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Poor accuracy - What's wrong?
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I am having accuracy trouble with my 7MM STW. It probably has 200-300 rounds through it as it hunting rifle and most of the rounds were load development or occasionally checking the sights. It’s my second rifle of this caliber and was 23 years old last summer. It has a Rem 700 action, a 1-in-11-inch twist Shilen barrel and a HS Precision fiberglass stock. My original load for it was a Nosler 140-grain Balistic Tip bullet over IMR 7828 powder at a little over 3,500 fps. Accuracy was generally about ¾ MOA or better. I used 8MM Rem cases sized to 7MM which were outside neck turned and nickel plated by a local metal plating company.

The original 3.5-10X Leupold scope worked fine for 400 yards or so but in 2006 (and for the next 8 years) Iowa had a rifle season for deer. I got a Leica rangefinder and had my .338-.378 Wea modified for long range shooting. Sometime in those years of rifle deer seasons I had the scope modified with Leupold’s “Custom Dial Reticle”. I mostly used the .338-.378 for long range deer but I did shoot a deer with the 7MM STW at nearly 600 yards and my grandson shot one at 540 yards with it. At those distances the 140-grain Balistic Tips made nickel size exit holes whereas the .338-.378 made 1-inch or larger exit holes at 700 yards. Anyway, I wanted a better long-range load so switched to Hornaday 162-grain ELD-X bullets at around 3,200 fps. A Hornaday rep said they would stabilize in the barrel with its 1-in-11-ich twist, even though the boxes the bullets come in say to use a 1-in-9.5-inch twist.

I developed the new 162-grain bullet load during the spring or summer but not when it was very warm. With that bullet and vary charges of IMR7828 (73, 74, 75, and 76 grains) I got a 4-shot group at 100 yards of less than 3/4 -inch. I selected a powder charge which gave about 3,200 fps. I like develop loads shooting groups with varying powder charges thinking it will better show powder/bullet combinations not sensitive to temperature/pressure variation. I have seen loads which are accurate in the summer but inaccurate in the winter. Using a fixed powder charge this load shot under a MOA at 200, 440 and 600 yards early in the fall but I did not have the gun perfectly sighted in with the new “Custom Dial”. I went to the range to get that done in October when the temperature was 12 deg F and got groups of at least several MOA not including bullets which missed the target paper entirely.

I figured the scope had shook lose and went back to the range in November with another Leupold scope on the gun and again got good groups. I did not write the temperature down as I usually do but am pretty sure it was warmer than 12 degrees. My assessment was that the previous scope had shook loose so I sent it to Leupold for repair but bought a new 3-15X Leupold scope with a “Custom Dial” reticle thinking the original scope might not be fixed soon. They sent the scope back saying that it “is” working properly but I don’t know that they meant that it “was” or if it was fixed. This morning I went to the range to sight it in with the new scope and it was 6 Deg F and again I got groups of several MOA at 200 yards with a couple bullets missing the paper.

I thought another potential problem was that I had added a silencer to it (before developing the 162-grain load) and had cleaned the silencer before shooting the bad groups in October. That is, the baffles would not have been oriented the same way as before. Then I shot a group at 100 yards with the silencer off the gun and also got a several MOA group. Now I am thinking that the bullets are not adequately stabilized at low temperature because of lower velocity and denser air. However, the holes in the target did not indicate key-holing and the groups were not just a little larger but many times larger. My next test will be to load up and shoot some of the original 140-grain loads to see if they still shoot OK. If so, I will probably assume the 162-grain bullets are not stabilized at low temperatures and try to develop a load with Hornaday 150-grain ELD-X bullets thinking a 1-in-11 twist might be OK for them.

Has anyone seen something like this where a rifle shoots good at regular temperature but “can’t hit the broad side of a barn” when it is cold? Or is there something else wrong with my rifle, scope or scope mount?
 
Posts: 253 | Registered: 25 November 2005Reply With QuoteReport This Post
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