How far off the lands 243?

S

Stillhunter

New member
i bought some 105 amax for reloading my 243 win. I thought it had a 1:9 twist,but it only has 1:10.
I was just wondering has anyone experimented with seating the bullet unusually farther back away from the lands to help stabilize this heavier bullet?
If so,how far off the lands did you try?
Did it help?
Or is it just wishful thinking on my part?
I,ll be using H4831.
 
I'd seat bullets so the cartridge OAL is about 1/16th inch less than magazine length. That'll allow easy and reliable feeding. If it's too long for easy chambering, cut it back 1/100th inch then try that OAL.

Accuracy should be good too. Bullet stabilization's caused by the correct twist; which your is good for 105's in 24 caliber. With the barrel life of a .243, I wouldn't worry about chasing accuracy with different OAL's. Jump distance to the rifling will increase about .001" for every 15 rounds fired anyway regardless of how much it is to start with. How much doesn't matter or stabilization.
 
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Competitive shooters got about 1500 rounds of good accuracy with the .243; hunters about 2 to 3 times that much depending on their accuracy needs and expectations. It is a quite overbore capacity round.
 
I have noticed my CZ 243 is falling off in accuracy has between 1000-1500 rounds thru her. Overbore is the main cause of low barrel life? That means lot of powder for bore size right?
OP as far as to lands I set all 3 of mine back about .15-.20 with good results.
Thanks for good info
roc1
 
Yes, overbore situations shortens barrel life

I did a barrel life comparison for 22 through 30 caliber cartridges used in competition to see what people getting best accuracy had for barrel life. Also asked Sierra Bullets ballistics man what barrel lives his test barrels got for best accuracy. Then looked at some things about bore size that related directly to their barrel lives. Learned that for cartridges getting about 3000 rounds of barrel life starting out at about 1/4 MOA at short range then getting worse to a 50% increase, about 3/8 MOA, which is the limit for competition and bullet accuracy testing, the round count was the same for all those calibers. The .222 Rem, .22PPC, 6PPC and .308 Win rounds all got about 3K rounds of barrel life to that standard.

The interesting thing was, the cross sectional area of the bore in square millimeters equaled the powder charge weights of these cartridges. So, I decided bore capacity was equal to the charge weight in grains that had the same number as the bore's cross sectional area in square millimeters. A 6mm bore's capacity is 6 x 6 x .7854; 28.2 grains which is what the 6PPC benchrest round typically uses for accurate barrel life of about 3000 rounds. One of the National Champions used a .243 for his cartridge winning it and he told me 1500 rounds is what he got for barrel life. A custom barrel maker predicted 1500 rounds for match barrel life when he started making them and that's about what all his customers got.

In comparison to what a lot of hunters get with their rifles and military standards for rifle accuracy standards, about 6000 rounds (double bore capacity numbers) for hunters and about 9000 or 10,000 (triple bore capacity numbers) for military service rifle barrels.

Comparing cartridges that burned about 41% more powder for each caliber, barrel life was half; if doubled, barrel life was 1/4th. Example, with a .308 Win getting 3000 rounds of super accurate barrel life with about 45 grains of powder, the .300 Wby Mag got about 750 rounds of accurate barrel life burning about 90 grains per shot. My .264 Win Mag target rifle got 640 rounds of accurate barrel life burning about 70 grains of power for each shot.

That varies somewhat depending on the heat intensity of various powder charges.
 
If so,how far off the lands did you try?
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It will depend on the rifle you are using. I use a Savage 243 and set my rounds to be 0.050 off the lands. I also use the Hornady 105 A-Max's.

Excellent results.

Jim
 
Bart B. thanks for the reply,your right,chasing the oal did not do improve anything at all.As matter of fact it got horrible worse,lucky to get one shot on a 12x12 target at 100yrds,and obviously signs of tumbling.
 
Bart B. said:
The interesting thing was, the cross sectional area of the bore in square millimeters equaled the powder charge weights of these cartridges. So, I decided bore capacity was equal to the charge weight in grains that had the same number as the bore's cross sectional area in square millimeters. A 6mm bore's capacity is 6 x 6 x .7854; 28.2 grains which is what the 6PPC benchrest round typically uses for accurate barrel life of about 3000 rounds. One of the National Champions used a .243 for his cartridge winning it and he told me 1500 rounds is what he got for barrel life. A custom barrel maker predicted 1500 rounds for match barrel life when he started making them and that's about what all his customers got.
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Bart, this statement interests me. Could you expound on it a bit? You say that bore capacity equals the charge weight in grains and is the same as the bore's cross-sectional area. A 6mm bore has an area of pi*r2, or 3.14*3*3=28.2 grains. When does a cartridge become "overbore"?

