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Thread: Seating off the lands

  1. #1
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    Seating off the lands

    At lunch yesterday with 6 shooters we had a discussion about seating depth. The caliber was a .308 Sierra 155 gn Palma bullet. The consensus was that you needed to start at .040 off and work back, up to maybe .100. I'm trying to visualize what's going on in the barrel after the primer pops and the bullet starts going forward that would result in better accuracy/group as the bullet is seated progressively more off the lands. Anybody got any theories as to why this happens? Is the bullet free of the case before it hits the lands? In our short range benchrest world we don't seem to operate in this arena.

  2. #2
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    is this a specific load..that is a palma bullet.
    i start at the lands work up a load and tune length,
    and try small powder adjust if i end up shorter.
    so far have always found a load much closer than forty off.
    i have heard those stories, just never fit for me.

  3. #3
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    Yes this is a specific case and some end up at 70 off to get rid of vertical. We do all this testing of different seating depths until we find one that seems to work and then go on. But does anyone have any theory as to WHY this occurs? Why does a particular load perform better at 70 off than 40 off? Don't know if anyone has done any research into this but its just piqued my curiosity.

  4. #4
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    two cents worth

    distance to the lands is time, time is velocity.
    maybe the bullet needs velocity to engrave/fit the bore squarely.
    maybe to little allows an off center start.

    ya get what you pay for.
    i have the similar lapua bullet, but have done
    very little work with it.
    mainly 175/185's

  5. #5
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    I'm of the mind

    that barrels can be tuned either by going off the lands or into the lands. For instance, I have a friend who use to tune his 30 BR off the lands but only .009" and it shot one bullet hole. Most of the folks I know tune slightly into the lands. Me, it's .006" in but I believe both would work in any barrel.

    Pete

  6. #6
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    Red face

    OP poses n interesting question
    I never understood the physics of this issue and choosing OAL by experimentation was kinda Art or alchemy.
    I have always wondered why OAL is an issue
    I have read it had to do with angular exit from the case mouth, but that would not explain how some get better groups OFF the lands
    Velocity is velocity, if that is true then your best group would be a given powder charge for a fixed distance or in said lands because you can vary powder charges and change velocities, but that group changes when you use a different OAL

    Thus if there is a given velocity and that is the answer, then it matters not where the bullet is with respect to the lands , provided velocity is constant for more than one OAL, but in practice that is not the case.

    Could it be the OAL has an effect on the barrel resonance for a constant velocity?

    Beggs on this forum says that the bullet should be just touching of just off the lands, and changing the barrel resonance with the tuner will control the group size, and if that is so, it suggests that OAL without the tuner, resonance will change by changing OAL.

    I would be grateful for some very smart and experienced shooters who could better explain the physics of Jump.

  7. #7
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    I have no foggy clue what "angular exit from the case mouth" means but IMO the reason seating depth plays a role is fairly simple....... getting the bullet going down the bore almost always involves some sort of "start/stop/start" cycle. For instance with a soft-seat and some jump the primer blast may advance the bullet to the lands where it will STOP until pressure builds enough to engrave/obturate the bullet and it continues down the bore. Any changes in the timing of this sequence will change vibration timing.

    Because tuning is based on control of vibration, playing with the vibration impulse in ANY way (seat depth/neck tension/tuner/primer/ogive etc) can be effective.

  8. #8
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    In my opinion

    Exact OAL" is so that each and every bullet will engage the lands and groves at exactly the same place every time. I have proven many times that Exact OAL's is the basis of real accuracy, that along with exact weigh powder charges. It's provable any time. Each bullet must be presented to the throat of the chamber the exact same place and time every shot. I have generally found that .006" into the lands, measured from where land marks are faintly visible on a polished bullet .006" is likely what some consider just touching but I almost always find that .006" in is where the magic comes.

    My current load for my 30 BR''s is 35.5g of H-4198 and .006" in. One single bullet hole groups. Both rifles love it. It's a mid node but shoots great and no worries about getting the powder in.

    Pete

    I also have a great load of Reloader 7 worked up with the same results: 34.6g. I have used Re 7 for years in both the 30 BR and 30-44 with great results. It doesn't load as easy as H-4198 but it has worked for me as well. I tested 35.6g of Reloader 7 the other day with good results but had about a quarter of a bullet sideways to the left out of the bullet hole so don't know if I pulled it there or it needs another tenth. I'll try it again when I get time, along with an added tenth. It is so important to have exact everything when testing to know exactly what the rifle is telling you.
    Last edited by Pete Wass; 08-21-2019 at 03:31 PM.

  9. #9
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    Quote Originally Posted by Pete Wass View Post
    Exact OAL" is so that each and every bullet will engage the lands and groves at exactly the same place every time. I have proven many times that Exact OAL's is the basis of real accuracy, that along with exact weigh powder charges. It's provable any time. Each bullet must be presented to the throat of the chamber the exact same place and time every shot. I have generally found that .006" into the lands, measured from where land marks are faintly visible on a polished bullet .006" is likely what some consider just touching but I almost always find that .006" in is where the magic comes.

    My current load for my 30 BR''s is 35.5g of H-4189 and .006" in. One single bullet hole groups. Both rifles love it. It's a mid node but shoots great and no worries about getting the powder in.

    Pete
    Did you mean H-4198?

