Velocity swings

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by shortpants, Mar 1, 2011.

  1. shortpants

    shortpants Well-Known Member

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    Just wondering how big of a swing you guys consider to be acceptable in your velocities? I'm testing different loads for my hunting rifle. I would like to find something that will shoot consistantly out to 800 yds. So far I have just shot for groups at 300 and to check velocities. I have some that group better than others but they have a much bigger velocity swing and I wonder what will happen when I try them at 800? Any input???
     
  2. Michael Eichele

    Michael Eichele Well-Known Member

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    You will get many answers here. My opinion is that for UPTO 600 yards 30FPS is acceptable IF the load to barrel harmonics are working well together. Harmonics will definately play into where your bullets will land on the target even with fairly hefty velocity differences. This does however have it's limits. For 600 yards you might not see any difference. 800 will still group fair and at 1K 30 FPS will be quite noticable. With good harmonics, you can shoot very tight at 600 yards even with 25-30 FPS extreme spreads.

    A quick look at a ballistic calculator will show you how much 25-30 FPS will wreak havoc on your 1k groups. The paper targets will verify this as well.

    I am spoiled with the Edge. I get about 8-12 FPS extreme spreads regularly where my 308 loads are all over the place. Anywhere from single digits to 30 FPS. At the lower 308 velocities.

    Inconsistent neck tension, varrying neck hardnesses, charge consistency, charge to primer relationship, rifle/burn rate/charge relationship, case volume, ogive to base differences, primer flash hole uniformity all among other things will play into extreme spreads.
     
  3. MontanaRifleman

    MontanaRifleman Well-Known Member

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    I think Michael is giving you some very good advice. But... you need to take your chrony readings with a grain of salt. It might show yo a 30 fps spread when in fact it's really a 10 fps thread. When I got a second chrony and set them up back to back, their reading varried anywhere from 5-60 fps difference on the same shot string... a real eye opener. Bottomline, if your load is grouping well go with it. If you have significant vertical dispersion at extended ranges then you probably have a velocity issue.

    I would be very hesitant to throw away a good load based on what my imperfect chrony was spitting out. Shoot some down range groups and see what that's telling you.

    -Mark
     
  4. Michael Eichele

    Michael Eichele Well-Known Member

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    Mark, what kind of chronies are you using? The reason I ask is because I have been shooting over 2 chronies of two different makes in tandem and I am getting readings with 10FPS of eachother. Sometimes identical, sometimes up to10FPS apart and anything between. When I run my edge loads over one or both, my extreme spreads vary between 8-12 FPS. Even on the same day between the 308 and Edge my Edge is really consistent and consistently low where the 308 will be all over the place. Between 0 and 30. So I was wondering which chronies you are using.

    M
     
  5. MontanaRifleman

    MontanaRifleman Well-Known Member

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    Michael, I have the Pro Chrono which is about 16 years old and the Alpha Shooting Chrony which is about 2 yrs old. The Pro chrono "seems" to be more accurate based on published and factory loads.

    My approach to this is to work up a good load based on 200 yd groups to give the bullets a chance to go to sleep. Any bad grouping loads we can throw out there, because no matter how good the ES, if it doesn't group well at 200 it wont group well at 1000. If it groups well at 200, send a couple of strings down range on a calm day and see what the result is, if your getting less than .5 MOA verticle dispersion, it's probably a good group. Make sense?

    Not all chronys are created equal and most people don't have the top of the line, so I think it's a safe bet to test what appears to be a good load @ 200 yds, downrange. If a chrony has a margin of error +/- 1/2%, at 3000 fps, that's 30 fps.
     
  6. Michael Eichele

    Michael Eichele Well-Known Member

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    That is very similar to how I 'work up' loads. I typically will start at 100 with the use of a chrony to make sure there are no red flags in regards to excessive velocities. After I have a 'range' of charges that are acceptable I will move to 300 and test my groups in .5 grain increments. I still shoot over the chronies and document the results but I learned along time ago that you cannot get excited about 20-30 FPS extreme spreads. Some of the best grouping loads have been in this range even out to 600+/-. At 1K though, good harmonics or not, there is pretty notable vertical spreads when dealing with 30 FPS spreads. It seems though that the higher BC bullets are a bit less affected but still notable. All that said, I still am of the opinion that good harmonics are worth more than low ES's but having both is much better.
     
  7. shortpants

    shortpants Well-Known Member

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    I was testing a few different factory loads. Two weeks ago I fired just a couple of rounds of each just to get a rough estimate of velocity so I could compare them on a calculator. I went back out Monday and the first thing I noticed was the HSM VLD's were reading over 100 ft. per sec. faster than 2 weeks ago. LIGHTING CONDITIONS WERE THE SAME AND CHRONY WAS UNDER SHADE THE ENTIRE TIME. Then when I went to test the E-TIP's they started out the same as 2 weeks ago (3150fps.) but I had 1 shot that read 3277fps!!! The next shot was back down around 3180??? I realize the 3277 might have been something weird with the chrony but do you think premium factory ammo will differ that much? If I wasn't getting such fast velocities out of these I'd be quick to toss them out but the ballistics are amazing out of a factory round if they are in fact more consistant than the readings. I know what your going to say try a different chrony and shoot some more. I'm going to do just that but in the mean time just thought I'd see what you guys think? After the E-TIP's I tested the Remington Scirrocco's and the velocity swings were less than 30fps. Can the chrony be accurate 1 minute and not the next when the lighting is the same? If the E-TIP's turn out to be inconsistant then I'll toss 'em but it sure would be nice to get 3150fps out of a factory 180gr. 300wsm!
     
  8. MontanaRifleman

    MontanaRifleman Well-Known Member

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    Shortpants, that is amazing velocity for factory 300 WSM loads. The best (average) I ever got was about 2990 with 180 AB's. With handloaded 180 E-Tips and RL17 I was able to get 3193.

    As for what load to keep and what to toss, I would base that on actual group results. First at 200-300 yds then as far as you plan to shoot.

    I still use the chrony as a general reference but I don't keep or toss a load based on it's readings.

    Something else to remember is if there is even a slight wind blowing, it can sometimes cause pockets of up and down drafts over uneven ground that will affect your vertical component. I've experience this on a couple of occasions. It's best to do this type of testing on a clam day.

    -Mark
     
  9. shortpants

    shortpants Well-Known Member

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    Thanks Mark for the reply. I am definatly going to keep the one that groups the best and these tests were done at 300yds. Right now all of them are grouping 1moa or less so I'm not ready to toss any of them yet but if it groups well at 300yds. but varies in speed by 100fps then I'm pretty sure I'm going to stuggle to shoot moa out to 800. Winds never seem to stop completly around here but my testing is done with winds under 5mph and I wait for it slow before breaking the shot.