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Thread: Hotter, Flatter, Higher, Lower?

  1. #1
    Member 8x57 Mauser's Avatar
    Join Date
    May 2006
    Southeast Alaska

    Default Hotter, Flatter, Higher, Lower?

    I'm having a physics failure.

    You guys are gonna think I'm a nitwit, but I can't get my head wrapped around this.

    Starting charge working up a handload put the center of the group about 3.5 inches above bullseye. Increasing the charge by a grain of powder lowered the center of the group to 2 inches high.

    This proceeded until the max load centered just about an inch below bullseye. All groups were fired from sandbags at 100 yards. No adjustments were made (scope, primer, seating depth - nothin'.)

    I'm confused. Generally speaking, more powder should equal more velocity. Greater velocity should equal decreased bullet drop over a given (fixed) distance. Doesn't that mean the point of impact should go up instead of down?

    Does this have something to do with the mass of each extra grain of powder? I have a feeling when you folks explain it I'm going to slap my forehead and say "D'oh!" but I just can't figger it on my own today.

  2. #2


    your thinking is totally logical but you are for getting that the rifle barrel is a moving insterment. barrel vibrations are changing, so your speed sould go up 500fps and you group move is all in the harmonics of the barrel.

    once you have a load you like, move the scope and you will be good to go, if you try to move your load to your scope you will pull your hair out before it happens

  3. #3
    Join Date
    Jan 2005


    Barrel vibrations do play a part in point of impact but I think there is a simplier answer.

    It is not uncommon for short barreled or light barreled rifles to have POI higher with lower velocity loads and lower POI with higher velocity. This is generally from a light grip on the forend and the bullet spending more time in the barrel and more time for recoil to lift the barrel before bullet exit. This is a common cause of vertical stringing of groups. What downward pressure we put on the forend must be consistant from shot to shot. This is very common with handguns, heavier slower bullets hit higher, as most of us know. But it shows up with rifles as well, particularly in rifles that shoot at generally lower velocities. (1800-2400fps)

    One of the reasons we test powder charges in a linear, systematic fashion, just as you are doing, is to watch this transformation of both accuracy and combustion efficiency of the loads come together. We can actually see the groups get smaller and in some cases get larger again as we go up a grain or half grain at a time. When your sights are on the paper don't worry about where they hit just shoot good groups. There are many things at work here and what we want to find is that sweet spot of barrel, bullet and velocity that works best. Barrel harmonics are invisible little gremlins that can make or break any and every shot. There are good gremlins and bad. Once we eliminate the bad ones we're on our way to a good load in a good rifle. However, the biggest and meanest gremlin of all is the shooter. It is the shooter that by bad technique can throw even the best aimed shot. Gremlins so subtle as muscle tension and so obvious as a spastic trigger finger can haunt us until we master them.

    You're on the way. You are observing the results and that is half the game. Keep shootin'.
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?

  4. #4
    Join Date
    Feb 2007
    Kodiak, Island


    Murphy is dead on.

    Find the group you like and load your rounds. Remember what he said and always hold the rifle in the same way. Best to bag it deep and not touch
    the stock forward of the trigger guard. If the rifle is not free floated then the scenario will duplicate itself more often without help. Such as changing temperatures. Where you put your hand on the forend. # 1 culprit for your problem in my experience is non-floated barrel and resting your free hand on the scope and across a solid object. ONE CAN LITERALLY CHANGE THE poi every shot if he wants to.


  5. #5
    Join Date
    Jan 2007

    Default impact height

    Was the powder charge compressed as you worked up?

    This can affect velocity.(like early .458 win loads that failed in Afrika)

    The other statements earlier are true as well about barrel harmonics.

    There is also barrel temp and barrel memory. As a barrel heats up they return to original condition before they were straightned. (cryo fixes this)

    Were all loads shot in the same day? Humidity and temp and barometric pressure affect this as well though they would have to be way different for that much change.

    Also what is your barrel length? Maybe you need a faster powder if you have got a short barrel. Sometimes a short barrel reaches max effieciency with slower powders sooner and more powder only gets in the way and is blown out as unburned propellent.

    lots of weird things to consider. Just trying to give you as many angles to look at as possible without having all the facts myself to help you solve this as fast as possible.

    Hope this helps and hope you solve it soon.



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