Distance off the lands

[Morrey]

Just measured the distance with ogive resting on the lands to be 2.790 with a bullet that I will use to begin a load workup. What is a good COAL to begin testing? How much should I back off the lands to prevent pressure buildup? Thanks!

 

[CarJunkieLS1]

Caliber and bullet is it a hunting bullet or a match grade bullet? IME standard hunting type bullets tend to shoot better with a little bit of jumo in to the lands. I know every rifle and barrel is a rule unto itself, but my rifles shoot best with .025-.050 jump.

 

[outlawjw]

The experts say ( & I am not one) to touch lands while working up your load to find max load. Then you know what max truly is & can stay away from that load . mine prefer about .030" on average & I just hunt & play around the farm

 

[Saintsfan1977]

Start with .020 off the lands. You can adjust forward or backward from there in .003 increments.

 

[Morrey]

This is a Sierra Pro-Hunter bullet, 150 gr, .30-06. I use a Hornady comparator and headspace gauges for measuring on the ogive and bullet shoulder/headspace. I'll try .030 backed off from touching lands. When I run a gamut of powder workups, I can then try adjusting the distance off the lands in small increments + and - Thanks!

 

[Morrey]

Honestly I couldn't remember if I used 30 thousandths (.030) or 3 thousandths (.003) in my previous load workups. My question was answered in that it was 30 thousandths! I added this value in my reloading notebook.

 

[Wildfire]

All different.

My .223 rifles have long twist rates of 1-14. They toss 40 grain bullets with extreme accuracy. The 55 grain bullets are also very accurate. I run the 55s at .010" to .012" off the lands and this has been the most accurate loads with no signs of PSI. 40s set back more yet.
My .308 PSS had an extremely long throat. .250". It was impossible to get close to the lands with 168 grain bullets. That rifle was still one hole at 100 yards.
My rule was, the hotter the round the more back set they got.
Touching the lands was an old idea. It creates PSI rapidly.
Once you find the powder you like, and your brass fits your chamber correctly, bullet set back is likely the next action required for fine tuning accuracy.

 

[ArchAngelCD]

IMO worrying about distance from the lands is something only benchrest shooters with very expensive rifles should worry about. In a hunting rifle, even a good one I feel its a wasted time and components. How do we explain the very long jump the bullet must make in most, if not all Weatherby rifles yet Weatherby rifles are known to be extremely accurate. An exception to the rule or does it disprove there is a rule?

Just a note, you mentioned when you get the powder charge right you are going to then change the COAL. Many times the optimum powder charge at one COAL will not be the best when you change the bullet depth. It's a viscous cycle I tell-ya. Lol

 

[Rule3]

IMO worrying about distance from the lands is something only benchrest shooters with very expensive rifles should worry about. In a hunting rifle, even a good one I feel its a wasted time and components. How do we explain the very long jump the bullet must make in most, if not all Weatherby rifles yet Weatherby rifles are known to be extremely accurate. An exception to the rule or does it disprove there is a rule?

Just a note, you mentioned when you get the powder charge right you are going to then change the COAL. Many times the optimum powder charge at one COAL will not be the best when you change the bullet depth. It's a viscous cycle I tell-ya. Lol

Even though I do not know much and I have forgotten what I did know about rifles, I agree with the above. I load my 223/556 and 308 to whatever Hornady or Sierra tells me and it seems to work just fine at 100 yards.

How many successful hunters out there use factory ammo??

 

[Andrew Leigh]

Distance from the lands is not the holy grail of reloading.

My research in a .30 calibre is that pressure will drop as you seat further out ..... until that is you get to within 0.040" from the land and then pressure begins to rise almost exponentially.

Pro Hunters and Game Kings have always done well in my rifles from 0.040 to 0.100. I keep them at 0.080" for some obscure reason. Always want them to cycle when hunting.

Many will get a good load, then play with the jump and get better results believing the jump to be the reason. Nope, it is the change in the pressure in the case that brings the load together not the jump.

