How do you lower S.D.?

Discussion in 'Reloading' started by KQguy, Feb 2, 2008.

  1. KQguy

    KQguy Well-Known Member

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    What is the biggest culprit of a large spread in S.D.?I am trying to lower mine on a 300 win mag load(180gr. SMK)I have tried IMR7828 and H4350,WLMR primers and CCI primers,various seating depths.The best I have gotten was about 60.I am shooting a Browning A bolt,maybe that is the problem?
    My groups are any where from 1 1/2''-2 1/2'' @ 100yds.I am sure I can tighten these groups up if I could get more consistant velocities,I am not sure where to start.
     
  2. britz

    britz Well-Known Member

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    If you want to reduce Standard Deviation you need consistancy. You might want to look at the rifle, if it is bedded and freefloated. I'm not sure if there is any one answer to your question. How do you prep your brass? Are you weighing each load of powder? Are your components stored properly? I don't load for the 300 win so I don't have any pet loads. I load for the 06 and 300 WSM and I really like H4350 for powder.
     

  3. KQguy

    KQguy Well-Known Member

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    I load as consistantly as I can,I use an RCBS charge master to dispense my powder,I trim all cases to the same size,and full length size all my cases.Maybe it is my gun,I didn't realise the gun could cause inconsistant velosities,I thought it was all in the load.I have a new savage coming next week,maybe if I run some of the same loads through it,that will answer my question.
     
  4. Willys46

    Willys46 Well-Known Member

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    Chrono?

    What Chrono are you using and was the light conditions changing while shooting. Mine will read different if the sun goes behind a cloud.

    Also, is there a load that is in the 90-95% densit (powder volume in Case). I have had better SD with full loads of powder. More uniform burn.

    I might be wrong, but the gun is going to have little to do with velocity spreads...BUt the group will suffer if not bedded correctly.

    One other thing i have read is using non magnum primers..I have not tried it in my 7 rem mag, but it was suggested to tighten groups up.

    Willys
     
  5. Natty Bumpo

    Natty Bumpo Well-Known Member

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    I've never owned or shot an A-Bolt, but I've heard they have thin barrels. Maybe if you give the barrel more time to cool between shots your groups will tighten up a little. I wouldn't worry too much about standard deviations. The load with the tightest velocity range won't necessarily be the one that shoots best in your rifle.
     
  6. comfisherman

    comfisherman Well-Known Member

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    double check the scale

    I would just for kicks check the zero on the scale and double check the powder weights. Some powders don't go through the chargemaster all that great, although I think 4350 would be fine. With the other variables youve eliminated by prep work its the only thing that would mess with sd.

    Unless your popping off rounds at a great deal of speed I don't think a thin barrel would make any difference in sd. The only way I think it could feasable would be if you rattled off some really rapid shots and heated up the barrel to the point that it heated the round and the powder to the point that its burn ration changed. I doubt that is a real big factor, although I could be wrong.
     
  7. goodgrouper

    goodgrouper Well-Known Member

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    I agree with the first part of your post but couldn't disagree more with the last part. If long range shooting is the goal, one must pay attention to the standard deviation of the load to avoid big vertical dispersions in group at distance. In fact, if using some of the longer range bullets that have long lengths, they may not shoot super tight groups at close range but if the standard deviations are low, they can shoot amazingly good groups at longer range.

    Btw, standard deviations would have very, very little to do with the mechanics of the rifle. Standard deviations deal with combustion and pressure among other internal ballistic properties. To get low Sd's, the brass must be uniform, have equal neck tension, and have deburred flashholes. Then the powder must be consistently charged, the primers seated same, and the bullets must be seated uniformly to maintain equal distance from the riflings. And even after all that had been done, the standard deviations can still be large if the wrong amount and/or the wrong powder was chosen. To fix it, go up in charge .5 grains until it improves. If it never improves, then switch to a different powder and try again. This should yield some load that has good sd's.
     
