OAL and accuracy (30-06)

[Peakbagger46]

I have been trying to get 180g accubonds to shoot in my Sako A7. 180g SSTs with rel 22 at 3.334" (max magazine length) has been great accuracy wise but I really like the game performance of Accubonds.

Accubonds seated to the lands measured out to a whopping 3.440"! The ones I have been shooting at max mag length (3.334) have proven inconsistent accuracy wise.

So... I intend to test shorter OAL to see what happens. What incrimintes should I be testing them at?

 

[murf]

.010" until you get close, than .005".

murf

 

[Bart B.]

What accuracy did the ones you have been shooting at max mag length (3.334) have? Remember few-shot groups can have a 3 to 4 times spread in group sizes.

I'd seat bullets to 1/16th inch less than magazine length.

Forget about tiny increments of different OAL then shoot the ammo. Unless you start out at 1/2 MOA range, changing seating depth for different OAL will end up using a few hundred rounds if you want over 90% confidence the OAL you like is valid.

 

[murf]

you might want to fiddle with the powder weight before you mess with the oal. if your powder isn't right, the oal really isn't going to matter.

murf

 

[Peakbagger46]

Yeah, I think I will mess with powder charge a little more first. Seating to max magazine length, I am jumping theses bullets .11"! Thinking that INCREASING that jump is not likely to help...

I'm getting 1.6" 3 shot groups at 100y. SSTs are more like .8 and have done well out to 400y.

 

[SARuger]

I have the same problem with my Ruger American 30-06. 3.33" is all my mag will take. I load Sierra 165g spitzers. I also see some great accuracy but wish I could reduce the jump. I'm seeing 1/2" at 100yds. Pretty sure I could one ragged hole with it if I could work with the mag.

I'm using Reloader 15

 

[Reloadron]

you might want to fiddle with the powder weight before you mess with the oal. if your powder isn't right, the oal really isn't going to matter.

murf

I have to agree with that. I really don't see where bullet seating depth will enter into things until the bigger factors are done and that would be what powder with your bullet choice. The powder ans amount of powder first and then bullet jump. Just my take on the subject but I agree with murf.

Ron

 

[Ifishsum]

I never could get Accubonds to shoot as well as SSTs in either of my '06s but you may have better luck than me. They may like a different charge weight, or even a different powder. I'd test out a little more down that vein first...even if they do shoot better seated long do you really want to limit yourself to single load while hunting?

 

[Peakbagger46]

No, I don't want a single shot for hunting. For deer that wouldn't be too bad, but elk and moose are also on the menu.

 

[Andrew Leigh]

I have to agree with that. I really don't see where bullet seating depth will enter into things until the bigger factors are done and that would be what powder with your bullet choice. The powder ans amount of powder first and then bullet jump. Just my take on the subject but I agree with murf. Ron

I agree with both Ron and Murf. It is a simple matter of cause and effect and it is with great amusement that I note (where I live at least) that jump is considered to be the holy grail of final group reduction. You get your load close and then tweak the jump for perfection.

So what happens when we tweak the jump, two things;

The first is that we alter the internal volume of the case thereby altering the internal pressure / velocity / elapsed barrel time.

The second is that we reduce / increase the inertia that the bullet requires to overcome the lands.

So changing the jump (not talking about high precision shooters here who seat into the lands) simply is a method of altering the load. So where some people are very close to an accuracy node a change either way in the jump could be sufficient to bring them onto an accuracy node. Now when you bear in mind that pressure lifts rather exponentially as you approach the lands then you will understand that you can significantly alter the characteristics of the load in question. In my opinion chasing the lands as a means of altering load is not the safest reloading practice.

Now there is a definite point at which the relationship between internal pressure and inertia. I believe that there are many misconceptions about the interplay between the two. All things being equal, if you have an extremely long leade, then as you seat the bullet further out you increase the internal volume of the case and you reduce the pressure and consequently the velocity.

Now this would be all good and well if we had a long lead but most rifles simply are not designed that way so we have to contend with the lands. An herein lays the rub. There is a point where the reducing pressure in the case as one seats further out is reduced / offset by the increasing effect of inertia.

After numerous debates on local forums on the matter I went out to test the theory. At the time I was doing load development for a friends 7.62mm Israeli Mauser. So to the OP the calibre is the same so probably very relevant to your current situation.

I prepared a series of loads with the only variable being the proximity to the lands. So I loaded at 2.00mm (0.080") from the lands coming down in 0.008" step until 0.008" off the lands.

I shot 3 shots per jump and averaged out the speed. I then fed the speed into QL and arrived at a pressure. I then for each group modelled the QL calculated pressure, take note QL does not take into account the effect of inertia other than when you are directly into the lands. So this is the plot;



The blue trace is the theoretical pressure and the red trace is the pressure based on the actual recorded velocities. What is also apparent is that at 0.040" that the effect of inertia is being experienced.

So while a change in jump may well close a group it is my considered opinion that this is not due to the distance from the lands but rather due to the change in the ballistic profile from the combination of internal pressure and inertia.

I would, for a hunting load, encourage staying away from the lands and loading just short of the magazine length so that cycling a new round into the chamber never becomes an issue, normally not a problem with a 30-06 due to the action length.

