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Hey guys. just got re signed up here. I am a old time reloader and target shooter{ 38 years}. but i picked up a used m70 338 win mag in excellent shape a year ago and have been trying different bullets and powders in it. The pattern is the same. It starts out shooting tight groups and then after about 20 rounds they really start to open up. alot . I guess it could be heat.But i think i am letting it cool down enough. Its a standard diameter 24 inch bbl and it has a factory muzzle break. I have tried different scopes so thats not it. I have not taken it apart yet to look at bedding.
 

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The Hog Whisperer (Administrator)
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What are you doing with a .338 Win Mag, that group size past 20 shots matters? Prehistoric ground squirrels..... ?

In all seriousness, if it goes back to good groups after cooling down, then there's your answer.

If it goes back to good groups after both cleaning and cooling down, then you have two variables, and it could be either, or both.

Good to have you back.....
 

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What are you doing with a .338 Win Mag, that group size past 20 shots matters? Prehistoric ground squirrels..... ?

In all seriousness, if it goes back to good groups after cooling down, then there's your answer.

If it goes back to good groups after both cleaning and cooling down, then you have two variables, and it could be either, or both.

Good to have you back.....
Thanks for quick reply, I found a range near me that has steel set up from 400 to 1000. I take my 708 varmint special rifle out there and shoot. I have hit all the way out to 900 and i think i have hit the 1000 with it. but sometimes its hard to tell your hits. there are other guys shooting at same time and you cant go down range. The 338 would make it easier to see and hear hits. But your point is well taken its gotta be heat or fouling, or a bad barrel. How long should i let it cool when it starts to go hay wire on me
 

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So I am understanding, you take it out, fire 20 rounds, things get wonky. When you go out the next time, are you trying the same loads and finding the groups to be good? Are you cleaning after each range session?
 

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So I am understanding, you take it out, fire 20 rounds, things get wonky. When you go out the next time, are you trying the same loads and finding the groups to be good? Are you cleaning after each range session?
Yea. clean it. same loads, and same scenario. only shoots good for about 20 rounds. then it gets nuts on me. like 6 to 8 inch groups. I let it cool but i guess i really need to make sure its cold before i keep going.. Sierra guys thinks its fouling. Told me to really get it clean
 

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For what it’s worth, you can get a bore camera on Amazon pretty inexpensively and get a look down the bore to see what’s going on in there. The easy thing is to skip cleaning it when you go home next time, then see what it does next time out. If it’s still patterning instead of grouping, I’d definitely get a camera or find a friend who has one and see what’s going on inside. If the groups go back to normal, I’d check free-float and bedding and anything else heat can affect.
 
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The Shadow
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Cleaning is the easy thing, and since you clean each time anyway; just actually clean it well.

Use a serious, modern chemistry cleaner, or serious elbow grease with old time stuff. Use a dedicated carbon cleaner, then copper cleaner, then carbon again. Then if she's moving still, it's either heat, or a seriously rough bore. But if it's that rough, you should discover that while cleaning.

Cheers
 

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Buy a BoreSnake. Put no more than a dozen drops of any powder solvent on the first brush and three drops of any good gun oil at the upper end of the loop. Drag it through the bore, same way as the bullet goes, three times. (There are supposed to incantations and humming with one foot off the ground, too, but I forget.
If the barrel is too hot to lay your finger on it and keep it there while aiming, it's too hot.
 

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Hey guys. just got re signed up here. I am a old time reloader and target shooter{ 38 years}. but i picked up a used m70 338 win mag in excellent shape a year ago and have been trying different bullets and powders in it. The pattern is the same. It starts out shooting tight groups and then after about 20 rounds they really start to open up. alot . I guess it could be heat.But i think i am letting it cool down enough. Its a standard diameter 24 inch bbl and it has a factory muzzle break. I have tried different scopes so thats not it. I have not taken it apart yet to look at bedding.
Hi Grantmauser,

You say your groups are opening up. Are you getting stringing or does the group move in a certain direction? I would be very suspicious of fouling in the factory muzzle break. Have you checked the barrel clearance when the groups open up? The barrel could be getting hot/warm and making contact with the stock. If you haven't checked barrel clearance when the groups open up, I would check it for sure.
Frank
 

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I'd suggest you try a good bullet jacket metal displacing solvent. Something like Sweete's 7.62. It has ammonia in it so don't expect a smell like Hoppe's.
Follow the directions on the bottle and if the patches come out green, you know its bullet jacket fouling. Keep at it until the patches come out without any green stuff, then oil the bore with an oily patch. The techs at Speer know their stuff, so I'd try what they suggest first.
 