  8. britz

    britz Well-Known Member

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    I wasn't sure if you were refering to standard deveation in velocity or distance from aim point (should have guessed that you were talking about velocity though). I agree with the above posts that you should let the gun cool between shots. Also, I have noticed a significant difference in velocities from a bad batch of primers. I had about 25 cases primed with some mag primers and decided to fire the primers and resize the cases inorder to use those brass for a test using CCI BR2 primers in my mag. I fired the brass into a bundle of old ripped bluejeans. I was shocked to hear the poof on some and a bang on others and a crack on some. I guess my primers were old and exposed to some bad stuff. I had been disgusted with the results of my chrono tests last summer using that load and I think I know why the SD was so large now!

    I am only going to use fresh primers from now on.
     
  9. Augustus

    Augustus Well-Known Member

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    What kind of scope,rings and bases do you have. Did you mount the scope yourself. Have you been able to shoot consistant sub-moa groups with other rifles that is as raunchy as this one. Some of the little skinny barreled sporters need some work on the rifle itself. I wished i had a dime for every dollar i had spent trying to find what is causing a rifle not to shoot as well as it should. Ammo can cause some of it but I would just about bet your problem is in your set-up somewhere. Many of the rings and bases are just not up to the task of keeping everything pointed in the right direction on any rifle let alone one like yours. Everything has got to be tied down.
     
  10. KQguy

    KQguy Well-Known Member

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    What does the rings and bases have to do with S.D.?:confused:
     
  11. davewilson

    davewilson Well-Known Member

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    don't claim to have any 300WM experience, but i've heard a lot of people say Reloader 22 is the ticket for the 300WM. i would try a powder that is a tad slower than what might be optimum to get better load density. this usually helps with consistent velocities. i recommend H1000 for your application. on Hodgdon's site, it not only gives the best velocity, but does it with the least pressure.
     
  12. J E Custom

    J E Custom Well-Known Member

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    KQguy

    Standard Deviation spreads are effected by poor or inconsistant
    combustion.

    Some causes of this are low powder densities( 70% to 80%) wrong
    primer choice,wrong powder choice, inconsistant powder charge and
    poorly prepped brass.

    And good groups with poor SD,s spell disaster at long range.

    A good way to show this is run the balistics at the lowest velocity
    with that load and then run the same test using the highest.

    Just for kicks I ran the 7/08 at 3000ft/sec and added 60 ft/sec
    to see what the spread was. and it came out to 14.2" @ 1000 yrds
    under perfect conditions (enough for a bad hit or a complete miss at
    long distance).

    In cases with around 100gr or less capacity I have found times that
    going to a standard rifle primer improved SDs but they also lost some
    velocity (25 to 50 ft/sec).

    Good luck on improving SD,s
    J E CUSTOM
     
  13. Augustus

    Augustus Well-Known Member

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    In answer to your question, Sd has nothing to do with rings and bases however you indicated in your original post you were not happy with the groups you were getting at 100 yds. Tight SD does not equal tight groups at 100 yds. Long range tight SD is a must. What good quality scopes, rings,bases,proper bedding, good barrel etc has is the power to make you smile or frown when you go down to look at your target. Why did I ignore your question about lowering your SD? Because you asked it in the context that you wanted to improve your groups, I suspect if you find a load with an SD of 0.00 you will still have the accuracy problem. Sorry if I offended you.
     
  14. Augustus

    Augustus Well-Known Member

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    In answer to your question, Sd has nothing to do with rings and bases however you indicated in your original post you were not happy with the groups you were getting at 100 yds. Tight SD does not equal tight groups at 100 yds. Long range tight SD is a must. What good quality scopes, rings,bases,proper bedding, good barrel etc has is the power to make you smile or frown when you go down to look at your target. Why did I ignore your question about lowering your SD? Because you asked it in the context that you wanted to improve your groups, I suspect if you find a load with an SD of 0.00 you will still have the accuracy problem. Sorry if I offended you.