confused 308 accuracy

[lt dan]

some of you might have noticed me asking questions or and help with the 308 i inherited, with regards with accuracy/groupings.

to make a long story short : i inherited a 308 build in Austria. the problem with this 308 is amongst other things that it is very light ( lighter than my ,22 bruno unscoped!!!). it had a free play between bullet and lands that could only be rectified with a 180grn flat bullet seated at more than max to compensate for what we call over here the : JUMP.

i did new scope ,new scope mounts, free floating of the barrel, cutting of a new crown, new target shooting trigger, electronic scale, and so on and so on......


at the end i hardly shot a 1 inch group at 100 yards.


two days ago, after testing numerous loads, i went to the shooting range and for the first time since owning the 308 i tested the best loads i have at 100 yards at 200yards. ie: for the first time i shot at 200 yards on the shooting range to see the results.

the grouping at two different times wasnt just better it was amazing!!???

i dont understand, i couldnt get moa at 100 yards yet i get much better than that at 200 yards. to ad insult to injury at 200 yards i dont have the luxury of a bench and the wind was above normal.


does anyone have a sniff of an idea what happened here?: better groupings at 200 than 100 yards!! i have since confirmed this so this is not a fluke.

 

[az_imuth]

Not sure, but I would try again at 100 yds without the bench and see if that makes a difference. Maybe the bench has some slight movement that isn't quite perceptible.

I would think if the rifle were getting good groupings at 100, then it should get as good or better at 200. Shoot at 100 in exactly the same manner as you shot at 200.

best of luck

 

[jrothWA]

Hello lt Dan,

Only thing I could think is the 100yds target the bullets are not fully stabalized in-flight??? At 200 they have finalized stabilty and that bring the group together.

I have observed the trace of a US Mil-MATCH cartridge, do a "barrel roll" on it way to a 600yds target, and get scored an "8".
The shooter looks at me questionly, all I could say was keep shooting, you wouldn't believe me.
After completeing his 20, and securing his gear. I told him. Though I was joking.

My I suggest, have the barrel shoulder faced-off the distance of on or two thread pitch and re-chamber the barrel to minimize the "jump".
Talk to a smith on this and get his view.

 

[lt dan]

az_imuth: i am going to shoot at 100m as you suggested. i did the 200m shooting in my land rover using the outside rear mirror as a rest.

jrothWA: you are the second guy(the other was a gunsmith) telling of bullets that stabilized better at 200 than 100. i dont understand that but i except it.

 

[lmccrock]

The bench rest guys claim that a boat-tail bullet design takes some distance to stabilize, and 100 yd groups are better with flat base bullets than with boat-tail, but out at 300, BT is better. There is probably some disclaimer or limiter to go with that, but I do not shoot bench rest so I take them at their word.

Lee

 

[Dood_22]

Probably just luck.

BTW, don't worry about the bullet jump, you want to seat your ammo off the lands for best accuracy anyway. I've noticed some of my loads shoot best about .040" off the lands.

 

[Bart B.]

If anybody really believes bullets can shoot smaller MOA groups at 200 or 300 yards than they do at 100 yards, then they can explain how bullets at the outside edges of the 100 yard group know which way to change their direction so they'll shoot smaller MOA groups further away and those dead center in the 100 yard group don't change direction at all; plus where the forces come from to make these directional changes.

If they can't explain the physics of this, then I don't think they really don't know what they're talking about.

The link "bench rest guys" has the following statement:

My guess is that velocity spreads within reason don't adversely affect harmonics or the distribution of nodes

One of the most inaccurate things published about shooting. A given barreled action has a fixed resonant frequency which has several fixed harmonics as well as fixed nodes (non-vibrating points) along its length. It never changes regardless of how fast the bullet leaves. As long as the physical dimensions remain the same, so will the frequencies it vibrates at.

 

[emcon5]

I tend to agree with Bart, but I have heard a few people make the same claim regarding the .224 80gr Sierra matchkings.

 

[Old Grump]

lt dan just a couple of questions, what is your rifle twist, what is your bullet weight and what kind of bullet is it? flat base, boat tail, HP, HC, Spitzer? Last but not least what was your velocity.