If jump was all important then my .375 would never be able to shoot as I cannot get closer than 0.25" from the lands.

 

[Metal God]

Andrew

Real question not snarkyness . If it's the pressure in the case and not the jump . Then why does ( making up numbers ) 43gr shoot better .005 off the lands and 42.6gr does not at .045 off the lands . I know there could be differences but those pressures should/could be the same ?? My point is I've done a few load work ups that started .045 off the lands or so in .4gr increments in 308 . Found a great charge of 43gr where I was shooting small groups . Then adjusted seating depth in .010 increments both back and forward and found .005 off the lands tighten the group up quite a bit . If pressure in the case was the actual factor why did .005 off the lands shoot better then the .045 off the lands with a powder charge of 42.6gr during the original work up ? shouldn't one of the lesser charges shot better during original development ? Now I believe the over all pressure of the complete internal ballistics had something to do with the results but I'll need some convincing it was only the start/case pressure that made the difference .

New question :
Don't you have to consider where you are starting first . If the book says the recommended COAL is 2.800 but you start 2.870 and are still .030 off the lands . Backing down the seating depth .040 to 2.830 is not going to change that case pressure much seeing how you have .070 of free bore fore gases to escape around the bullet before it enters the lands ? It was my understanding that the bullet really needs to start to take up a good amount of the case volume before pressure starts to go back up . Now if the book says your COAL should be 2.800 and you seat your at 2.760 . I can see that causing an issue .

If you are out past the recommended COAL and still way off the lands . You should have some wiggle room to play with seating depth in both directions with out pressure going off the charts at either end . Now of course that does come with a big assumption . That is you are not at the max pressure for that cartridge when you start adjusting seating depth .

One should never change any aspect of a cartridge with out reducing the charge if the cartridge to be adjusted is already at max pressure to start with .

Sorry OP . .020 or .030 off the lands is a good place to start .

 

[Andrew Leigh]

Andrew

Real question not snarkyness . If it's the pressure in the case and not the jump . Then why does ( making up numbers ) 43gr shoot better .005 off the lands and 42.6gr does not at .045 off the lands . I know there could be differences but those pressures should/could be the same ?? My point is I've done a few load work ups that started .045 off the lands or so in .4gr increments in 308 . Found a great charge of 43gr where I was shooting small groups . Then adjusted seating depth in .010 increments both back and forward and found .005 off the lands tighten the group up quite a bit . If pressure in the case was the actual factor why did .005 off the lands shoot better then the .045 off the lands with a powder charge of 42.6gr during the original work up ? shouldn't one of the lesser charges shot better during original development ? Now I believe the over all pressure of the complete internal ballistics had something to do with the results but I'll need some convincing it was only the start/case pressure that made the difference .

I can only relate to my experiences and those of people I have coached. But before that. Please explain what you think the difference is when a bullet engages the lands from 0.04 or 0.05" and how this could affect accuracy. The lands profile is the same, the bullet profile is the same so how would the barrel determine which bullet was seated further away and that this should now be more accurate than the previous bullet? What mechanical considerations could possibly cause a significant change in accuracy. To my mind the only thing that changes is the inertia, for most of us the bullet engages the lands before it leave the neck. So what happens is that in the one scenario the bullet has more time to build up a head of steam before engaging the lands. In both cases the internal pressure of the case is affected which affects the velocity which affects the elapsed barrel time which affects the point at which the bullet leaves the muzzle which affects the accuracy.

IMHO most people attribute the change in accuracy to the change in COL but completely discount that they have altered the internal ballistics, the very reason for working up a load is to alter the internal ballistics such that a sweet spot is found. So when close to a node people will then tweak the COL, all this is achieving is a fine tuning of the internal ballistics which can be achieved by tweaking the load. So in the work I have done it is not the proximity to the lands that is meaningful but getting the internal pressure correct so that the optimum barrel time can be met to ensure that the bullets exits the muzzle at the optimum sweet spot.

