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OAL vs Seating Depth

Discussion in 'Reloading' started by glockaviator, Aug 9, 2010.

  1. Hodgdon's website give an OAL for my .308 in 155gr SIE HPBT (anyone know what SIE is?) as 2.775. But different bullets are different lengths. The Lapua Scenar's in 155 gr must be made of lighter metal (less lead, no lead?) I don't know but they are PREMIUM bullets, The 155 gr Lapuas are actually LONGER than the 168 gr Sierras by quite a bit (.065" to be exact). The same design Sierra's in 155 would be even more difference!

    Then I read that deeper seating depths lead to higher pressures.

    It seems to me that they should specify a maximum bullet seating depth and then we can go from there. Specifying OAL (I assume it is a MINIMUM OAL, you can load longer, making for LESS seating depth? making it ok pressure wise) is ambiguous because different bullets are different lengths!! Yet everyone uses OAL.

    I mean what the heck?
  2. fredj338


    Dec 22, 2004
    No, the bullets aren't made w/ lighter metals, just slightly diff design to the ogive & BT. You have to first load to the OAL that fits your mag & chamber. You don't raise pressures much in a rifle by seating deeper but if you get the bullet into the lands, pressures will go up. It's not like a small pistol case & fast powders. Don't worry about pressure issues from changing seating depth, just stay out of the lands, you'll be fine.

  3. Has anyone ever sawed a bullet in half vertically with some kind of saw?
  4. fredj338


    Dec 22, 2004
    The easier way is to put them flat onto a grinding wheel or embed them in resin & cut the block in half. Match bullets vary in construction. They have hollow noses. So the shape of the nose (ogive) & shape of the boat tail & then exactly how much lead core determines OAL & weight.
    Last edited: Aug 9, 2010
  5. PCJim

    PCJim Senior Member

    Aug 4, 2008
    SIE is an abbreviation for Sierra bullet.
  6. steve4102


    Jan 2, 2009
    This, as a matter of fact Barnes did a study on OAL vs pressure vs accuracy and found that seating deeper in many instances reduced pressure. Here is a link.

    Here is what Accurate and Ramshot has on their web load data page.
    The “COL” values should be seen as a guideline only.
    The reloader is free to adjust this dimension to suit their particular firearm-component-weapon combination.
    This is determined by various things like magazine length , freebore-lead , ogive or profile and position of cannelure.

    Not a direct quote, but you can find it here.
  7. When I started loading 308 I had the same questions. OAL's all over the board. I chose a length to start that was similar to factory 308 and worked from there. I don't have a chrono so I couldn't measure any velocity differences. And I still can't hit the broad side of a barn so not sure if accuracy was affected with slight variances in OAL. Never worried too much about differences in pressures on account of what Fred mentioned.
  8. Bullet manufacturer's should publish the length and maybe other dimensions of their bullets.

    Also, there are TOO MANY different powders! Wow. How do you narrow it down (I am loading for .308). Using Varget right now.

  9. Powder selection vary confusing. Many flavors. I choose one that could load 223 and 308. Tac filled the bill - it meters well and is relatively inexpensive.
  10. Jim Watson

    Jim Watson

    Jul 10, 2001
    SIE = Sierra
    The Scenar is long because it is full of air, it has a large open area in the nose, more so than the Sierras.
  11. Gunnut 45/454

    Gunnut 45/454

    Jun 20, 2002
    Easiest way to figure OAL with a particular bullet is to make a dummy -use a fired case , put the bullet into it finger pressure then chamber the round in your gun! It should force it into the case by pressure from the groves! From that back it down by .010-020" and shoot some at that OAL ! Most guns shoot very accurately at about .010" off the rifling!
    Of course you must use case trimmed to the proper length! Then if you really want to get picky start weighing/segragating your bullets by weight +/- .1 then weigh your cases/segragate by headstamp,prep primer pocket etc. Then you get to loading MATCH grade ammo!:supergrin:
  12. fredj338


    Dec 22, 2004
    When I load for a new cartridge, I look to the reloading manuals for a pwoder tha gives me the vel I want w/o pushing max pressures. then decide how I will measure it. Ball powders work better in volumn measures & progressives than stick powders. Buy 1# only! Then do load workups. If I like the powder, I'll but a 4# or 8# & stay w/ it.
  13. hoffy


    Jun 12, 2007
    This is why manuals say to start low and work up a load, if you are using exactly the same components the manual uses(how often does that occur) then you may start a little heavier, but not much. The best way I have found is to load 5-10 cartridges, varying the powder a little, and shoot groups from a bench or other solid rest. On bolt guns, usually one gets best results with bullets nearly, or just touching the lands, but this is not written in stone, you have to experiment, and keep good records. OAL, powder charge, bullets, it can be a challenge, especially in some rifles, I have some rifles that shoot anything well, and others that are picky, good luck.
  14. steve4102


    Jan 2, 2009
    Sorry, but it is impossible to use the "exact" same components as those used to compile the test data. Not only must the components be the same, they must also be from the same lot. Not to mention the number one component of all, The Firearm. As we all know each firearm is an individual unto itself.