Help with velocity variance

Discussion in 'Reloading' started by Striker77s, Dec 29, 2010.

  1. Striker77s

    Striker77s Member

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    I just started reloading about 4 months ago for my 300 win mag. I would like to get into long range shooting and I’m 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’ve kept the brass and now I’m reloading it. It was 25[FONT=&quot]°[/FONT][FONT=&quot][/FONT]
    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’m shooting at a elevation of 7000 ft. Here are my results.
    FC at 25° (3152, 3180, 3214)
    FC at room temperature (3151, 3151, 3145, 3185)

    W-W at 25° (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’m 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’t annealed the brass.
     
  2. SBruce

    SBruce Well-Known Member

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    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: Dec 29, 2010

  3. Striker77s

    Striker77s Member

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    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
     
  4. SBruce

    SBruce Well-Known Member

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    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: Dec 29, 2010
  5. Mikecr

    Mikecr Well-Known Member

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    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.
     
  6. WRG

    WRG Well-Known Member

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    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!
     
    Last edited: Dec 29, 2010
  7. nheninge

    nheninge Well-Known Member

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    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.
     
  8. boomtube

    boomtube Well-Known Member

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    Neither your velocity spreads nor case variations are off base.

    Temp varations are most effective in beer sessions around the campfire, there's much less effect on the range than common BS would have you believe. It's the burn temp of the powder that matters and a few degrees of ambient temp really doesn't change as much as some folks say. I suspect a LOT of the variations we hear about actually come from other factors.

    As suggested above, put the chronograph at least 10 feet from the muzzle for small cartridges, go 15 for those as large as yours. And, if you want your data from different trips to mean anything, keep the distance identical each time.
     
  9. Boss Hoss

    Boss Hoss Well-Known Member

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    I beg to differ sir---have done the testing in real conditions not a simulation as was done here. It does make a difference with in some cases it gets ugly. At long range a 60FPS ES will mean a not winning. So many factors go into this it would take 15 minutes to type it out but in short just trying to heat the ammo does not work. Ambient is the key---everything must be the same temp but hey if it makes people feel good by testing this way then have at it.
     
  10. Striker77s

    Striker77s Member

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    I am new to reloading so I fully admit I have a lot to learn. That is why I'm trying to test and take a systematic approach. So much of what is said in these forums are based off of gut feelings from personal experience. I would be wise to listen to such experience since I'm so green, but occasionally such gut feelings are wrong. I'm a scientist so I admit I have a bias towards controlled systematic experiments.

    When I did my last test a large snow storm was coming and I was in a hurry to shoot and get out of there. So I didn't have the luxury of time to really spend time on my experiment. From my background in physics it seems to me that Boss Hoss is right in regards to velocity, it does matter. Almost everything we do as reloaders is to produce consistent bullet velocity. All the ballistic calculations are based on physics to predict the behavior of the bullet. The behavior of the bullet depends upon many environmental factors (wind, outside temperature, humidity, etc), angle, bullet design, and velocity. Outside of having a quality firearm (barrel, bedding etc) the factor we try to control as reloaders is velocity. Annealing, brass volume, brass strength, brass shape, primers, powders, flash hole consistency, cleaning, cartridge temperature, etc all lead to better consistent velocity. I'm sure there are a few but the only factor I can think of the effects the bullet flight beyond velocity that reloaders mess with is meplat trimming. If having more consistent velocity doesn't lead to better groups it isn't because velocity doesn't matter it is because others factors are causing problems that mask the velocity improvement, such a barrel wear, barrel harmonics, bullet design etc.

    I have looked for data on how each one of these reloading techniques effect the velocity in relation to each other but I can't find it. For example a list in order of impact on velocity of each reloading factor, reloaders could decide which factors to worry about first. As a shooter gets more serious they can start worrying about the minor techniques. Right now as I read and ask for answers I always get conflicting responses, some saying US cleaning of the interior of the brass matters and other saying it doesn't, some saying a low velocity SD matters and others say it doesn't. I'm sure they all matter to some degree the question is how much. Which factor are the most important in relation to others. I bet a few of the more experienced reloaders could come up with a reasonable list and I would love to see. But what I really want to see is a controlled experiment where each factor is tested and rated on how much it impacts the velocity. From there shooters can use ballistic software to see if at the distance they are shooting to if it is worth it to deal with.

