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?

I would try another powder.

Haven’t really seen that much difference because of the temperature variation, we’ve had excellent results with RL26 in a couple of different rifles, but they’re 9 twist and we’re shooting 168 and 180 grain Bergers.

I would try a bullet that is more suitable for a twist rate closer to what you have. Hornady reports shooting their loads with 1:10 twist barrel, and suggest a 1:8.5 for the 175 grain ELD-X bullet.

Sounds like you may be on the edge.

1:11" twist is really slow for that long of a bullet. I'd ask a different Hornady Rep because there's no way that he should be recommending that bullet when the factory says it requires a 1:9.5". You might get a way with a 10" twist but I can't believe that an 11" twist would work.

I would look at the throat with a borescope. In my opinion STW's life is around 400 shots anyway. Maybe less maybe more depending on how hot the throat has gotten, but that will tell the story.

Of course you have checked every screw, bedding, crown, etc.

God Bless, Louis

If you are using new brass for these loads, you may want to anneal the cases.

quote:

Originally posted by youngoutdoors:
I would look at the throat with a borescope. In my opinion STW's life is around 400 shots anyway. Maybe less maybe more depending on how hot the throat has gotten, but that will tell the story.

Of course you have checked every screw, bedding, crown, etc.

God Bless, Louis

You could also use a Stoney Point (Hornady) tool to measure the OAL to the lands. I keep a log of all my rifles; one of my .300 RUMs that I measured recently didn't hit the lands at all; a borescope showed two are completely gone.

Thanks for the comments. I just talked with a Hornaday representative and he said that the 1-in-11 twist of my barrel is far from what they would recommend. Also, I looked up stuff on air density and at 20 deg F with about any humidity the density is about 25 percent greater than air at 70 deg F at 60 percent humidity which is probably average for Iowa at mid-day. And bullets are less stable in denser air. I checked the throat recently and found that the 140-gr Nosler bullets touch the lands about 0.04-inch further down the barrel than they did when the barrel was new. At that point the accuracy had not noticeably changed. I think a new fast twist barrel would be a good solution but it’s not just the cost, there is also the time it takes to get a rifle re-barreled, and all the trouble, and the chance the next barrel might be a lemon. Anyway, I will check it again for accuracy with the old 140-gr loads and if that works out OK will try some of the Hornaday 150-gr ELD-X bullets and see what happens.

In the olden days someone came up with a "formula" that there was ≈0.005" throat erosion per 50-100 rounds in target rifles of .224/.30 cal as measured with an throat erosion gauge...going to the larger, hotter cartridges would increase that number considerably. Some of my hot 22/6mm/25 cal would do two to three times that number before accuracy started falling off and I would re-measure/re-seat out longer.

A 7mm STW is a LARGE cased, HI-VELO round...I wouldn't be surprised if it ate up 0.0001"-0.0005" of throat EACH ROUND. I've played with "Hot Rocks" for 50 odd years and I can tell you from first hand experience I've "worn" out barrel throats in 500 or so rounds...AND 7-8" of rifling to boot...a couple I never did get a good load before the barrel was wasted.

I't part and parcel of shooting Hi-energy cartridges and new barrels rather quickly comes with the territory.

A 1-11 twist will stabilize a bullet ≈1.25" long at 3200 fs in .284 cal...at 3000 fs that length drop's to ≈1.2". It's NOT the bullet weight that counts, it's the bullets LENGTH that is the limiting factor...twist rate formulas ask for CALIBER, VELOCITY AND LENGTH and if it is boat tailed.

Stick with bullets that are ≈1.2-1.25" long and keep the velo above 3200 fs and your accuracy will return...AND seat out closer to touching the lands...single load if you have to or accept the accuracy for magazine length cartridges.

Good Hunting

I think some of the other posters hit the nail on the head.

Your load was probably "on the edge" in warmer temps but still shooting well. As temps got cold and 7828 was affected and velocities dropped off a bit, along with all the other things mentioned, the stabilization just isn't there with the 11 twist barrel.

When loads are on the edge but still shooting well at 100, they might go straight to hell at longer distances as the velocity falls off. As you know, this is one of the reasons we should all shoot at distance and so very many folks do not.

I think you're barking up the right tree by moving back down with your bullet weight (length).

Zeke

quote:

In the olden days someone came up with a "formula" that there was ≈0.005" throat erosion per 50-100 rounds in target rifles of .224/.30 cal as measured with an throat erosion gauge...going to the larger, hotter cartridges would increase that number considerably. Some of my hot 22/6mm/25 cal would do two to three times that number before accuracy started falling off and I would re-measure/re-seat out longer.

That sounds like something I've heard from "BartB", (Bart Bobbitt).

He also had barrel life figured out to "pounds of powder" for various cartridges.

Bart is known for shooting a 20 round, 3.325" group at 800 yards.

https://search.aol.com/aol/sea...ards&s_it=loki-tb-sb

Id prefer the gun in my lap to make any kind of suggestion..too much information..

With the specs you gave at 60 deg F (just an estimated temp I picked), that bullet at 3200 fps has an Sg (gyroscopic stability) of 1.291.

The minimum for accuracy is considered to be 1.3. Berger recommends when evaluating twist rates that you get to at or above 1.5 because below that you start compromise the BC of the bullet.

So at 1.291 you were just below/essentially at minimum which is why it probably shot "ok".

Drop to 12 deg F and your Sg is 1.172.

The above is likely your problem.

Just FYI, using the 150s and assuming 3250 fps Sg jumps to 1.405 at 12 degs and is 1.548 at 60 deg