Thanks for this info.
 
Continuing with my question:

Since smokeless loads for the .243 win are someplace in the 40-50 grain field, this indicates an "overbore" because the standard loads are more than 28 grains?

The .30-06 has a bore diameter of 7.82mm, so 3.91*3.91*3.14=48 gr. Yet standard loads for the .30-06 are in the 50-60 gr range. Does this mean that the '06 is slightly "overbore" as well?

Thanks again.
 
Distance off the lands varies with every rifle. Not every brand of rifle. Every rifle. It's a trial and error thing that really doesn't make much difference in a hunting rifle. There is no formula that applies either. No hunting anything but varmints with A-Max's though.
"...Overbore is the main cause of low barrel life?..." Nope. How hot the load is matters far more. Over bore has to do with the ratio of the calibre to case capacity. The case capacity in grains of water divided by the area (in square inches) of the bore cross-section is the magic formula. Don't worry about it. It really makes no difference except for target rifles.
 
Wyoredman,
You're on the right track. Except I use "bore" diameter, not bullet or groove diameter to keep it in line with "bore capacity." A .243 Win barrel has a bore diameter of .237" or 6 mm. 30 caliber barrels have a .300" or 7.62 mm bore diameter and a .308" or 7.82 mm groove diameter. And square millimeters is the unit of measure to use for the bore's cross sectional area that equals a max, normal peak pressure producing powder charge weight to get 3000 rounds of barrel life. My bore cross sectional area is different than SAAMI's specs for it which is the groove diameter's cross sectional area then the area of the lands subtracted from it. Their 30 caliber bore cross sectional area is spec'd at .0736 square inch compared to mine of 0.0707. The difference in square millimeters for each one is small and not critical anyway, as long as one uses the same way to calculate. The bore capacity and barrel life numbers are approximations, generalizations empirical calculations made to closely parallel observed barrel life numbers.

Bore capacity for a 30 caliber barrel's about 45 grains. Any charge weight over that's "over bore capacity" and an "overbore" 30 caliber cartridge is one that burns more than 45 grains of powder.

I gave my formula to a guy some years ago who modified it to include an input for powder heat indices so that also could be an input. I think he used 3500 rounds as the barrel life for a cartridge at bore capacity instead of 3000 that I used. His formula's barrel life numbers are a little more than mine. He used the powder heat index numbers used in Quickload.

Note these formulas are based on a rifle starting out shooting no worse than 1/4 MOA at 100 or 200 yards and the barrel's useful life ends when the groups get bigger than about 3/8 or 4/10 MOA. Double the life numbers if the rifle starts out shooting around 1/2 to 3/4 MOA and if it starts out at about 1 to 2 MOA, you can triple the numbers. A 30 caliber service rifle's barrel life is near 10,000 rounds; that's when the M1 and M14 throat erosion gauges will read "10" which is the maximum number on it.

T. O.,
I don't think maximum loads make any significant difference. I've observed many .308 Win. barrels shooting 60% reduced loads and 40 percent maximum loads give the same 3000 or so round barrel lives as others that shot 100% maximum loads. Higher heat indexed powders do cause shorter barrel lives. However, if you're loading to pressures above SAAMI spec, then those "extra hot" loads will make for shorter barrel lives.

If the case capacity in grains of water divided by the area (in square inches) of the bore cross-section is the magic formula, please explain the following:

A .243 Win has a case capacity in grains of water of about 54 grains of water. Divided by SAAMI spec bore cross sectional area of .0453" square inch, the result is 1192 rounds.

A ..308 Win case capacity is about 55 grains and its SAAMI spec bore area is .0736"; the math gets 747 rounds.

Why does the .243's barrel life have round counts 60% more than the .308 with your formula? That contradicts too many facts. Accordingly, the more powder burned for a given caliber, the bigger your formula's answer is.
 
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Twist Rate Stability Calculator.

105 amax 243 win. has 1:10 twist .
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http://www.bergerbullets.com/twist-rate-calculator/ Twist Rate Stability Calculator.
243Bullets6mmA.jpg
My 1-10 likes these bullets in a 27" barrel. My 18" barrel that produces less RPMs does NOT like the 100 gr Sierra BT.
 
I don't shoot 243, but both my .30 cal R700's (.308 Varmint, and .300WM Sondero) have shown the best accuracy and consistiancy from loading off the lands about .025" .
 
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