    Now in the end I wish to state again that I'm green and inexperienced. All of what I said may be a bunch of garbage and way off. I do sincerely value and appreciate those who have taken the time to respond. I read each post carefully and compare it with other posts to decide what action to take next.

    Thanks again for all the generous help given. For example those who pointed out the fact that I need to put my chrony even farther away and that my car heater attempt to heat up the cartridge was flawed is very helpful. I will try to correct it the next time I do it.

    Thanks,
    Mark
     
  11. WRG

    WRG Well-Known Member

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    If your talking long range hunting 300 yards and beyond, it does. I thought you were talking about normal hunting distances..sorry about that. The further you need to shoot the more theses variables effect accuracy and certainly your SD in velocity will have a great impact. Simple logic tells us that the POI has to be effected if your not at or near the same velocity every shot. However, under 300 yards there is little if any effect.

    May I suggest you look into a copy of Quickload to help you close up that SD. You don't neccessarally need to pay this kind of money "$150" for a ballistic program but it is the best I have found and I don't regret buying it myself. It comes with Quicktarget that is all tied into the loads you develope with the software. If you decide to download the sample version before hand, it doesn't do the full program any justice. But the full version would be a nice addition to your library!

    Good luck.
     
  12. boomtube

    boomtube Well-Known Member

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    "From my background in physics it ..."

    Okay, with your background in physics, convert the temps you tested to absolute temps and calculate the percentage of potential pressure change. And then consider that the temp:pressure variations you calculate are static, they will be valid ONLY for the ignition phase, just a few nanoseconds. The rest of the massive delta temp:pressure rise will be the normal powder burn temps for your load. The end temp is really hot by anyone's standards so the start temp is a fairly minor factor!

    So, yeah, sure it "matters." But not a lot.

    Again, physics for the chronograph/muzzle distance thing; Air in the bore is ejected in a straight line ahead of the bullet. That puts a very high density blast of air, a shock wave, out of the bore before the bullet exits. The chronograph's photo cells look for a change in light intensity to trigger the start count. IF the frontal density change in that air blast causes the cells see a light change we will get false triggering; not good. The photo screens have to be far enough from the muzzle to let that air blast dissipate and let the bullet do the job; 12-15 ft is usually okay.
     
    Last edited: Dec 30, 2010
  13. cowboy

    cowboy Well-Known Member

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    Couldn't agree more with SBruce's statement. I like to place my chrony at 20'0" from the muzzle. You can easily then go to a ballistic calc. and back up to get your muzzle velocity. How sturdy is your tripod - any movement at all raises he**. Never noticed where you are from but was there any snow cover under your chrony? This at times has given me readings on a tried and true load where I was getting really poor ES etc. Solved that by laying a tarp down when we have snow.
    Keep digging and when you get all the answers it'll be just in time to be able to tell them to your great great grandkids. Fun sport isn't it. Good luck.
     
  14. Striker77s

    Striker77s Member

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    All great comments, thanks again for your help. Clearly I need to move my Chrony back and try again. Unfortunately minutes after I finished the last test a snow storm moved in and dumped 8" of snow. So I won't be shooting for a while. I'll have to watch for any snow under the Chrony when I do finally make it to the range.

    Boomtube: You are right in that fact that the delta in the gas temperature change is much larger than the initial temperature, hence the initial temperature gets lost in the noise. However I think the temperature impacts the powder burn rate which lasts up to 1ms. So it is a chemistry issue not a gas expansion issue. Which makes it much harder since I'm not a chemist. Each powder reacts differently to temperature variations. I chose Hodgdon 4831 since it is supposed to be temperature insensitive, but I've seen data from other shooters that suggest even temperature insensitive powder can vary the velocity at extreme temperatures. I wasn't thinking at the time but this is why my car heater experiment was flawed. Boss Hoss said that the cartridges have to be at ambient temperature for a while. I'm guessing it is because you aren't worried about the brass or air temperature but the powder temperature. It takes a while for the heat to flow to the powder even after the brass has been heated from a car heating vent.

    WRG: Right now I'm more interested in long range target shooting. Eventually I would like to start shooting at 1000 yards but I'm still working on 300. I'm not happy with my loads yet to bother going to 1000. Quickloads looks very interesting, thanks for the tip.