Generally accepted sweet-spot distance to the lands.

P

Pond James Pond

New member
I remember seeing a thread once that stated that, in the case of .308, seating distance from the lands was 0.020" (or 0.5mm).

Is that correct?

I'm updating my records and I want to put that in as a point of reference. Most of my loads are restricted by the magazine size, but my most recent bullet purchase: a Norma Oryx SP is rounder than my other bullet choices and so could be seated right up to max chambered length and still fit in the mag.

So I want to check!!
 
Unfortunately that is a persistent myth that gets applied to all calibers. Seating depth sweet spots change with chamber and freebore. It is individual to the gun and bullet combination, as this bit of wisdom from Berger describes.

Writing in the Precision Shooting Reloading Guide, the late Dan Hackett described an example of this. He changed bullets to a 50 grain Nosler during loading for a .220 Swift varmint rifle one day. He he knew the Nosler required 0.015" difference in seating die adjustment from the previous bullet to stay at 0.020" off the lands, which he'd been given to believe was ideal, but he turned the seating micrometer the wrong way, putting the bullets 0.050" off the lands, instead. He didn't discover this before he had 20 round loaded. He debated pulling the loads down and reseating them, but decided just to shoot them in practice. The best loads in this rifle had previously been unable to group five shots into better than 3/8" at 100 yards. That sounds good to most of us, but to a nationally ranked benchrest shooter it was not so satisfactory. To his amazement, when he fired the erroneously seated practice loads, he got two 1/4" groups and two bugholes in the ones (between 0.1" and 0.2").
 
Distance to lands a given bullet seating depth has will increase about .001" for every 5 to 30 shots fired depending on powder charge weight and bore diameter. That's caused by the throat erosion at the origin of the rifling.

I've watched a given .308 Win load's OAL stay the same 2.8" over 3000 to 4000 rounds of barrel life. The bullet's jump distance to the rifling started out at about .050" but increased to over .130" over the accurate life of the barrel. Accuracy through 300 yards didn't change any significant amount for the barrel's life with that fixed OAL and variable jump distance. Accuracy started out at 200 yards about 1 inch at worst then opened up to about 1.5 inch for 10-shot groups. At 300, it started out at 1.5 inches then opened up to a little over 2 inches.

If I set the round's OAL to .010" longer than the origin of the rifling allowed so they set back in the case neck when chambered, 10-shot test group accuracy was 20 to 30 percent better, but it still opened up about the same fractional amount over the life of the barrel.

I'm convinced there's a bullet jump distance to the rifling that can have a 1/10th inch spread where accuracy will be quite good over the life of the barrel. One thing's for sure; Federal Gold Medal and Black Hills match .308 Win ammo shoots very accurate across all sorts of barrels with varying bullet jump distance to rifling. So did 7.62 NATO M118 and M852 match ammo in M1 and M14/M1A chambers and barrels with a wide range of bullet jump distances.

My bottom line is, if you can shoot your stuff into no worse than 1/3 MOA at 100 yards all the time, then try .005" increments in seating depths to change bullet jump distances that amount. And use 10 shots or more per group to get statistically significant results. Cut the powder charge a grain or so then seat bullets to set back when they're jammed into the lands to see what that does for accuracy. Sierra's bullets often do best with seated into the lands; Berger's VLD bullets need a few ten-thousandths jump. I'm convinced the throat angle (slant of the origin of the rifling) plays a part in this too, but have not seen any test data for a given load's dimensions being tested with barrels whose only difference is the throat angle.

Otherwise, do other things to improve the accuracy you get that mask what a few or several thousandths difference in jump distance makes. Minimizing human intervention with the rifle's performance helps a lot.
 
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Bart you clearly know a ton about specialized hand-loading, but I question the figure about the jump increasing .001" every 5-30 shots.

In a 3000 shot life, that would mean that the freebore increased by a tenth inch if it eroded .001" every 30 shots, or sixth tenths if every five shots.

Really?
Between a tenth and a half inch+ of rifling eaten away after only 3000 shots?
Can that be?