We called it putting the bullet to sleep and is why we only used our heavy 178 gr bullets at long range, (beyond 500 yards) because whether some want to believe it or not they were moving in a orbit around the path of flight and when they finally went to sleep they were on the flight path and the only turning of the bullet was the twist. Up to 300 yards we could shoot better with standard issue ball ammo as long as we went through the ammo first and sorted it by length, each shooter getting one length and that is what we sighted our particular rifle for, (match M1 Garands with 308 barrels).

I don't really see this happening at the close ranges you are dealing with and I am more likely to think something in your gun has changed, settled in so to speak. Just for chuckles and giggles what was the temp like on both days and where was your ammo stored, in the sun or in a box under cover? Did you clean the gun, was one time the action full of oil and the next time running fairly dry?

 

[lmccrock]

then they can explain how bullets at the outside edges of the 100 yard group know which way to change their direction so they'll shoot smaller MOA groups further away and those dead center in the 100 yard group don't change direction at all

No one said bullets change direction; they wobble, and that wobble can open groups enough for them to lose. The bullets stabilize and do not wobble at greater ranges.

Lee

 

[GeauxTide]

+5 BartB

Well said.

 

[Bart B.]

Old Grump says:

We called it putting the bullet to sleep and is why we only used our heavy 178 gr bullets at long range, (beyond 500 yards) because whether some want to believe it or not they were moving in a orbit around the path of flight and when they finally went to sleep they were on the flight path and the only turning of the bullet was the twist.

Well, I don't believe that. Spark photographs have proved that the bullet's tip cones or nutates about the trajectory axis but the center of bullet mass stays dead on its trajectory. Good info's in this web site:
http://www.nennstiel-ruprecht.de/bullfly/index.htm

Having seen thousands of 30 caliber bullets going down range in all sorts of conditions, I've never seen a bullet take a circular path about its mean trajectory axis. With the wind blowing, the "trace" is as straight as an arrow. But in near dead calm conditions when the air movement is random, the bullet still goes straight down its trajectory but the "trace" caused by it does sometimes appear corkscrewed in shape. That's caused by the random air movements the bullet goes through. And this is what makes folks think the bullet takes a spiral path down range.

Sierra Bullets' has proved that the bullet's coning motion due to unbalance has damped out by 100 yards. Their own tests with both flat base and boattail bullets of the best quality shoot sub 1/4th MOA groups through 300 yards.

 

[Old Grump]

Great site and I have it bookmarked now. Didn't mean to make it sound like the bullet was doing a lunar orbit around the flight path but slow motion film I have seen the tip is well out of the center of axis but this was with 178 grain bullets and this was in 1972 so I would have to hope that they put some improvements into their bullets by now. On the other hand my rifle shoots 180 grain bullets and my group size doesn't differ much from 200 to 300 yards either. Back in my competition days a coffee cup size group at 300 yards with military issue ammo was bragging groups. Today I would be disappointed.

I still think his gun had more to do with his group than the ammo, sometimes it just takes a few boxes down the barrel before it starts to behave.

 

[Bart B.]

Lee (lmccrock) comments:

No one said bullets change direction; they wobble, and that wobble can open groups enough for them to lose. The bullets stabilize and do not wobble at greater ranges.

I know they stabilize a few dozen yards downrange, but they're flying in a straight trajectory although it be curved vertically due to gravity. And they all have a tiny bit of residual "wobble" due to their unbalance which causes their drag (BC) to vary about 1% resulting a slight amount of vertical shot stringing. But I don't think they wobble enough to enlarge groups horizontally; I've seen poor quality bullets shoot very straight through 600 yards into groups the size of wash tubs.

In my own tests (and that of others) on how much groups open up as range increases, starting at 100 yards, they open up about 10% at every 100 yard point further down range. Vertical shot dispersion is mostly caused by muzzle velocity spread and horizontal dispersion mostly by subtle wind changes of less than 1 mph.

