Seating Depth & Magazine Capacity

Discussion in 'Reloading' started by bill123, May 8, 2014.

  1. bill123

    bill123 Well-Known Member

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    Ok, I did a dumb thing today. I am working up a load for 155.5 Berger Fullbores. I loaded the bullets to .015" off the lands but found when I got to the range, I had to feed the cartridges by hand because they were greater than the magazine length.

    I found a good node, came back, measured the mag. length and found that in order to magazine feed, I need a jump of at least .066"

    Is all of my data wasted? I assume that I can't just load to .066" jump without a pressure spike. Should I start over or should I back down progressively from .015" jump until I'm in the Berger recommended seating test sequence (and below mag length):

    .010 off the lands
    .050 off the lands
    .090 off the lands
    .130 off the lands
     
  2. Kennibear

    Kennibear Well-Known Member

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    Shorter COAL raises pressure in pistol cartridges not rifle cartridges. Jamming the bullet into the rifling raised pressure in rifles. Most of the slow burning powder charges are already compressed so a small amount more does not change much.

    To keep the mag from battering the bullet nose the COAL must be shorter than the Mag OAL. How much shorter is dependent on the gun itself. Your jump is going to be more than 0.066".

    KB
     

  3. MudRunner2005

    MudRunner2005 Well-Known Member

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    What kind of rifle is it, and what caliber?
     
  4. bill123

    bill123 Well-Known Member

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    GA Precision. 308.
     
  5. bill123

    bill123 Well-Known Member

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    So it's ok to start at .066" and work shorter?
     
  6. MudRunner2005

    MudRunner2005 Well-Known Member

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    If it's a .308 Win, it should shoot fine with the bullets pressed in as far as SAAMI spec, which is an OAL of 2.810". I'd try them like that to see how they shoot. Then work your way further out till you find a good OAL that is still within mag length.

    If the rifle is based off a Rem 700 style footprint, you could always send it back to them and have them mill out the reciever and install a Wyatt's Outdoors extended mag box and follower, which will allow for longer OAL cartridges.
     
  7. bill123

    bill123 Well-Known Member

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    Just to be sure, can I go right to SAAMI with the powder charge I'm using? Why the difference between pistol and rifle cartridges?
     
  8. MudRunner2005

    MudRunner2005 Well-Known Member

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    I don't know...What does the Berger manual say is max charge for that load? All feloading manual loads should be tested at SAAMI spec OAL to ensure you don't blow anything up. So, I'd check the manual to see what it has to say.
     
  9. kcebcj

    kcebcj Well-Known Member

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    Will disagree with Kennibear on the pressure issue when setting the bullet back giving it more jump. It does increase pressure. For example in QuickLoad my 06 with a COAL of 3.345 has a chamber pressure of 50,144psi. If I set it back .045 to a COAL of 3.400 the pressure increases to 51,602psi about a 1000psi.

    So if your load is in the upper limits and you start setting it back without a powder adjustment it could get nasty.

    One more note. If the bullet is against the lands with no jump the automatic increase in chamber pressure is about 4000psi. So if you started against the lands and worked up a load then you could set the bullet back till it ate up the 4000psi then you would need a powder adjustment if the load was at max.

    Bottom line Quickload is just a guide so pay attention to pressure signs when working up and know where the bullet is in reference to the lands and of course all this is rifle dependent.
     
  10. Kennibear

    Kennibear Well-Known Member

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    Weatherby uses a freebore where the bullet runs about 1/4" before engaging the rifling to reduce pressure. The primer drives the bullet into the lands before the pressure rises very much in a rifle with rifle burn rate powders. So even when bullets are seated deeper they pretty much run to the rifling when the primer pops. The extra confinement only increases the ignition effect somewhat. Not denying the predictions of Quickload but the NRA experimented with this in the '60s using the old copper crusher pressure measuring equipment.
    Now admittedly copper crusher measures accumulated time + pressure data without showing definitive peaks, their conclusion was that unless the load was on the ragged edge it wasn't very significant. I believe that kcebcj has the correct amount but when the mean average variation is around + or - 400psi (a good figure) the load will vary 800psi.
    This is not to say it can't raise pressure but unless the load is off the chart (like those who venture over book with their loads) you are unlikely to run into problems. I have fired 556 with max loads of 69gr SMK/ WW 748 that had the bullets telescoped into the case when feeding in a match AR. The primers and cases showed no adverse effects even though it was a max book +0.2gr load.
    Being a 308 I would think that cartridge would be somewhat tolerant. As long as bill123 is inside book values then seating to SAAMI would do nothing bad with the pressure. At least not horrendous. But if bill123 is living in the outer limits then all bets are off anyhow. The 308 will not tolerate an overload for the same reason they all won't- brass is brass.

    I would ask those professionals who are members here and work with internal ballistics chime in please.

    But I haven't run into anything catastrophic when it happened to me.

    Pistol powder burns so fast that the extra confinement drive the burn rate to a faster speed than the powder would burn otherwise. The deeper seating also represents a much higher percentage of the powder space, as high as 50% or more. Rifle bullets do not take up 50% of the space when seated deeper.

    KB
     
  11. bill123

    bill123 Well-Known Member

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    Thanks for all if the responses. I think I'll restart my test back at sub magazine length. Any suggestions on how far below mag length a bullets should be to clear the mag and not get beaten up?
     
  12. Kennibear

    Kennibear Well-Known Member

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    The DB mag on my Savage is 3.5" and I load out to 3.4" so a tenth (0.1") should work. My DB mag has a pretty strong follower spring and it only holds 3 belted mag cases, two really without jamming, so that holds the cartridges against the recoil.

    KB
     
  13. bill123

    bill123 Well-Known Member

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    Thanks KB. Will I likely get the same node or should I expect that an increase in pressure (thus velocity) will throw off the node? In other words do I have to do the entire process all over?
     
  14. Kennibear

    Kennibear Well-Known Member

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    Probably not. But + or - 0.1gr for accuracy might be worth it. If you fire 3- 3rnd groups that is enough so < ten shots to see if it's worth chasing. I would try to shoot at least 300 yards for those 9 shots. After finding the charge weight I would consider seating a little deeper (0.010"?) to see if that changes.

    Another approach is to use the Hybrids. The members here claim they are a lot less sensitive to seating depth. So at least you might get really decent hunting loads that feed through the mag.

    Just a thought...

    KB