I don't know if you're right or not, I haven't examined a bore that closely, it just sounds like an awful lot of erosion of a relatively small number of shots.
 
distance to the lands (jump )
this is measured from the oglive ( where the bullet first touches the lands
in the barrel to the barrel ( many companies make the tools to make these measurements )
that distance can vary from bullet to bullet in the same box of bullets so you will need the tools to make this ammo
the best jump varies with manufacturer of bullet and barrel combination
and you must find that sweet spot by varying the jump with each brand bullet
barrel combination
the jump for Sierra Match Kings in most of my rifles -- 10 to 15 thou.
jump for Hornadys -- 10 to 15 thou.
I have an older Bushmaster AR15, with Sierras and Hornadys it likes them 25 to 30 thou. off the lands

I have not seen a noticeable difference in the pattern on the target, between
magazine length ammo and the special jump ammo at 200 yards and closer,
at 300 yards and farther it will make a small difference( if you are shooting in a match every little bit helps )( hunting ammo, you and the game will never now the difference )
 
Kframe, my experience using match grade barrels including four 7.62 NATO Garand barrels, three 30 caliber magnum barrels, several .308 Win. Hart, Obermeyer and Kreiger barrels, along with that of other competitive shooters winning our share of the matches, my erosion numbers are typical.

All the .308 Win barrels' throats eroaded away .090 to .100 inch after about 3000 rounds; that's when accuracy had degraded by about 50% from when new.

The 30 caliber magnums lasted about 1200 to 1300 rounds of equal accuracy but the same throat erosion numbers.

Worst was my .264 Win Mag long range rifle that, when in 1969, it shot Norma 139-gr. nickel plated match bullets into 10 inches at 1000; Sierra's at the time didn't even come close to that. Bullets had to be seated out about 1/10th inch from new barrel throat length with bullets touching the lands to barely being held by the case mouth when it went from 6 inches at 600 yards to 20 inches across only 5 shots; barrel was gone after 640 rounds.

Few people measure their throat erosion rate, so there's few people that have data. One of TFL's forums mentioned that the erosion rate he got with his .308 was the about the same as mine. The erosion rate varies somewhat with the powder's head index; hotter ones erode barrel steel faster than cooler ones. Extra high peak pressures also erode the steel faster than normal max ones. Sierra Bullet's first ballistic technician reported to me their match grade .308 Win barrels used to test Sierra's bullets for accuracy erode throats about that much, too. He's probably worn out more barrels than anyone I know.

There's a saying that some folks use to describe the process of always seating bullets out to just touch the lands; it's called "chasing the lands."
 
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Bart, thank you for the response.
Those numbers sound excessive but I'm sure you're right.
I've never tried to quantify throat wear so this is new and surprising to me.

I've always heard that the .22-250 and 220 Swift eat barrels quickly, just never considered it a factor in "standard" pressure .30 cal rifles.

On the other end of the spectrum, I wonder how much .22 Long Rifle throats erode, I've got some that I know have fired over 100,000 rounds and can still hit pop cans at 50 yds.
 
I generally load about .025" off the lands on both my R700's in .308 and .300wm . Seems to definitely have helped my accuracy vs loading to Lyman's 49th OAL spec. I'm still a newer precision reloader, and the reason I picked .025 is to help my reloading consistency by removing another potential variable within the process. I load about 10 different .30 cal bullet weights and profiles.
 
Kframe, top ranked and scoring 22 rimfire shooters rebarrel (or set them back 2 inches) after about 30,000 rounds. Rimfire barrels used to last over 50,000 rounds for top competitors. Their indicator was the darker black throat color form bore erosion that starts at 6 o'clock in the throat then works its way up each side. When it gets half way up to 3 and 9 o'clock, that's when best accuracy degrades by about 50%; the point at which the barrel's worn out for them.

But that changed in the 1980's when the match ammo makers changed their primer compound with more abrasives and other chemistry in it to make it safer to produce. Barrels didn't last as long and accuracy wasn't as good since then. Proof's in the NRA 100 yard prone records fired in the late 1970's that still stand today; they've yet to be bettered.
 
Kframe, you wonder about .22 erosion. A lengthy article from Precision Shooting magazine a number of years back really nailed it. The author was given a tour of the Ely ammo plant in England, and among other observations was that they used a battery of 4 Anschutz rifles for accuracy testing of their ammo. Most interesting, was that they changed guns or barrels every 700,00 rounds, or around 175,000 rounds each.
 
