CBTO question

Discussion in 'Reloading' started by Vandy321, Oct 20, 2019.


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  1. Vandy321

    Vandy321 Well-Known Member

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    My 300wm has a CBTO with berger 215s of 2.802

    99% sure that goes for any .30 cal bullet, right? Or do differing lengths of the bearing surface or anything else effect CBTO.

    Looking to load up some 225 ELD-M...not sure if I need to measure CBTO again or the ogive is the ogive regardless of what bullet it is?

    Also, cant find a book starting charge weight for H1000 and 225 ELD-M...probably going to use 69 gr unless someone has a better idea? Based on the berger 230/H1000 being 65.5 for starting.
     
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  2. WeiserBucks

    WeiserBucks Well-Known Member

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    Every different bullet in any given caliber will have a different CBTO measurement given same distance from the lands or loaded to same OAL. Different weights , brands and types of bullets all have a different shape that will effect CBTO.

    Long live Remmy700
     
  3. Vandy321

    Vandy321 Well-Known Member

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    If I'm just looking to find max CBTO...I'm confused on how it wouldnt be the same...how is the length from base of the cartriage to the ogive not the same for a given 30 cal? If the bullet hits the lands at the ogive, wouldnt it be the same as the diameter or the ogive is the same on each 30 cal? I get that COAL will be different due to dif length bearing surface and ammount of bullet before the ogive...

    Not intending to to start debate or disagree... more just trying to understand what I'm missing?
     
    Last edited: Oct 20, 2019
  4. WeiserBucks

    WeiserBucks Well-Known Member

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    I'm not sure how to explain it better. Maybe another member will chime in and have a better explanation .

    Long live Remmy700
     
    Last edited: Oct 20, 2019
  5. ShtrRdy

    ShtrRdy Well-Known Member

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    The difference has to do with the bullet's ogive being a certain shape and the barrel's chamber leade being a certain angle.

    Basically CBTO can be different with a different bullet.
     
  6. entoptics

    entoptics Well-Known Member

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    Technically, the distance from the bolt face to the 0.300 (lands diameter) is going to be the same in any given rifle. The difference is, the bullet "ogive" is somewhere just before 0.300. So what you're measuring isn't exactly the distance from the bolt face to the bore.

    For example, my Hornady comparators are 0.001-0.002" under bore diameter. So for a shallow sloped ogive, the distance between 0.300 and 0.299 is further than one with a steep ogive angle.

    Also, since you're not actually trying to measure bolt face to bore diameter distance, but instead, where the bullet actually starts touching something in the throat, the throat dimensions and lead angle will affect where the bullet actually starts to touch, which isn't necessarily the bore diameter.

    The difference is likely to be minimal (a few thousandths) with similar bullet designs, but as an extreme example, imagine a wadcutter bullet and an extreme VLD. The wadcutter would hit the throat at the 0.308 bullet diameter, and the VLD would hit the throat at the 0.300 lands diameter somewhere much further into the chamber.
     
  7. Vandy321

    Vandy321 Well-Known Member

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    Awesome explanation...big thanks for that, explained in a way even a simpleton like myself can understand.
     
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  8. Havard

    Havard Well-Known Member

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    If you’re like me, and sometimes need a visual, take a look at this. Both are 165gr, but the ogive shape of the Game Changer (left) will cause it to contact the lands differently. There is a good read on the Berger site somewhere explaining the difference between Secant vs Tagant Ogive which explains it pretty good for me. Also, if loading close to the lands, better check your overall and be sure you can magazine feed if so desired. It may influence your CBTO desired measurement. I learned this the hard way
     

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  9. DUSTY NOGGIN

    DUSTY NOGGIN Well-Known Member

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    save a copy of this to your computer , in a reloading folder
    https://saami.org/wp-content/upload...99.4-CFR-Approved-2015-12-14-Posting-Copy.pdf

    to add to entoptics, look at the difference between like the noslers @3degree angle compared to 264WM @2degree & your 300WM @1degree ,

    the steeper the angle of the bullet the like the tangent , more it will hit the ogive before the bearing surface ,

    i think i would prefer it to start engraving/smearing from the outside in toward the center rather than the front at ogive towards the back. i wonder why nosler went with a 3 degree when alot of bullets are getting steeper angled ... i dont understand that but im sure they know what they are doing
     
  10. Lone Hunter

    Lone Hunter Well-Known Member

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  11. Lone Hunter

    Lone Hunter Well-Known Member

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    Also, You might want to get a reloading book for explanations and loading data. Not saying guys have wrong explanations but each weapon is different even though they have the same chamberings they are like people and will be different. Unless I know the person I will never take another persons word for reloading data but start low and work up on your loads. Just because it's safe in another guys rifle means it is in yours. Get a good reloading book and read and learn the basic steps then progress slowly.
     
  12. aushunter1

    aushunter1 Well-Known Member

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    Exactly how I would have explained it.

    It becomes very clear when you have an Hornady OAL gauge & modified case then start pushing different projectiles of the same weight into the lands & measure with a comparator.

    If you don't have this gear I would highly recommend it!

    Cheap really in the scheme of things & its so useful giving you all the data you need.

    The headspace gauges are also a great addition as well.
     
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  13. crystalgayleguy

    crystalgayleguy Well-Known Member

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    what is CBTO
     
  14. Lone Hunter

    Lone Hunter Well-Known Member

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    Case Base To Ogive