Sierra 180gr SP, cut wind like butter?

Discussion in 'The Ammo & Reloading Forum' started by The_Rifleman, Feb 17, 2010.

  1. The_Rifleman

    The_Rifleman Active Member

    Feb 10, 2010
    Keep in mind, where I take the shot from, the wind is strongest, and as the bullet travels the wind speed decreases.
    Every time it was kinda windy, I would shoot a 300 yard shot, not a 400 yard one, so it has been maybe 2 weeks that i have been shooting 400 yards in whatever wind condition and with the 30-06 i was using the 220s exclusively. So I am new to using the Sierra 180 SPs in wind at 400 yards.

    Today I took a shot with a Hornady 220gr RN at 400 yards in about 10mph 10:30 wind. My ballistics card fell out of my lens cap for the 220s, therefore I held left for the data for the Sierra 180gr SP bullets I have. Which I held over 8" left while the wind blew it 16", missing the center of the target by about 6" right.

    That is all good, that lets me know I made a mistake and my set-up is still working fine, I am the weak link.

    Then I annealed the cases, and loaded up some Sierra 180gr SP bullets, I did everything as I normally would...
    I was going to wait until Wednesday to get a zero at 400yards with them but there was a snow storm and I just couldn't help myself.

    So I took 3 shots with the Sierra 180gr SPs at 400 yards. The first shot I aimed a little left of the target, when I inspected it I did find a mark in the snow at the bottom just above where I aimed. I put the backing up so I would see where the shots landed and took the next shot a little more left than before; I thought I must have hit too much right. The rest you can see on the video.

    I have checked and double checked everything, the rifle is perfect, the scope is solidly on the rifle, everything seems wonderful with the whole set-up.

    If I would have went for a group, (without adjusting the scope higher 3 inches on the last shot, and the holding left different distances,) I think all the bullets would have fell less than 4 inches apart. So it seem that the bullets are flying well, the scope isn't malfunctioning, and the rifle is doing what it should...

    Why then are the bullets not drifting in the wind? You can clearly see the wind blowing the snow.

    Here is the bullet info.
    Sectional Density: 0.271
    Ballistic Coefficient:0.407

    I have noticed, the bullets are flying better or right with my Hornady 190grSPBTs at up to 600 yards.
    Sectional Density: 0.286
    Ballistic Coefficient:0.491

    And they really shouldn't be.

    My questions...
    Of those familiar with these bullets, are yours flying this good?
    Or is it more like some weird fluke that these fly left more than the 220gr?
    End questions...

    It is probably something to do with the scope or something. It is really calm out there tonight, so this morning hopefully it will still be calm, then I will take a few shots and see what the results are.

    Very confusing... I'll update tomorrow morning with my results.
    Last edited: Feb 17, 2010
  2. JLA

    JLA Well-Known Member

    Feb 26, 2007
    Heart Of Texas
    Thats a good inquiry rifleman, I load sierra 174 gr matchkings for my .303 enfield for 300 and 600 yds. They dont seem to drift nearly as much as the 174 gr Hornady RN for the same cartridge and arent near as accurate as the Sierras. Keep us posted on what you find...

  3. The_Rifleman

    The_Rifleman Active Member

    Feb 10, 2010
    When I adjusted my scope 4" to the right, I remembered when I last adjusted the left/right adjustment; when I started using the 220gr bullets again. It doesn't make sense to me, but I think those bullets fly differently than the others. Maybe someone will be along that can explain it.

    But, tomorrow I think I'll be on target again!

    180gr Sierra, Scope Off; NO, Gyroscopic (spin) Drift

    (About 4 hours after I wrote the above.)
    I figured it out!

    I couldn't sleep thinking about why there is an at least a 4 inch difference at 400 yards. There had to be a reason, and I had to know that reason!

    Barrel twist!

    That is what it is! Barrel twist!

    ***********************incorrect info******************
    The 220 RN bullets are in the air almost twice as long as the 180s and the round nose slows it down considerably. So the 220s spin many more times. Coupled with the more spins and the greater area spinning, I think a 4 inch difference at 400 yards may be in order.

    The comment above, "when I last adjusted the left/right adjustment;" is what got me thinking. I made a 2.25" left adjustment when I switched from shooting the 190s to the 220s at 300. That is 3 clicks, one more click would put a bullet that gets there a little faster, (180s,) centered.

    I thought to myself, which way would the twist need to turn for my theory to be correct. I thought the bottom of the bullet would have more resistance/friction. Therefore the bottom would need to be in a left direction to force it right. If my twist was clockwise, my twist theory would have to be the factor. I couldn't remember of which way it turned so i got up and tested, Sure enough it is a one in ten clockwise twist!
    ***********************incorrect info end******************

    I always knew about the twist making the bullets trajectory head in a lateral direction, but I never seen any numbers on the subject or a detailed explanation; so I thought it was a negligible amount. It seems as if I was in error!

    [EDIT] As I am uploading the video, I am searching for info on bullet drift and twist rate. If I find some info I'll post a link to it.
    Found one! (Example: A .308 Win shooting a 175 Sierra MK in a 1:12 barrel will experience about 8" spin drift in 1000 yards; the same bullet will drift 12" (a 50% increase) in a 1:10 barrel.)
    It doesn't say which direction but I think I must be correct.
    I am correct, Bullets fired from right twist barrels drift to the right, but for the wrong reason. It makes better sense now. Much more drop, much more drift.

    I love missing targets! I learn so much!

    Guess what?

    All my 600 yard shots were all right about the same amount, (around 6 to 8 inches from center.) I was kicking myself in the behind for not calculating the wind drift correctly. Now I suspect it may have been a little wind and a lot due to twist!

    I would have never came up with this epiphany if it weren't for me deciding to be more precise in my shooting at these shorter distances to get better at longer distances. 4 inches difference at 400 yards, most people wouldn't have thought anything of it.

    I'm going to hit that paper plate at 600 yards next time, you just watch!

    Gyroscopic (spin) Drift, who could have guessed?
  4. FTK87

    FTK87 New Member

    Mar 1, 2009
    Chouteau, Oklahoma
    I belive you figured it out. I just watched a sniper show on the history channel yester day and they were talking about spin drift, they're illistration showed the bullet spining clock wise and the bullet climbing up and to the right because of it(only slightly up, with gravity and all ;) )
  5. The_Rifleman

    The_Rifleman Active Member

    Feb 10, 2010
    I knew there was a reason as to why I should be watching more TV! If only I knew that as a kid. ;)

  6. JLA

    JLA Well-Known Member

    Feb 26, 2007
    Heart Of Texas
    ill be durned. Thats why my enfield seems to shoot better when the wind is out of the left of the range. 1:10.5" Lh twist.. damn things drift like crazy when the wind is out of the right of the range Spin drift pushed by wind drift. When its blowing from the left it counters the spin drift and my hold over isnt near as much. I thought it was just a fluke deal, but the flags are always pointing right when my shots hit center... Holy crap... Thanks rifleman;) I woulda never considered it a factor....
  7. The_Rifleman

    The_Rifleman Active Member

    Feb 10, 2010
    This is why I like to post range reports as I was saying in my introduction post. You never can tell when a scrap of information can be of help to others.
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