Calculating OAL

Discussion in 'Reloading' started by andy6149, Dec 27, 2012.

  1. andy6149

    andy6149 New Member

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    Hi Folks,

    this is my first post so be gentle ....

    I have been reloading .308 for a few months now and have been sticking to reloading manuals for OAL. I have just tried to use an empty case with the neck sized down to hold the bullet (.308 Sierra HPBT Match #3156). I start the bullet in the case OAL is about 2.970 at this point - i then lock the bolt down (full battery?) and eject the round - i believed that the lands should push the bullet back giving an accurate OAL as I have seen in videos - but nothing happened ...... is this normal? The rifle is a Howa 1500 varmint which i bought new earlier in the year.

    Any help would be great

    thanks

    Andy
     
  2. Bart B

    Bart B Well-Known Member

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    Your rifle's chamber may well have a longer throat than normal.

    Are you sure it's a Sierra 3156 bullet? Their web site lists the 155-gr. match bullet as a 2156. And that bullet will not reach the lands in many commercial sporting rifles even when barely held by the case neck.

    Here's what you can do.

    First....Seat a bullet in an empty sized case, then chamber it. Push a small rod down the bore until it stops against the bullet tip. Mark the rod at the muzzle. Pull the rod out then measure the mark to the end of the rod that stopped against the bullet.

    Second....Remove the bolt, point the rifle muzzle down then drop in one of those bullets so it falls into the chamber and stops against the rifling at the front of the chamber. Put something in the chamber to hold the bullet firmly in place, then put that rod gently back down the muzzle until it stops against the bullet. Mark it at the muzzle.

    Third....compare the difference between the two marks. That shows the difference between the seated bullet's tip in the test case and where the bullet actually first touches the rifling. It may be 2/10ths of an inch between the two marks. But at least, it'll let you know the situation.
     

  3. boomtube

    boomtube Well-Known Member

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    OAL causes lots of confusion new guys and quite a few old hands....that should not be. And there is NO way to "calculate OAL", chambers vary too much for that. There are three ways we can establish an OAL for rifles.

    First is the max length that will feed and function through the magazine OR go by factory length - that's the SAAMI standard length.

    Second is book OAL length and all that means is the length the guys who developed the listed data used in their rifle. Our's ain't theirs so ....

    Third, we can start somewhere off the lands and develop the best shooting lenght for our rigs. There's no need to be highly precise about where the lands are because it's not going to be exactly there anyway. Most factory rifles are going to shoot best with a bullet jump to the lands from about 20 thou to as much as five times that much. ONLY experiments on the range will tell you what works best with YOUR rig and YOUR load!

    A lot of us agonize over getting a specific amount of jump from seated bullet to lands contact and that demands a bullet comparitor/gauge that references off the bullet's ogive. Fact is, we can do quite well with our experiments by simply measureing the actual OAL unless the bullet noses are highly variable - most aren't that bad. There's a range of seating that will work well and it seems to run from 10 to 20 thou wide for most rifles so try to find where the 'good' range is and seat in the middle of it so small variations won't matter much, if at all.
     
  4. andy6149

    andy6149 New Member

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    Correct Bart the Sierra is #2156 155 Match, Thank you both for such great answers I will try the process with the rods and see what happens

    thanks
     
  5. Bart B

    Bart B Well-Known Member

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    Note that as the barrel wears out, the lands will start further away from the bolt face. Such is life as steel erodes away from hot gasses. .308's erode their lands about 1/10th inch over 4000 or so rounds; depends somewhat on the peak pressure of a given load and the powder type used.
     
  6. Mikecr

    Mikecr Well-Known Member

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    I agree with posted except the 10-20thou 'range' of seating for optimum performance.
    With every gun I've worked with, including factory, best seating depth represented a huge change in the gun's performance. Grouping is sometimes outright halved.
    Through further testing from this point(wherever it is), the biggest window I've found to hold this performance has been around +/-3-5thou. Far tighter than 10-20thou.

    I have not had to keep chasing normal land erosion. Once that initial best seating depth was determined, I've never needed to adjust it again. Just seat very accurately to that exact same depth and everything holds.
    A qualifier to this though; I don't continue shooting barrels past their peak. It ain't worth it, new barrels are cheaper..
     
