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Thread: Seating off the lands

  1. #16
    Join Date
    Dec 2017
    Posts
    572
    quickload has a generic 7200 psi add for seating at the lands vs off.

    Quote Originally Posted by Lou Murdica View Post
    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.

  2. #17
    Join Date
    Jul 2007
    Location
    Fort Worth Texas
    Posts
    1,531
    Quote Originally Posted by K Hope View Post
    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
    Wonder if the Pressure Trace II system would provide detailed enough traces to show the difference between seating depths. How would you describe what happens if the bullet is seated with a hard jam?
    Last edited by adamsgt; 08-23-2019 at 10:32 PM.

  3. #18
    Join Date
    Nov 2007
    Location
    La Belle Province
    Posts
    150
    Quote Originally Posted by adamsgt View Post
    How would you describe what happens if the bullet is seated with a hard jam?
    I do not believe there is any one, fit all answer to your question and is dependent on the specific cartridge. Eg Compressed loads versus ullage, bullet design and distance of travel to obturation.

    I defer to others more qualified to answer your question than I am.

    I shot a bit with a couple of brothers from VT. Two top notch shooters; one was a strong believer in hard jam and spent a lot of time with his dies and in setting his loaded rounds straight. The other brother was a strong believer that more powder was the answer when he lost his tune. I tried both these methods but never had much luck. That being said these guys had years of experience and were using select bullets and blended powders. It certainly worked for them but for me it was like living on the edge.

    BR

    Ken

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