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Thread: Seating depth

  1. #1

    Seating depth

    This one is probably a question for the more technically minded of you out there.
    I have been reloading for several years, started out reloading pistol ammo, that was before handguns were banned, more recently rifle ammo. I have had some good results and some not so good. Many of the threads on this website and others give testament to the importance of seating depth on accuracy and group size, which I following the perceived wisdom have also found. However I have never come across any information as to why, (some people would say just accept it, but thatís not the way my mind works).
    So my question is why, why does seating depth have such an effect on accuracy and group size?

  2. #2
    Quote Originally Posted by DCG View Post
    This one is probably a question for the more technically minded of you out there.
    I have been reloading for several years, started out reloading pistol ammo, that was before handguns were banned, more recently rifle ammo. I have had some good results and some not so good. Many of the threads on this website and others give testament to the importance of seating depth on accuracy and group size, which I following the perceived wisdom have also found. However I have never come across any information as to why, (some people would say just accept it, but that’s not the way my mind works).
    So my question is why, why does seating depth have such an effect on accuracy and group size?
    google is your friend
    http://www.reocities.com/CapitolHill...21/seating.htm

  3. #3
    DCG, being a newly afflicted reloader, I read all I could about reloading. There seems to be a weight of opinion that seat depth is really about distance of bullet ogive to the leade. From what I can gather, it varies not just from rifle to rifle but in my case from one type of bullet to another through the same rifle.

    When I started reloading I put on a post on this forum that i was following Richard Lee's dictum, the nearer that you seat to the rifleing the more accurate the rounds you will produce. This does not mean that they must be jammed into the rifleing, but close to, it seems to vary by bullet type, what suits some does not suit another.

    I seated my 129gn SST's with much less case neck holding the bullet, much nearer the leade, and it improved my rifles performance no end. But I was slated for following this advice, by some who can produce very accurate loads by just seating to the cannelure. I too have found I can seat to the cannelure and produce reasonably accurate loads, but have done so by crimping, but that's another contentious issue. I will continue with my original load recipie when stalking, for now, untill I find an even more accurate load.

    If you find a scientific answer to what is the optimum seating depth, please post it, I would love to read it!

    ft
    Blindness to suffering is an inherent consequence of natural selection. Nature is neither kind nor cruel but fiercely indifferent.

  4. #4
    Simples it's all to do with nodes and barrel timings. get on the right timing node and you get small groups. get ont eh wrong node and they scatter.

  5. #5
    DCG:-simply put, different makes of rifle have differing lengths of throat/leade, factory rounds are seated well back to allow for this, when reloading you have the option to alter this, from touching the lands to seating back 015"-030" or whatever length that suits your particular rifle, basically what happens is the further back from the rifling the more chance you have of having the bullet being out of concentricity when entering the rifling so this shows up down range on your target, the closer to the rifling the bullet is,ie.015" compared to say 040" as in a factory the more chance there is in the bullet entering the rifling as concentric as possible and giving smaller groups on target, there are other variables to take in to consideration but in a nutshell thats about it, hope this helps.....callie

  6. #6
    I would guess it changes the pressure curve within the chamber giving fantastically small barrel travel time differences. BUT this could be the difference between the barrel being in an upward vibration, downward vibration or a desireable mid way vibration.
    Only a thought. Many of the black arts of reloading are proven in action but really hard to prove empirically.

  7. #7
    longlowdog:- harmonics and resonancies, as you say "black arts".

  8. #8
    Quote Originally Posted by longlowdog View Post
    I would guess it changes the pressure curve within the chamber giving fantastically small barrel travel time differences. BUT this could be the difference between the barrel being in an upward vibration, downward vibration or a desireable mid way vibration.
    Only a thought. Many of the black arts of reloading are proven in action but really hard to prove empirically.
    Ahhh if you go to a mid way part of the movement you in for larger groups. Think of a pendulum it stops of a split second before reversing direction off movement so for that tiny bit of time is stationary unlike the middle where it's constantly moving. The barrel moves and whips in a sort of sign wave with the concussion of the powder and primer going off. It can be worked out the timings and travel time of the shock wave. It's beyond me and my maths but have friends who thrive on this stuff. one has even brought his own pressure trace system but then he is a scientist and laps this stuff up.

  9. #9
    I am not certain about the barrel vibration theory with regard to this seating depth question. I don't dwell on things that I can't prove, or directly manipulate.

    What I do know about seating depth is that it controls "pull weight" of the bullet, bullet alignment in the throat, and case capacity. I recently pointed out to a friend that at one time, reloading manuals told you how deep top seat the bullet, not what OAL to load to. Seating depth of a thirty cal bullet would be listed as something to the effect of " seat to .338 inches" or the like. The OAL was not an issue. The distance to the lands was not a consideration. The emphasis was on the items I mentioned, not how far off the lands the bullet rested. Many articles were written on the proper technique for adjusting the bullet to the proper seating depth. As many will be quick to point out, the resulting seating was probably done in just the same matter as we do nowadays. So, what's the difference? Maybe none for practical purposes, but the datum was measured by the base of the bullets' relationship to the case mouth of a trimmed case, not the bullet tip to the headstamp. This might lead one to think that the importance of controlling the grip on the bullet, the case capacity, and alignment were well considered. JMHO ~Muir
    Last edited by Muir; 15-09-2010 at 11:56. Reason: Clarification

  10. #10
    I'm new to reloading and have been reloading for a .243 with speer bullets. I have been following speers OAL as a guide for bullet seating. So how do you go about deciding how deep or not to seat the bullet? Where do you get measurements from to seat it .015 from the rifle lands?

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