I have found over the years that solid is a must for long range. This is even more so past 1000. It all started while I was in the midwest and was shooting prone off some wet soggy pasture one day. My bipod was getting a little deeper with each shot. My groups were noticably larger that day. Later it showed up again with a horrid group when I rested my rear bag on top of a shooting mat. Removing the mat tighten things up a lot especially in vertical. Since then I have played with this a bit and have found this to be a huge help to me, especially at ELR. I do these things, once prone I take my left hand and grab the bipod legs and scratch them into the dirt to get them on solid ground. I only use a dense rear bag and set up to be low on the target and work the rifle butt down to be as solid as possible. You won't find any sort of mat or padding under my rifle or bags. I want them solid as can be in the dirt. From this point I ever so slightly pre-load the bipod forward just a bit, just enough so I can feel the recoil pad on top my shoulder. Then I check my cheekweld like you would with your anchor point on a bow. I have found that when all is right I can feel it is and becomes a confident feel for me with consistant practice.
Another point along this line is that while attending the Defensive Edge long range class Shawn made it known he is very fond of pod locks. I think this is also to keep things solid durring recoil.
One of my shooting buddies, "Montana Marine" has been using the aid of a monopod on the rear of his stock to stiffen things up. He shared with me at our last long range shoot that it has noticably reduced vertical spread for him.
So, I just wanted to pass this along. Take it or leave it. But for me I can not get things solid enough and the more rock solid the better.
Broz, You've covered an aspect to accurate shooting that I've been wrestling with for a while now (usually dismally unsuccessful I might add); a firm, unmoving setup. The weak point in my technique has usually been supporting the butt of the rifle. The bags I've tried with the exception of one, are too soft, too small or just the wrong shape or size. Doesn't matter how tight the bag is held there is still some movement - perhaps creep would be a better term. The one bag I do like is an old leather benchrest bag I filled tightly with a mixture of #8 lead shot and used corn tumbling media; still way too heavy to carry in the field while hunting or practicing. Some of my other bags are light but have a density resembling a baggie full of jello. In a word, useless.
I've also tried using plastic ammo boxes and blocks of wood. These are perfect when they are the correct size but can be a bunch of trouble when they are not.
Might I ask what rear bag you are using? Do you know what it is filled with and is it filled to the max or is there some space left.
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My favorite bag is a RedTac I purchased from Defensive Edge. My second favorite is one, I made from a led shot bag I filled with rice. I sewed a 1" wide seam in the middle with no rice to allow it to be doubled if needed. I like the bags that are filled pretty full with less space to shift. Also if needed I will grab a flat rock for a platform to lay the bag on if more height is needed. Where I live rocks are not usually far away. A rock scratched into the dirt a bit makes for a good solid rest to place the bag.
Broz, Great post for a while now I have been using the same techniques you have described to ensure I have a solid rest. It took me a few years to figure out what worked best for me but there was a noticeable difference in long range grouping. It seemed the more solid I was the more my groups shrunk, along with a consistent shooting position and technique, and something else that seemed to make a large improvement was applying the right amount of pressure of the butt stock to my shoulder. A few years ago I was just not holding the rifle tight enough and it showed at longer ranges. Similar to your experience I was awakened when I was shooting with my bi pod legs sitting in sand and sinking more with each shot. For me this also went the same way if I had something like a jacket or shirt under my rear bag instead of solid ground, it just was not as solid and my hold was slightly more inconsistent vs with solid ground under the bag.
I have found the same effect on 700+ yard, long range shooting, particularly the loading of the bipod legs with forward pressure. I use a Harris 9-13 notched leg, swivel model and found that in order to sufficiently load the legs with forward pressure, I have to have the legs extended at or near the fully extended position. At shorter lengths the legs will not flex, and the rifle will be subjected to whatever surface irregularities the bipod base pads are subjected to, including the spring loaded, low position of the legs. Unless I get 1/8-1/4" of flex in the legs, my groups suffer. I think this enables the first few milliseconds of rearward movement of the barrel under recoil to move straight back against the flexed legs. I use a corncob filled rear bag about 6x4" and make sure it doesn't interfere with the sling/ swivel. A solid surface is critical as Bros stated. Add numerous hours of consistent cheek weld, trigger control practice, and I have been able to reproduce bench rest results at long range.
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Harper, I use Harris 9~13" swivel bipods. I always set them as low as possible. If I need to raise them for field conditions I only raise them as little as possible to keep them stable as I can.
Grey Fox brings up good points. Much of which I have seen too with the exception of the amount of preload on the bipod. I only load it slightly. Just enough to take out slack and feel the location of the recoil pad on my shoulder. I used this " light preload" method while developing my consistant hold with cheekweld. The reason I opted for a light preload is, I felt just enough to remove the slack was the easiest to repeat with every shot consistantly. I wanted everything the same as I could get for each shot.
Also I am going away from the notched legs for the reason Greyfox mentioned. When set at the lowest level they have about 1/8 to 1/4" of spring loaded travel. Depending on the rifle weight this can really induce some verticle stringing and open groups. I have seen it and better groups were obtaned by changing to a friction lock bipod or raising the notched legs one notch to get off the spring.
I don't muscle my rifle. I let it ride the bags. My left arm is always cradled with my left hand on the rear bag. I use slight pressure to adjust for elevation. If I need very much squeeze I have found it much better to raise the bag somehow. I do not want a bag that is ready to drop in height durring recoil. A bag that needs you hand to make it a lot taller will almost always induce verticle spread.
About the difference in the preload that I use and Greyfox uses. My first guess is that he might be shooting a lighter rifle than I do. I do not care for light rifles. My " light gun" is almost 12 lbs with bipod. So lighter rifles might take a firmer hold. With a firmer hold beware of what I call "muscling a rifle"
Stiff muscles or firm holds are harder to control. To develope this I recommend these exersizes. Get your hold on target solid and steady. Then close your eyes and count to five. Then open your eyes and see if you are still on target. If you are using too much muscle it will be off target. Now do the same thing and dry fire. Did the rifle stay exactly on target? If not it is most likely due to too much muscle forcing the rifle to point of aim and not properly set up bags and preload.
While we are on this lets touch on form a bit. It is said to be straight behind the rifle. I agree with this but I teach it this way. Be as straight behind the rifle as is comfortable. I have broken my back before and it is a fight to get perfectly straight. However I do just fine with a slight angle of my body to the rifle. Remember that the angle can cause the rifle to jump right or left durring recoil. If you are consistantly jumping off target and not spotting shots try to straighten the line you and the rifle make. I can spot most all my hits at any distance. With a consistant shooting discipline and form the rifle will jump up slightly and return to target and you will not have to fight it back to target to see the hits.