 

[lt dan]

sorry for leaving you guys hanging there for a while. i have been out for a short hunt and when i got back i saw that there were a couple of good replies to my questions. it seems only fair that i respond to the questions that old grump asked. so here they are:

i dont know my rifle twist. the barrel was shortened on two occasions. my bullet weight is 180grn flat base. i find that only a 180grn flat base allows me to seat the bullet so long that i can compensate for the "jump" i mentioned in my opening post. having said that i can compensate for the jump with a 168grn m/king sierra as well but this bullet simply dont give me a grouping and what is even more confusing is that the 168grn(boat tail) hits the paper lower than the 180grn(flat bottom)!!!!???? hence the opening topic: confused 308 accuracy. i dont know my velocity but what i can tell you is the best groupings i get is with max loads minus a half.

the difference between my usual 100m bench rest shooting to check loads and THIS 200m shooting was the rest and the temperature. at 200 m i dont have a rest. it is unusually cold the last 12 days. and my barrel is very thin(very, very thin). so after 3 shots on a normal day it is so hot that you cant touch it. though under these cold conditions i could get 5 shots off without it getting so warm. remember that the barrel is free floated by a gunsmith.

at the moment the rifle zero"s at 220m and shoots 3 inches high at 100m.

(180grn flat base)

 

[Jim Watson]

There was a "benchrest guy" who set up one of those Oehler Accoustic Targets, which determines the placement of a passing bullet by triangulating on its shock wave, at 100 yards; and a paper target at something over 300 yards. That way he could plot shots at two different ranges for the SAME bullets.
He said he NEVER had a group smaller in terms of MOA at 300+ than at 100.

I do not credit the "sleepy bullet" theory.

There is an old joke about the hillbilly first hearing about a Thermos bottle, that it keeps hot things hot and cold things cold. He was puzzled, "But how do it KNOW?" As Bart said, how do the outside bullets know to curve back into the center?

 

[Old Grump]

It isn't a sleepy bullet. It was called putting the bullet to sleep which only meant that it had a wobble that made groups wider and unpredictable but at longer ranges the bullet had stabilized. I'm talking 10's and X's here not 12" groups.

lt dan: Since your gun likes 180 gr and not 168 gr, (mine is the same way), best guess is you have a 1/12 twist. Instead of going way down to 168 gr bite the bullet and buy some match 175 or 178. Also might want to try 185 or 190 with a sightly reduced load. Or just a different brand of bullet. Mine likes 180 gr Remington Corelokt yours might prefer Speer or Hornady or Barnes. Before you do any of that I would just try it again and see if you still have the same results, it might have just been a bad day.

Last but not least you might want to try a different powder. A wide variety out there. Mine prefers 4350, not the fastest powder out there but it gives me great accuracy and that's my main concern. You said you shortened the barrel and recrowned it. Maybe you need to change the brand of your powder or have you done that?

Biggest thing that jumps out at me though is you shot from a rest at 100 and without a rest at 200. I will bet if you shoot prone at both 100 and 200 your groups will look a lot better. Sometimes its the little things like pressure on the forearm when you are slinged up and laying on the ground vs shooting from a sandbag rest with your forearm free of pressure. Whatever it is it will be the last thing you look at then comes the head slap and the "Doggone it, why didn't I think of that before?" or whatever you say when the light shines through.

 

[azredhawk44]

having said that i can compensate for the jump with a 168grn m/king sierra as well but this bullet simply dont give me a grouping and what is even more confusing is that the 168grn(boat tail) hits the paper lower than the 180grn(flat bottom)!!!!????

Makes perfect sense. lighter bullets tend to hit lower than heavier bullets at close-in targets. Has to do with dwell time in barrel and recoil impulse. Heavy bullets cant the barrel up higher during recoil.

I've heard the wobbling and "putting a bullet to sleep" thing before with boat tails. At short-range targets (less than 200 yards), my M14 .308 shoots flat base 150 and 165gr bullets best. At longer range targets (300 yards) it begins to shoot boat-tails better. I need to get out to longer ranges than 300 to get a feel for what happens to flat-base bullets at 500+.

I've also heard that bullets can spin off their designed center axis, along their true mass center axis. I can imagine a bullet being affected aerodynamically while wobbling due to a bad mass center axis, and shifting aim a fraction of a minute.

Frankly... I'm intrigued by your excessive jump from chamber to lands. What's your cartridge OAL?

 

[HiBC]

My 2 cents,

It's not like this is a custom made match /benchrest rifle.
Its an inherited skinny barrel featherweight hunting rifle.
It sounds like it is giving you less than 1.5 MOA
That means at 400 yds,it will strike within 3 in of point of aim.
Sounds like a very sweet rifle!!Enjoy!!There is nothing wrong.

 

[lt dan]

azredhawk44 my OAL is: 2.901569 inch that while LEE says it should be maximum 2.8 inch.