  10. #10
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    Yes

    Quote Originally Posted by adamsgt View Post
    Did you mean H-4198?
    I'm a bit dyslexic at times, sorry.

    Pete

  11. #11
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    Quote Originally Posted by alinwa View Post
    I have no foggy clue what "angular exit from the case mouth" means but IMO the reason seating depth plays a role is fairly simple....... getting the bullet going down the bore almost always involves some sort of "start/stop/start" cycle. For instance with a soft-seat and some jump the primer blast may advance the bullet to the lands where it will STOP until pressure builds enough to engrave/obturate the bullet and it continues down the bore. Any changes in the timing of this sequence will change vibration timing.

    Because tuning is based on control of vibration, playing with the vibration impulse in ANY way (seat depth/neck tension/tuner/primer/ogive etc) can be effective.
    Al, some thoughts running around in my mind about this. Does the position of the bullet in the case neck at the times it enters the lands a factor in accuracy? For the 6PPC I've read that people use a 1.490 to 1.510 case length. I measured the chamber length on two of my 6 PPC barrels and IIRC the chamber length was 1.525. So if my brass is trimmed to 1.490 there is .035 of chamber where the bullet might veer, maybe. Cause at that point you now have space around the bullet for gas to flow as it is no longer supported by the case. It's probably as important as the number of angels that can dance on the head of a pin. But, I'm still curious as to why a bullet that is already seated off the lands can shoot noticeably better when seated significantly more off the lands.

  12. #12
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    Quote Originally Posted by adamsgt View Post
    Al, some thoughts running around in my mind about this. Does the position of the bullet in the case neck at the times it enters the lands a factor in accuracy? For the 6PPC I've read that people use a 1.490 to 1.510 case length. I measured the chamber length on two of my 6 PPC barrels and IIRC the chamber length was 1.525. So if my brass is trimmed to 1.490 there is .035 of chamber where the bullet might veer, maybe. Cause at that point you now have space around the bullet for gas to flow as it is no longer supported by the case. It's probably as important as the number of angels that can dance on the head of a pin. But, I'm still curious as to why a bullet that is already seated off the lands can shoot noticeably better when seated significantly more off the lands.

    Hokayyyy, this is gonna' pi$$ some folks off, and I doubt I'll take my thoughts very far as that's just asking for the jackals to swarm in.... but your timing of this particular question is freaky so I will share --- I JUST ORDERED TWO IDENTICAL 6PPC REAMERS, ONE AT 1.500 AND ONE AT 1.550!!!! Both with with my currently f'rav'ritest .268 nd

    So that makes at least TWO of us interested in this subject.

    I call that 35thou you've noted above "gapspace"..... and I have spent time exploring this "gapspace" via my collection of 6BR reamers, all different..... but this is the first time I've actually ordered two otherwise identical reamers, but one of them 50thou longer than the other.

    I don't have a way to answer your actual question above, " Does the position of the bullet in the case neck at the times it enters the lands a factor in accuracy?" because I don't understand what you're actually asking but YES, I explore "position of the bullet in the caseneck" a lot.

    Do I have any "answers" to post here?

    Absolutely NOT

    LOL

  13. #13
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    I will say this.... my findings agree with Pete's. Without even entering the argument of "where" or "how much" I will opine quite strongly that THE SAME EVERY TIME is crucial. And add that this is a moving target.

    Most of my newer grinds are "cupped leade" or 3-2 angles ground into the leades, generally ogive-specific, and I'm currently completely cornfused as to which angle, or combination of angles is "better"

  14. #14
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    Quote Originally Posted by fx77 View Post
    I would be grateful for some very smart and experienced shooters who could better explain the physics of Jump..
    Lacking on both counts but do have a few theory's:

    Pressure and bullet motion in relation to time and location:

    Itís the primer output that drives a stress wave / powder compression through the powder to the bullet. A shower of hot particles and gases start the bullet into the engraving process largely by a passive medium push. With no ullage the powder acts similar to an imcompressible fluid transmitting the primer force to the bullet with no significant chamber pressure mid case forward. This initial bullet movement reduces the amount of brass / bullet surface contact and the amount of tension or grip.

    After the primer push at some point between engravement, and obturation, gas pressure is in short supply with somewhat of a pressure plateau or static load; bullet movement lacking. Shortly, significant gases and pressure build in the case from the rear to the base of the bullet and then the same at the neck. Pressure builds from the rear expanding the neck as it moves forward until obturation.

    The brass not only has to withstand high internal pressures but, it also undergoes deformations in order to seal the chamber; case material properties e.g. strength and ductility consistency maintain the required level of tension.

    Initial bullet movement and Initial Burn Rate have to match every time. Any pressure changes through case volume uniformity (e.g. compressibility % and Temperature) or neck tension (e.g. sizing, maintaining and detaching) and when blow by starts (seating depth) make a big difference as to the peak pressure value. (time and position)

    Burn Rates are about how long pressure stays high along with velocity but, Initial Burn Rates and Initial Bullet Motion (Impulsive Force) are more about where the bullet is located in relation to peak.

    Not mentioning vibration..........

    BR

    Ken

  15. #15
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    great reply KEN
    just to add the difference between in the lands and out, adds about 4000 to 6000 PSI to your load and changes the spike in the ramp up
    in the case.

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