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Help with velocity variance

 
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  #1  
Old 12-29-2010, 06:34 PM
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Join Date: Nov 2010
Posts: 15
Help with velocity variance

I just started reloading about 4 months ago for my 300 win mag. I would like to get into long range shooting and I知 slowly building up my equipment list to get me there. I received a chronograph for Christmas and today was the first chance I had to try it out. In the past I purchased some cheap Winchester and Federal ammunition. I致e kept the brass and now I知 reloading it. It was 25
Fahrenheit outside and my ammunition was in the garage so it was at outside temperature. I thought it would be a good opportunity to test how sensitive my powder is to outside temperature. I setup my chrony on a tripod and began to shoot through it. On the second shot the wires frame flew apart. My heart dropped as I figure I had just shot my brand new chrony. After finding the part and no nicks on it I put it back together and with each shot I could see it move and three shots later it came apart again. I then realized that since my Chrony was only 5 feet from the end of my barrel the Chrony was getting blown apart from the exhaust gases. I decided to put on my muzzle brake which ended any movement on the Chrony. I heated several cartridges by using my vehicle heater to approximate room temperature. The Federal brass is stamped with FC and the Winchester brass with W-W, so I will refer to them as such. I reloaded 155 grain Berger VLD bullets with 76 grains of Hodgdon 4831SC. I was expecting a velocity around 3050 fps from Hodgdon reloading data. I have a sensitive electronic scale and measured each load of powder very carefully and I知 shooting at a elevation of 7000 ft. Here are my results.
FC at 25ー (3152, 3180, 3214)
FC at room temperature (3151, 3151, 3145, 3185)

W-W at 25ー (3155, 3150, 3122)
W-W at room temperature (3148, 3146, 3124)

I was surprised at the higher than expected velocity and the extreme spread (ES). Due to the large variance there is no way to say if temperature affected the velocity. I decided to measure the weight of each case to see if it indicates a large variance in the volume of the cases. Here is what I found in grains.

FC (257, 256, 255, 258, 245, 256, 255, 255, 257, 253, 254, 257)
W-W (244, 240, 242, 245, 243, 244)

After that long diatribe I知 interested if anyone has suggestions for why the velocity varied so much. Is the case volume or could it be something else? This is the 3rd time this brass has been shot and I haven稚 annealed the brass.
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  #2  
Old 12-29-2010, 07:28 PM
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Location: Thunder Basin, WY
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Re: Help with velocity variance

Question: Did you weight sort and uniform any of the brass prior to testing?

If not, that's likely the problem. If that's the case, I'd try that before the annealing. There appear to be some weights of cases that I would sort out if it was me.

What kind of primer are you using? Cooler primers often give lower ES's. Due to the higher than expected velocity you mentioned, I am assuming you're using a fairly hot primer..........magnum primers by any chance? Match primers are usually some of the cooler ones and more consistent. Even normal large rifle primers might be more consistant.

Neck thickness can be a factor also. If the neck thickness is uniform, you'll get better ES's too. This also applies to neck or case length. Uniform lengths are important, as are uniform primer pockets and flash holes.

IMO, these steps all contribute to low ES's and consistency of your loaded round.

IMO, annealing cases that aren't consistant size and weight is a waste of time.

Last edited by SBruce; 12-29-2010 at 10:46 PM. Reason: I now see you did mention the cartridge
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  #3  
Old 12-29-2010, 07:57 PM
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Re: Help with velocity variance

I did not sort any of the brass by weight before shooting. On the Winchester W-W brass the weight varied by 5 grains but the velocity ES was 33 fps. How tight of a variance on the brass weight is acceptable?

I did size and clean the primer pocket by hand. No tumbler or ultrasonic cleaner. I am using CCI magnum primers.

I just ordered 200 Remington brass cartridges from Midway since they were on sale. I'm hoping that standardizing the brass will help instead of using once fired brass from multiple brands. I'll post after a few weeks to say if the brass made a difference.

Mark
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  #4  
Old 12-29-2010, 08:32 PM
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Re: Help with velocity variance

Quote:
Originally Posted by Striker77s View Post
I did not sort any of the brass by weight before shooting. On the Winchester W-W brass the weight varied by 5 grains but the velocity ES was 33 fps. How tight of a variance on the brass weight is acceptable?


I did size and clean the primer pocket by hand. No tumbler or ultrasonic cleaner. I am using CCI magnum primers.

I just ordered 200 Remington brass cartridges from Midway since they were on sale. I'm hoping that standardizing the brass will help instead of using once fired brass from multiple brands. I'll post after a few weeks to say if the brass made a difference.

Mark
33 fps is pretty good for most normal cartridges. I've always had good luck with Winchester Brass.

For a large case, 5 grains is not alot, but I try to sort to within +/- 1 to 1.5 grain on smaller cases........or about 1 percent. I only do this after the cases are uniformed in length and size, and after primer pockets have been cut to uniform depths and flash holes have been de-burred.

The new brass may help alot, just be sure to get it all as consistent and uniform as possible before loading it. If you're still have issues with new brass, try a different primer.

I don't know how accurate your chronograph is, but even 99.5% accurate means possibly 15 fps variance on a round that shoots the velocity you mentioned.

One other thing, try to get your chronograph about 12-15' from the muzzle. If your chrono is moving as the bullet passes through it............ it's probably gonna give some errors.

Best o Luck.

Last edited by SBruce; 12-29-2010 at 10:47 PM. Reason: I now see that you did mention the cartridge
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  #5  
Old 12-29-2010, 09:44 PM
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Join Date: Aug 2003
Location: NC, oceanfront
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Re: Help with velocity variance

Quote:
Originally Posted by Striker77s View Post
I知 interested if anyone has suggestions for why the velocity varied so much
Cause your chrono is lyin on the ground over there buddy...

I think you're doing pretty good considering all the chaos in your process.
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  #6  
Old 12-29-2010, 10:33 PM
WRG WRG is offline
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Posts: 133
Re: Help with velocity variance

It has been my experience that searching for the lowest SD has no correlation to accuracy. I have had loads with as much a 50 fps out perform loads I have that run in the low teens. I use to spend more time trying to tune my loads to have the smallest SD, some are under 5 fps and are no better. If you can get away with just neck sizing that 300 mag, I would suggest measuring a fired form case for volume "H20" with the bullet seated where you think is a good place to start. Then worry about trying to get the best case fill that will give you the desired velocity your looking for. As long as you understand what the harmonic nodes / barrel timing are for that barrel you should have a target velocity in mind. You can do the same if you have to FL size also but I prefer to neck size as long as I can get away with it in my 300WM. Sure having a low SD with a super accurate load is great however you could end up beating your head silly trying, especially in that 300WM.

oh, foregot, most chronographs should be at least 8'-10' from the barrel. You are most likely getting interferance from the muzzle blast at 5'. Try moving it out further.

IMAO!
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Last edited by WRG; 12-29-2010 at 11:00 PM.
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  #7  
Old 12-29-2010, 11:25 PM
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Location: Bryan, Tx
Posts: 284
Re: Help with velocity variance

Waaaaaaaaaay to close to chrony. Each time it shakes you get false data. Move at least ten feet out, and 12 to 15 ft with muzzle brake. Simple things first.
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