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Thread: Determining COL

  1. #1

    Determining COL

    As per my recent thread on the Nosler Partition I am approaching the stage where I will be thinking about fine tuning a load and almost without fail everyone has recommended working on COL so that is what I'm going to do.

    I've loaded some partitions for testing to the 308W max of 2.800 inches and looking at my magazine on the Blaser I'm not sure I will be able to go significantly longer than this.

    I've never messed with COL before and have no idea how far the lands are from my bolt face or how long I need to make my OAL using the partitions to set the end of the ogive a given distance from the lands. I am aware there are various "make shift" methods for doing this and also a range of things that can be bought to do the job (the precision mic for example).

    So, what methods are recommended for measuring the distance to the lands and converting this to a correct COL taking the ogive into consideration etc? Is something like the precision mic worth the money as it looks like it will give repeatable results while many of the make shift methods appear, to be, to be prone to some vagueness?

    As ever all views, advice and opinions are gratefully received.

  2. #2
    I'll tell you how I did it, but obviously there are many ways to skin a cat so standard disclaimers apply!

    I made myself a 'gauge' using a case I had resized and trimmed. I polished out the inside of the neck slightly with a dremel so that a bullet would seat slightly loose, but not so loose it could easily slide in and out just by handling it.

    Then I experimented by seating that bullet in further and further until it got to the point that I could comfortably chamber it whilst feeling a tiny bit of resistance whilst closing the bolt. At that point the action of closing the bolt was pushing the bullet into the case against the lands. You can put a bit of engineers blue or pencil on the bullet to confirm that it is indeed pushing against the lands if you like.

    I then repeated the process several times, closing the bolt and seating the bullet into the case, then very carefully extracting and measuring using calipers and a suitable comparator.



    That gives you your baseline. Then every thou smaller (using the same calipers/comparator) is a thou off the lands.

    Alex

  3. #3
    The depth of detail of the answers depends on how much detail YOU want.

    The BEST way to get the dimensions of your chamber is to cast it. You can use a sulphur (yech! stinks and is a mess, but is cheap), or you can use CerroSafe. It is expensive, but is simple, safe, and reusable.
    Next best, and a very close second to casting, is the use of the likes of Hornady's Chamber-All. I understand there are some UK competitors. From what I read, "six of one, half a dozen of the other".
    Next are the "make shift" methods. They are cheap, but less precise and generally more 'complicated' operationally. If done properly, they are precise enough.

    The "mic" isn't necessary, bit it makes the process 1) repeatable, and 2) "easier". That's usually what you get when you "pay for it". Unbeknownst to many, you can buy one mic body, and only have to buy new inserts as you exapnd the number of calibers (CALIBERS, NOT cartridges) you load.

    If you want detailed info from me, send a PM, but I'm sure there are plenty here that can help you.

    Regards,
    Paul

  4. #4
    Instead of polishing out the neck you might split the neck using a fine tooth blade such as those on the Junior hack saw. Then debur the cut. The splits will allow the neck to give slightly yet hold it tight enough to get a reading . I have cases adapted like this for most of those that I reload I do not have a comparitor liek the Stoney point.

    Now to my mind the comparitor is only of much use in measuring the bullet jump IF the comparitor is the same diameter as the bore and leade of YOUR rifle and as rifles will differ in this dimension it makes me laugh when someone posts they seat their bullets say 0.005" off the lands. For one thing the postion of the ogive can and will often vary more than that is a box of bullets. Makers use more than one bullet forming die in production so the chances of gettign bulelts formed in different dies in one box is more than likely . Then add to this that they are most likely using a commercially made comparitor and the chances of it being the same diameter as their barrel is very slim.

    Tin hat doned and securely tucked under the manlet .

  5. #5
    Quote Originally Posted by Brithunter View Post
    Now to my mind the comparitor is only of much use in measuring the bullet jump IF the comparitor is the same diameter as the bore and leade of YOUR rifle and as rifles will differ in this dimension it makes me laugh when someone posts they seat their bullets say 0.005" off the lands.
    I'm not sure I agree with this. So long as the comparator is measuring to the same point on the bullet each time then it doesn't matter whether that point is exactly the same point that the lands touch the bullet because the readings you are taking only have to be relative.