Don't you have to consider where you are starting first . If the book says the recommended COAL is 2.800 but you start 2.870 and are still .030 off the lands . Backing down the seating depth .040 to 2.830 is not going to change that case pressure much seeing how you have .070 of free bore fore gases to escape around the bullet before it enters the lands ? It was my understanding that the bullet really needs to start to take up a good amount of the case volume before pressure starts to go back up . Now if the book says your COAL should be 2.800 and you seat your at 2.760 . I can see that causing an issue .

If you are out past the recommended COAL and still way off the lands . You should have some wiggle room to play with seating depth in both directions with out pressure going off the charts at either end . Now of course that does come with a big assumption . That is you are not at the max pressure for that cartridge when you start adjusting seating depth .

One should never change any aspect of a cartridge with out reducing the charge if the cartridge to be adjusted is already at max pressure to start with .

Sorry OP . .020 or .030 off the lands is a good place to start .

I have posted this before but will do so again. Actual results with a .308 same load but increasing COL.



So above you can see the effect of a load at 2.0mm from the lands (0.080") down to 0.008" from the lands. Here you can see the relationship of seating depth. The blue trace discounts the effect of inertia and is the theoretical internal case pressure as you seat further out. The red trace is the calculated pressure form velocities recorded on the range.

So depending on which side of the turning point of 1.00mm you are, the same change in jump will have a different effect.

I wish I still had the source data of the recorded velocities but a laptop theft denies me of this. I had uploaded the pic to Photobucket so fortunately have that.

 

[cheygriz]

I've had excellent results starting .005 off of the lands and working back. Most of my loads shoot best somewhere between .005 AND .020.

 

[Metal God]

Please explain what you think the difference is when a bullet engages the lands from 0.04 or 0.05" and how this could affect accuracy.

The longer jump can up set the bullet forcing it into the lands at a cant or crooked not unlike having a huge run out to start with . There are a few factors that could contribute to this possible cause . Case prep , neck concentricity can both start the bullet out at a cant and the longer the jump the larger the cant gets as it travels through the jump . The angle of the forcing cone may have an effect on this but that's getting above my pay grade . My thought on that is a more tapered cone "may" re-center the bullet as it enters . Rather then a cone the bullet abruptly enters .

IMHO most people attribute the change in accuracy to the change in COL but completely discount that they have altered the internal ballistics, the very reason for working up a load is to alter the internal ballistics such that a sweet spot is found. So when close to a node people will then tweak the COL, all this is achieving is a fine tuning of the internal ballistics which can be achieved by tweaking the load.

I don't disagree with that . In fact I very much agree with that and even said so

Now I believe the over all pressure of the complete internal ballistics had something to do with the results but I'll need some convincing it was only the start/case pressure that made the difference .

My issue is with the cause being case pressure and I'll admit I may not be understanding what you mean by case pressure . To me case pressure means the first/initial pressure as the powder starts to burn but before the bullet has really started to move . Once the bullet starts to move the case volume is increasing as the bullet moves from the case . Between initial ignition and the bullet engaging the lands . Gases can actually escape around the bullet reducing pressure . So as your graph seems to show as long as you have a jump . You not only get gases escaping around the bullet . The space behind the bullet increases as well creating more volume for the gases to fill . In general those two combinations help keep us in a safe pressure zone . How ever when you seat the bullet right up against or even in the lands . You remove both those variables . So now you don't have the bullet freely moving forward increasing volume behind it . You also removed the ability for the gases to escape around the bullet . Resulting in the pressure spike you see in your graph .

I'll also admit after going back and re-reading your first post

My research in a .30 calibre is that pressure will drop as you seat further out ..... until that is you get to within 0.040" from the land and then pressure begins to rise almost exponentially.

I completely misread that to say seating the bullet deeper in the case results in higher pressures then seating closer to the lands . As we can see it does not even come close to saying that . Not sure why I interpreted it the way I did but I got that wrong and that's the very reason for the question . All this may be for not , Sorry about that .