Wide FPS range using Chrony

[RobertInIowa]

I recently purchased a chronograph to try and better my reloading tecnique as well as watch for pressure signs etc. At first I put the chrony about 6 feet from the muzzle and that gave me a wide range of FPS so I did some research and found that it should be 12' or better from the muzzle. So now I have the chrony as far as my data cord will allow, which is about 12'-14' from the muzzle.

My problem is that I am getting variances from 500 to 800 FPS. I try to load all the ammo the exact same way but I don't understand why I'm getting such a variance. Any ideas?

Here's some more info. Single stage press's. All use the same dies, primers, sizer/decapper. ll are processes on an RCBS trim mate. I chamfer and debur the necks. I apply no crimp as I'm shooting from a single shot bolt Cooper model 21 in .223 with a 1:12 twist and a 24 inch barrel. I hand weigh on a high precision scale with no drafts or temperature issues and I'm positive the charges are as identical as they can get.

Do you suppose neck tension could be the issue? If so, how do I improve it? What is the normal variance range in FPS for handloaded ammo?

I shot my pistol (.40 s&W Sig) through the chrony and had variations of only about 15 to 20 fps so I don't think it's the chrony.

I'm sure i've forgotten to provide some important info, so ask anything you need. What do you guys think might be the problem?

 

[Lost Sheep]

Just to be clear. The variation is 500-800 fps as in the difference between 1500 fps to 2300 fps or 1800 fps to 2300 fps or are your bullets actually clocking at 500 fps and 800 fps?

Neck tension would be my first guess, but primer seating might be the culprit.

What powder and primer are you using? If a hard-to-ignite powder and standard (not magnum) primers, you might get a lot of pressure variation there.

The "high precision scale". Is it electronic or balance beam? Some electronics are sensitive to things like the electronic emanations from flourescent lights or being moved or line voltage fluctuations. The better ones are less sensitive, but still...

I would put a baffle (plywood?) in front of the barrel with a small hole in it about 12" away to catch the muzzle blast and any stray powder granules that might be confusing your sensors. You could move both the Chronograph and the readout unit another few feet away to furthur reduce the chance that muzzle blast is the culprit.

Do you get the same sort of variation with factory ammo in that gun?

Does the powder fill the case pretty well (like 90% of the way to the base of the bullet) or is there a lot of room for the charge to be in different positions? Sometimes a considerable variation in velocity can be produced if the powder is piled up against base of the bullet (far from the primer) or right close to the primer. Though that is unlikely as a source of your puzzle, as the .223 case is not that large.

That's all I can think of off the top of my head.

Good luck, Thanks for asking our advice.

Lost Sheep

 

[GP100man]

Indoors or out ?? , clean the lens & use larger difussers out side, they don`t like direct sunlite or flouresent lites .

Big calibers with big blast I usually set up at 20 ft. with my chrony.

PS: I type too slooooow

 

[RobertInIowa]

Lost sheep, thanks for your reply.
The variance is like 2800 to 3300.
I'm not sure about neck tension but I think my primer seating is top notch.
Powders are either Varget, IMR4198 or Benchmark. Primers are Rem. BR.
The scale is an electronic Mettler Toledo Lab scale, very precise and reliable and there are no flourescent lights.
I havent fired factory ammo in the rifle, but I'll pick up a box and test it.
The powders fill the case from about 80 - 90%. (.223 Rem)

Thank you for the advice,
RobertInIowa

 

[RobertInIowa]

GP100MAN,

I am shooting outdoors using the large difusers. No direct sunlight or light bulbs.

Thank you for your reply

RobertInIowa

 

[Lost Sheep]

Three more questions:

Bullet weight. If you are near the maximum for the bullet/powder combination, strange velocities can creep in as the powder burn gets erratic (spikey). This could be a danger sign. Not necessarily, but maybe.

Are the primer flash holes uniform?

Seating depth (as measured from the base of the bullet to the case web). I know, once the cartridge is assembled this cannot be measured directly, so we use the cartridge overall length. But if there is variation in the nose shape you can get some uncontrolled variation.

I love a puzzle.

Lost Sheep

 

[mrawesome22]

Powders are either Varget, IMR4198 or Benchmark.