  7. andy6149

    andy6149 New Member

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    Thanks for the comments, I have tried dropping the bullet into the chamber and then measuring etc - it seems that the bullet loosely pushed into the case is 2.955 oal and the same head dropped straight into the lands gives a read of 2.990
    - I had been loading these to 2.775 - I have also heard that the Sierra 2156 is a bit picky about the rifles it works in and I was advised to use the 2155, I was thinking of moving up to 168 match.

    any tips?
     
  8. Bart B

    Bart B Well-Known Member

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    Andy, seems to me your chamber's got a long leade, so your best bet for those 155's is to set either one out as far as you can, then maybe add one grain to published loads for it. I doubt there's any difference between either 155 from Sierra; the newer one has a higher BC but that's all in my opinion.

    IF your barrel's got a 1:12 twist, you may be in great shape if you load 190's with 42 grains of IMR4064 or 44 of IMR 4320. That longer bullet may just be the ticket to great accuracy in that long leade you've got.

    David Tubb shot Sierra 250-gr. HPMK's in his .308 for long range matches one year. His 1:8 twist barrel had an extra long leade/throat so he could get enough IMR4350 in the case to shoot it out at 2150 fps. Bucked the wind very, very well. Very accurate, too.
     
  9. boomtube

    boomtube Well-Known Member

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    "the biggest window I've found to hold this performance has been around +/-3-5thou. Far tighter than 10-20thou."

    Just out of curosity, what factory rifles, bulk bullets and cartridges have you found that narrow 6-10 thou seating window in?
     
  10. Bart B

    Bart B Well-Known Member

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    There's another part of the loaded round that'll increase any spread a bullet's reference ogive point measured for seating depth. If a bunch of rounds have a .003" spread in seating depth to that reference, the case headspace spread on rimless bottleneck cases will add to it. Case head to shoulder reference spread is hard to keep under .002". It often has as much as .004 or more depending on the case lube used, how much of it's on the case to be full length sized and how much the reloading press springs when the case is all the way up into the sizing die. After all, those rounds have their shoulder hard against the chamber shoulder when they fire.
     
  11. Mikecr

    Mikecr Well-Known Member

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    This is mostly what I've messed with over the years;
    *223- Cooper (1/4moa)
    223- Browning
    300wsm- Savage
    *6br- Browning rebarrel (3/8moa)
    6dasher-Savage rebarrel
    6XC- Tubb2000 (McMillan)
    *6.5wssm- BAT IBS LG (3/8moa)
    6.53Scramjet- Savage (Lazzeroni)
    *what I currently hold

    Bart I have no problem holding headspace, fired or otherwise, to 1.5thou. MINE doesn't change on primer firing.
    Boomtube I have no problem holding desired seating depth to lands under 1thou tolerance.
    This, because I qualify bullet ogive radius, use only Wilson inline seating, and my neck tensions are measured & matched.
    Also, my primers are set to the same depth at exactly the desired crush.
    Man, I must be a miracle worker...

    To me it seems ridiculous to generalize that these things can't be controlled. They can be, and when they are, it can payoff. If I were to let slip the seating for my Cooper by 6thou either way, grouping would immediately double in size.
    Fortunately, I could not possibly be off by so much as 1thou & not catch it through measure.
     
  12. Bart B

    Bart B Well-Known Member

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    Mike, the 3/8th accuracy you're getting with your tools and techniqes, is that MOA at 100 yards?

    If so, others have got that accuracy (often better) at 200 yards with no case prep, no load workup, and a few thousandths case headspace spread from standard full length sizing dies since the 1950's. Friend of mine shot his Win. 70 using a single case loaded 57 times full length sizing that case every time and put all of them in under 3/8ths center to center at 100 yards. He probably had 3 to 4 times the slop spread in any dimension you crave.
     
  13. boomtube

    boomtube Well-Known Member

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    "Friend of mine shot his Win. 70 using a single case loaded 57 times full length sizing that case every time and put all of them in under 3/8ths center to center at 100 yards. He probably had 3 to 4 times the slop spread in any dimension you crave."

    And therein lies the message; it's usually not all that critical. What I posted about the seating window was just a generalization, not meant to be an iron clad standard for all times and places, amen; saying "I have one that requires ...." is statistically meaningless to anyone else. If we try to make every comment we post include the total of range of extremes nothing we could say would help anyone at all, would it?? :D