5 in XD:

940

946

930

922

937

So Im thinking, why the big swing for the 6 in as compared to the 5 in. So 5 more from the 6 in:

1115

931

934

1040

932.

Still a big varience. Any suggestions as to why I am getting such differing results?

[Trident]

OK, here is my experience at the range with the crono.

I loaded 9mm with 147 gr MG bullets, 3.6 gr N320, sm pistol primers, mixed brass.

I am using a Dillon 550, with no adjustments made during the reloading session.

So I shoot 5 loads out of my 6 in sti limited:

893

920

1348

940

935.

Then I shoot 5 loads out of my 5 in XD:

940

946

930

922

937

So Im thinking, why the big swing for the 6 in as compared to the 5 in. So 5 more from the 6 in:

1115

931

934

1040

932.

Still a big varience. Any suggestions as to why I am getting such differing results?

When I get this it has been the chrono location being too close and effected by the muzzle blast and/or due to overhead sun and not having the chrono in the shade as the chrono reads the reflection of the bullet. Some color the bullets with a black Sharpie in the sun here in Florida.

[The_Vigilante]

So what was the SD? The high fps shots appear to be the exception out of the 6" barrel.

[njl]

My guess would be the longer barrel is putting you just too close to the chrono, and you're getting inaccurate readings. 893 - 1348 is just too big a spread for the "same load" from the same gun unless you're loading incredibly badly...and 1348fps with a 147gr from a 9mm handgun just doesn't sound plausible. That'd be a 198PF. Did that shot kick like a mule or blow up the gun?

Your first string, minus the 1348 shot, looks reasonably consistent and in the expected FPS range.

[XDman]

I was back about 10 feet from the crono, and was using the shade. I will try it again this next week. Maybe it just likes my XD

[Hi-Power Jack]

XD, 5 shot strings is not enough.

You really need to fire 20 rounds of

each load to get an accurate reading.

If you drop that one high reading, the

rest are pretty consistent.

Each gun will be "faster" or "slower"

than another gun - just a fact of life.

[CeeZer]

With longer barrels you'd want to use slower burning powder... at least for more consistent bullet velocity.

N320 is fast. And the faster you go the more affect you get with diff length barrels.

Try Win 231 as dah fastest, or slower.

[HoMiE]

I've had mixed brass the crimp not be the same and get bullet setback which shortens OAL and higher pressure higher velocity. Just something to check when I see high velocity like that.

[CeeZer]

Bullet set back = not good enough crimp.

Crimp should be good enough to chamber reliably and to NOT leave marks (cut jacket/plating) on bullet (if pulled). No set-back should be there for at least 4-5 cycles.

[G-ManBart]

With longer barrels you'd want to use slower burning powder... at least for more consistent bullet velocity.

N320 is fast. And the faster you go the more affect you get with diff length barrels.

Try Win 231 as dah fastest, or slower.

Untrue. Lots of people use Clays (faster than N320) for incredibly consistent PPC loads in 6" 1911s, and a ton of folks use N320 in 6" Limited guns with excellent consistency. In fact, going to a slower powder will very frequently hurt consistency because you don't get 100% burned before the bullet leaves the muzzle (there are also other factors such a where the pressure peak is etc).

[G-ManBart]

Bullet set back = not good enough crimp.

Crimp should be good enough to chamber reliably and to NOT leave marks (cut jacket/plating) on bullet (if pulled). No set-back should be there for at least 4-5 cycles.

Untrue, and potentially unsafe advice.. Setback happens when you have insufficient bullet pull (commonly called "neck tension"). Neck tension is created by proper resizing paired to the bullet diameter, not crimping (unless you're talking a revolver round loading a bullet with a cannelure and using a roll crimp). If there isn't enough neck tension to prevent setback before crimping, it won't be there after crimping.

[XDman]

I am using Dillon dies, just enough expansion to get the bullet started, and a tapered crimp.

[njl]

I've had mixed brass the crimp not be the same and get bullet setback which shortens OAL and higher pressure higher velocity. Just something to check when I see high velocity like that.

G-Man has already set the record straight on crimp having nothing to do with it (at least for auto-loader handgun ammo), but I think people are ignoring the simple explanation already given....The velocity was so high for that one shot because the chronograph gave an incorrect reading. There's no way he had 4 shots at 900 and change and then one with the same load/same gun at 1348fps. I kind of doubt you could get that velocity from that bullet in a 9mm handgun.

Edited May 28, 2012 by njl

[Steve RA]

Agree, just a bad reading, the rest are reasonably consistent.

[HoMiE]

I've had mixed brass the crimp not be the same and get bullet setback which shortens OAL and higher pressure higher velocity. Just something to check when I see high velocity like that.

Whatever the cause for setback, it can happen. And as far as 9mm shooting 1300+, it happens all the time with major open shooters, even up into the 1500 fps with 115 gr bullets.

G-Man has already set the record straight on crimp having nothing to do with it (at least for auto-loader handgun ammo), but I think people are ignoring the simple explanation already given....The velocity was so high for that one shot because the chronograph gave an incorrect reading. There's no way he had 4 shots at 900 and change and then one with the same load/same gun at 1348fps. I kind of doubt you could get that velocity from that bullet in a 9mm handgun.

[Sin-ster]

I find the ES and SD drops in my loads using N320 as the velocities get higher. But I shoot a 124 grain MG JHP, so that might not be an option for you at all! BE SAFE.

I'll echo what others have said here about being too close to the chrono, and the issue of "glint" off of those shiny brass jackets on the Montana Golds. Step back another 5 feet the next time, and take a black sharpie to your projectiles before shooting them. You may also consider putting down some kind of dark cloth beneath the sensors, if you're shooting off of something that may be refracting the sunlight. I've found that even dry, lightly colored clay can throw things off by 10-15 FPS in both directions. (Water and snow are the worst obviously-- don't shoot in puddles!)

And a +1 to a small data pool-- 10-20 rounds is a much better representation. Every now and again, I will drop the fastest and the slowest readings to get an even better picture of the performance (if they're so dramatic as to cause real problems with the numbers). You can almost rest assured, when they're way off the average, that something wonky was going on with the chrony and not so much your loads.

To be sure of this, check your powder drop by averaging 10 loads. Then do it again. And again. And again. It will either amaze you how consistently that extruded powder measures (despite Dillon's warnings against using it), or you'll find out that you've got a problem!