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Discussion Starter #11
Cleaning is the easy thing, and since you clean each time anyway; just actually clean it well.

Use a serious, modern chemistry cleaner, or serious elbow grease with old time stuff. Use a dedicated carbon cleaner, then copper cleaner, then carbon again. Then if she's moving still, it's either heat, or a seriously rough bore. But if it's that rough, you should discover that while cleaning.

Cheers
ok guys
 

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I have had a similar problem with my 45-70 shooting 250 gr Hornady GMX's. For some reason that bullet or lot of bullets would put down enough copper fouling that the groups would start opening up at about 10 rounds and then keep getting worse until it cleaned it. After cleaning back to a little over an 1" for about 10 more rounds.

I do not have the same issue with 325 gr Hornady FTX's. They shoot well longer than my shoulder can.

So maybe you have a problem similar to that. I don't know if you have tried more than 1 bullet in your rifle or not.

Just a though.
 

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I'm with MikeG, but, I will add that a excessively tightened muzzle brake has caused me problems, in the past, when the barrel heated up. I would try shooting/heating it up, with and without the muzzle brake.

You may be surprised,.... I was!

Since then, I don't have brakes on any piece I own!
 

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The Troll Whisperer (Moderator)
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Ya know, 20 shots thru a 338 Mag from a bench or prone would make most folks start to wobble a bit in sight picture. No wonder the groups start to open. Also, 20 shots will most likely lay a copper fouling buildup in a 338 mag bire. Probably the bore snake ritual at the bench will benefit both the bore and the shooter's shoulder and mental being.
 
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Elk Whisperer (Super Moderator)
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What are you doing with a .338 Win Mag, that group size past 20 shots matters? Prehistoric ground squirrels..... ?



Good to have you back.....
.


HEY!! I resemble that remark !

A good cleaning with more than a bore solvent may be in order. For stubborn copper I use WipeOut foaming bore cleaner, sometimes a couple applications. In the field I use dry bore snakes every 20 rounds or so. A good field day usually runs 150-200 rounds each in 4-5 rifles (prehistoric ground squirrels and/or prairie dogs) The larger the caliber, the fewer the shots.

With a large caliber like a 338WM I would suspect copper build-up is affecting your accuracy if you are running 20+ rounds through it during a range session. Also, too much coffee can be a contributing factor 😉

Welcome back!

RJ
 

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Discussion Starter #16
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HEY!! I resemble that remark !

A good cleaning with more than a bore solvent may be in order. For stubborn copper I use WipeOut foaming bore cleaner, sometimes a couple applications. In the field I use dry bore snakes every 20 rounds or so. A good field day usually runs 150-200 rounds each in 4-5 rifles (prehistoric ground squirrels and/or prairie dogs) The larger the caliber, the fewer the shots.

With a large caliber like a 338WM I would suspect copper build-up is affecting your accuracy if you are running 20+ rounds through it during a range session. Also, too much coffee can be a contributing factor 😉

Welcome back!

RJ
Ok guys. Yea gonna go with fouling first. then let it totally cool down after 15 or 20..I might loosen the brake up, but i have no plans to shoot it without it. will not be fun.
 

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You don't have to make a decision on "brake or no brake", now.

Just losening it a bit, is kind of a half answer.

Just try it with and without,... then you can decide what you want to do, if it is the problem. Point is, I wouldn't eliminate it as the culprit, until I did so,... imho.

If it turns out to have a negative effect, and if I remained adamant on having a brake,... then I would experiment with how tight, is good to go.
 

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I thought that the mating surfaces between the barrel and the break was what keep it aligned with the barrel. There is always some play in the threads so loosening it might cause the bullet to hit the break. That interference is likely to cause more than inaccuracy. A bullet at 3000 fps hitting a break is likely to cause real damage - is it not?
 
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