I shoot 180gr. Accubonds as my primary 30-06 hunting load at 0.080" from the lands (only because I saw an article on this as a recommendation). I shoot consistent 0.75MOA groups and I am not a great shot. 180gr. SGK's I shoot 0.040" from the lands and get slightly better grouping but I have found Sierra's weight for weight to be more accurate than Accubonds, if only by a small margin. This I have found with 140gr. (6.5x55mm), 180gr. (30-06) and 300gr. (.375).

One final argument for jump. On my .375 I cannot get to within 0.250" from the lands, here is a group I shot with 300gr. SGK's.



Now granted I can't repeat this 0.240MOA shooting every time I pick up the rifle but if jump was so crucial then explain how being so far from the lands could yield such results?

 

[TimSr]

you might want to fiddle with the powder weight before you mess with the oal. if your powder isn't right, the oal really isn't going to matter.

Agreed. Before you can do fine tuning you have to be on the right channel.

 

[Bart B.]

Keep in mind that the .30-06 erodes away the rifling at the bullet's contact point in the throat .001" for every 30 or so rounds fired.

When you think you've found the perfect OAL for some accuracy level, are you going to seat bullets .001" less every 30 shots to keep the accuracy the same?

To say nothing about the fact that the bullet's jump distance to the rifling is not controlled by the case head to some point on the bullet. The critical measurement is from a reference point on the shoulder to that point on the bullet that first contacts the rifling. The case shoulder's hard into the chamber shoulder and there's a few thousandths spread in head clearance between the bolt face and case head.

I'll explain how being so far from the lands could yield such results as that tiny 3-shot clover leaf group pictured above. All 3-shot groups are not the same size. They vary as much as 5 times in size from smallest to largest. If one thinks a given change to the load will produce that size groups all the time, they should prove it by shooting 10 groups. Then see if they're all that size. Otherwise, that tiny group's part of a whale's tail; a fluke.

Single 3- and 5-shot groups are extremely popular for testing ammo for some parameter. Few people verify their reality by testing several times to see if it's for real. Even the best bench rest competitors see a spread near 5X across the same load's sizes for both 5- and 10-shot groups, several are shot each day. Most people think what a load's single few-shot group first shoots is what that load will shoot all the time.

 

[Peakbagger46]

Thank you Andrew, that was very enlightening! I'm going to try going up and down in either .3 or .5g increments and keep OAL the same for now.

 

[Silent Sam]

It's pretty well established that Accubonds shoot better with a "longish" jump. Chasing the lands with Accubonds will likely lead to frustration.

 

[SARuger]

I'm going to try going up and down in either .3 or .5g increments and keep OAL the same for now.

This is what I did with my Ruger American, the mag was my hindrance. I did a load work up in .5 increments (RL-15) and found one that was the most accurate then went up and down by .2 increments. I have it shooting MOA, sometimes sub MOA on Sierra 165g spitzers now. I think most of that is falling on the shooter.

The jump on mine is more than yours. I don't have my notes in front of me to post it though.

Since this is a hunting rifle, I'm okay with it.

 

[Peakbagger46]

"It's pretty well established that Accubonds shoot better with a "longish" jump. Chasing the lands with Accubonds will likely lead to frustration."

That's good to hear! I will not be chasing the lands any CLOSER as I don't want a single shot elk rifle.

I got some loaded in .5g increments and hope to test soon.

 

[Gtscotty]

So while a change in jump may well close a group it is my considered opinion that this is not due to the distance from the lands but rather due to the change in the ballistic profile from the combination of internal pressure and inertia.

Interesting post, sounds like reasonable theory to me.

 

[Gtscotty]

Keep in mind that the .30-06 erodes away the rifling at the bullet's contact point in the throat .001" for every 30 or so rounds fired.

When you think you've found the perfect OAL for some accuracy level, are you going to seat bullets .001" less every 30 shots to keep the accuracy the same?

I guess that depends on the size of the OAL sweet spot you've found. If you are at the short end of a 0.02" sweet spot, you might not see a difference for 600 rds, even though the throat is constantly eroding.

 

[Peakbagger46]

Think I found a load!

After all the advice, I kept OAL the same and shot two three shot groups of different charge weights .5g apart. The mid range loads shot the best, averaging 0.84" at 100y, much better than the 1.5" groups I had been getting.

I will play with it some more between 200-400 (maybe 500 eventually) but this should be a good load for pronghorn-elk.

I did adjust the scope between groups. This new model VX 2 3-9X40 CDS is really nice, especially for 175 bucks.

 

[murf]

nice shootin pardner!

murf

 

[GooseGestapo]

Bart B.;
I'm going to call "bunk" on the .001" erosion per 30 shots in the .30/06.
If that is the case (it isn't!) then 3,000rds will extend the throat by erosion by 3 Inches !!!

In otherwords, the throat would be shot-out in 300rds or less.

Recheck your math and "facts"! Maybe add another zero to the right of your decimal point.

 

[Bart B.]

I rechecked my math and facts. Facts are from people tracking rounds fired and throat erosion in 30-06 match rifle barrels from new to .100" advancement which is when their barrels were worn out for serious competition.

Formula: throat advancement divided by round count equals erosion per shot.

.100 inch erosion / 3000 rounds = 0.000033 inch.

0.000033 X 30 rounds = .001 inch.

3000 rounds X 0.000033 = .100 inch.

.308 Win's erode .001" for every 40 shots.

30 caliber magnums erode the throat .001" for every 20 to 10 shots; .300 Wby Mag being at 10. My .264 Win Mag barrel eroded .001" for every 6 to 7 shots. Barrel lasted 640 rounds after .100" of bullet contact point erosion from when new.