And I am the fan of the running start, I want my bullets to hit the lands a-running. I do not want my bullets to even think about slowing down.

F. Guffey
 
experimentation is the only way to find the right jump for a rifle and a load.softer bullets usually like a short jump .001 off the grooves but that is only theory.

anywhere from .050 to .001 is a good start and .035 is the most reliable theory as far as jump unlike the online belief that closer is always better.i have shot .035 for years
 
@James Pond

your orynx bullet might work well off the grooves i bet.you cant know untill you try but for tougher bullets sometimes do well a bit farther off.

start at .050 for the orynx and .035 for fast expanding spitzers but like i said you will never know untill you try.
 
When helping to determine a big problem with a custom built rifle I made tools to determine the distance from the bolt face to the lands. The bullet in the case came out of the case and continued .250" through the throat before hitting anything. With the longest cases and the longest bullets nothing got better.

I felt helpless when ask "How did this happen?". I rationalized someone misunderstood Weatherby, instead of having free bore they added free bore.

F. Guffey
 
Mr Guffy brings up something that I've often wondered about. It's the Weatherby freebore. I understand the logic of having the freebore with the Weatherby cartridges, but if we (most of us) are so in to having the bullet just off the lands while in the search for accuracy, how does Weatherby get top accuracy with their freebore approach?
 
I used to shoot matches in California with a stock maker who designed some of Weatherby's stocks. He said Roy Weatherby wanted a long freebore so the heaviest bullets made for his calibers could be used without sacrificing case capacity. And that Mr. Weatherby was a fan of longer freebore so initial pressures could be higher to shoot bullets as fast as possible into the rifling; all according to his reasoning.

Weatherby rifles do not produce top accuracy; in my opinion. If they really did, they would out shoot Savages in F class matches. Do they guarantee 1 MOA accuracy at 100 yards for more than 3 shots?
 
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zplinker said:
Kframe, you wonder about .22 erosion. A lengthy article from Precision Shooting magazine a number of years back really nailed it. The author was given a tour of the Ely ammo plant in England, and among other observations was that they used a battery of 4 Anschutz rifles for accuracy testing of their ammo. Most interesting, was that they changed guns or barrels every 700,000 rounds, or around 175,000 rounds each.
Click to expand...

z, that's interesting! Thank you, I'll have to look for the text of that piece. :)
I makes total sense that the light, soft, low pressure .22 round would lead to much less barrel wear than a centerfire, but that's amazing!

Especially considering that the barrels were not completely 'shot out' at 175K rounds, just that the accuracy was no longer good enough for testing the match rounds.
 
weatherby's were never intendid for match grade accuracy as we know today.but no one else can give you inch or inch and a half groups at 100 with calibers like .378 wby mag.no factory model 70 in .338 win mag could do that.

custom built rigs in .338 lapua that weigh 13 lbs with massive muzzle brakes can hit a 50 cent piece at 1500 meters. a .338-378 that weighs 8 lbs with a sporter weight barrel can kill an elk at 600 yards and that is still very impressive.

weatherby is not a match a or tacticle gun,its a lightweight moutain rifle that can kill big things at 500 yards.and i dont believe Savage who im a big fan of and have bought 3 savage rifles over the years.has ever claimed sub moa at 100 yards with calibers like .338 RUM
 
kframe: I do have that article...somewhere in the stack! As it happens, I have 308 issues of PS magazine, and am looking to sell the whole lot. Tons of info, stuff you won't google! From what I'm seeing on ebay, there isn't much interest in the magazines though, so for now, here they sit.
 
Bart, you've answered many questions, and plenty for me. One thing that lingers is; let's say I found the most accurate charge weight for book coal, which for my 308 is 42.5-43 grains varget. Then, I jump it with the same charge to .030" off lands. I find great accuracy there, but by that jump I've lost 150 fps which will lead me to be transsonic before my 1000 yard goal. Have you found a rule of thumb to correlate jump and increase of powder charge to maintain velocity before you start to increase pressure unsafely as you get closer to the lands? I'd rather not jump .010 increments with 5 charge weights every time to find the sweet spot of accuracy/velocity/fps spread. That would be hundreds of bullets with increasing charge weights the a possible reduction in charge weights as I approach the lands.
 
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