Different powders have different characteristics and give different velocities.

 

[RobertInIowa]

Lost Sheep,

Bullet weight is 52 grains. Pretty much top end for the 1:12 I have. It keyholes from 60 grains up.
Primer flashholes are deburred.
My COAL is about 2.260, maximum per SAAMI. I was trying to get as close to the lands as possible.

RobertInIowa

 

[mrawesome22]

Varget is a relatively slow powder compared to IMR4198 and Benchmark.

You can't shoot a bunch of different powders and expect the same velocities.

If you could, there would be no need for different powders.

 

[RobertInIowa]

mrawesome22,

The FPS variances are within batches of the same powder. I appologize for not making that clearer.

RobertInIowa

 

[Sport45]

I could be wrong, but I don't read it like he's expecting the same velocity from different powders. I think he's getting velocity variance with the same load of the same powder. He just gets the same wide variance regardless of the powder he used. Is that right, Robert?

Mixed brass, or is it all from the same lot? Full length sizing or neck sizing?

Do you have enough neck tension that you can't push the bullet into the case any farther by pressing the tip against your workbench withot trying really hard?

 

[RobertInIowa]

Sport45,

You are correct, I am getting the variances within the same loads, regardless of the powder I use.

Sometimes it's mixed brass, other times it's pure Winchester. And I've been neck sizing.

As far as neck tension, I will have to test that on my next loads, I'm plum out of reloads right now.

RobertInIowa

 

[mrawesome22]

Thanks for clearing that up.

Were the 40S&W loads factory loads?

If they were factory loads, I can only guess your scale is deviating substantialy.

 

[RobertInIowa]

the 40 S&W's were handloads 180 gr Hornady FMJFP on either 5.5 Power Pistol or 4.7 Tite Group. My scale is checked everytime I weigh and it's always dead on. I'm pretty sure we can rule the scale out.

 

[AllenJ]

500 to 800 fps variances and you are weighing each charge? I'm sorry but I think you have a chronograph issue. The pistol bullets are both bigger and moving slower, so using that as a test of the chrono may not be telling you all you need to know. Have you tried shooting factory ammo from the rifle over the chrono?

 

[mrawesome22]

I agree.

Sounds like the chrono is at fault.

Thank you for giving clear, prompt answers.

Sent from MIUI using Tapatalk 2.

 

[Mal H]

I also agree. You would have to really work hard at bad reloading practices to get a 500 to 800 fps differential. I honestly don't think you are, reloading incorrectly, that is.

The first thing I would do is eliminate for good the possibility of ultrasonic gases tripping the chrono. Back up to at least 20 ft and try 3 shots. You said you can't get any further than 12' to 14', but unless the wires are tied to your rifle ( ) you should be able to back up as far as you want to. It just means the control box will be a few feet in front of (and to the side of) the muzzle.

If you still get a huge variation in velocity, then you should start looking at the rounds themselves. You mentioned that you are trying to get close to the lands. For a 3 rounds test, back off a bit (shorten the OAL by .010") and see what happens.

Do you have a .22 rifle? If so, see what kind of velocity and variation you are getting from it.

 

[Wyoredman]

I was having the same problem with my chrony and my .223. I found that when I went to full length resizing instead of neck sizing, I got better numbers and groups!

I always set my chrony at 15', and sometimes with hotter loads, I get some variation, but never more than 100 fps.

Try full length resizing. It will make your neck tension more consistant. Just my personal experience.

 

[RobertInIowa]

AllenJ, I have not tried factory ammo. I'll pick up a box and see what I get.

MrAwesome, I will try all the other suggestions before I consider sending the Chrony in for a checkup, just to be sure.

Mal H, I'll extend out the chrony to 20 feet as you suggest. Good idea on the 22, I have one I'll use to see if I get more consistant results.

Wyoredman, I will try some full length resized loads and see what happens.

Everyone, thanks for the abundance of great ideas, I very much appreciate it.

RobertInIowa

 

[Arub]

Variability in lighting, like under trees on a bright sunlit day, could cause the variations. As the wind moves the tree limbs the light hitting the sensor changes.