    When you take the modified case/bullet out you know it is touching the lands. When you seat that bullet X thou further into the case it has to be X thou back from the point of contact in the rifle chamber. All the comparator does is try to give a consistent point of reference on the bullet.

    I agree that you won't know the true distance from bolt face to the lands, but if all you want to do is seat the bullet X thou back you don't actually need to know.

    I agree that bullet inconsistency will affect it though... but what can you actually do about that?

    Alex

  6. #6
    Quote Originally Posted by caorach View Post
    As per my recent thread on the Nosler Partition I am approaching the stage where I will be thinking about fine tuning a load and almost without fail everyone has recommended working on COL so that is what I'm going to do.

    I've loaded some partitions for testing to the 308W max of 2.800 inches and looking at my magazine on the Blaser I'm not sure I will be able to go significantly longer than this.

    I've never messed with COL before and have no idea how far the lands are from my bolt face or how long I need to make my OAL using the partitions to set the end of the ogive a given distance from the lands. I am aware there are various "make shift" methods for doing this and also a range of things that can be bought to do the job (the precision mic for example).

    So, what methods are recommended for measuring the distance to the lands and converting this to a correct COL taking the ogive into consideration etc? Is something like the precision mic worth the money as it looks like it will give repeatable results while many of the make shift methods appear, to be, to be prone to some vagueness?

    As ever all views, advice and opinions are gratefully received.
    You hit your own answer. Your starting point is the length that allows a cartridge to freely feed from your magazine. Worrying about how much longer you than that is silly unless you want to load single shot. Start at this length and move inward in increments. Seating out is not always the route to the most accurate load. I helped a kid reload 30-06 once using 150 grain bullets. He found that his groups tightened markedly as the OAL decreased. His final groups were half the size of his initial groups.

    I also chuckle when i hear .005" off of the lands. Few die/press/ bullet combos will allow that accuracy on a repeatable basis.~Muir

  7. #7
    Quote Originally Posted by csl View Post
    I'm not sure I agree with this. So long as the comparator is measuring to the same point on the bullet each time then it doesn't matter whether that point is exactly the same point that the lands touch the bullet because the readings you are taking only have to be relative.

    When you take the modified case/bullet out you know it is touching the lands. When you seat that bullet X thou further into the case it has to be X thou back from the point of contact in the rifle chamber. All the comparator does is try to give a consistent point of reference on the bullet.

    I agree that you won't know the true distance from bolt face to the lands, but if all you want to do is seat the bullet X thou back you don't actually need to know.

    I agree that bullet inconsistency will affect it though... but what can you actually do about that?

    Alex
    If this is the case then one could use a .22" bullet comparitor for all calibres after all once you measure your test bullet/case with it you can then move it back "X" number of thou...

    Somehow I get the feeling that all those extra calibres ones are like most fishing equipement. Designed to catch the angler and his wallet and not more fish .I will give this some thought and see if I can knock up some gauges to check the exact diameter of the leade and bore on the .280 then make up a suitable comparitor just out of interest. Although I have never even looked at the Stoney point of Hornady I am sure I can make one up.

    Now as to if it's worth the time hmmm that requires thought.

  8. #8
    Quote Originally Posted by Brithunter View Post
    If this is the case then one could use a .22" bullet comparitor for all calibres after all once you measure your test bullet/case with it you can then move it back "X" number of thou...
    Absolutely. The only reason for the various comparator sizes as far as I can see is to try to get the datum point roughly in the area of the ogive where the bullet dimensions are supposed to be more consistent, as opposed to towards the tip, or indeed ON the tip!

  9. #9
    Quote Originally Posted by caorach View Post
    I've loaded some partitions for testing to the 308W max of 2.800 inches and looking at my magazine on the Blaser I'm not sure I will be able to go significantly longer than this
    i load my .308 r93 to 71.5mm to the tip (from memory) with 125 b tips - thats as long as the mag allows and is still some way from the lands. i have all the measuring stuff but in the blasers it doesnt seem to make that much difference, compared to a turn bolt.

  10. #10
    I load my .308's to 2.8" which is just about the max for the magazine although the lands are quite some ways further - don't recall as it seemed academic at the time. As Muir points out, I don't want a single shot...

    P.S. I borrow a stoney point OAL gauge and buy the case whenever I get a new rifle...

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