Consistent seating depth

Discussion in 'Reloading' started by nitro901, Feb 11, 2011.

  1. nitro901

    nitro901 Member

    Dec 18, 2008
    Has anyone ever used an extra seater plug to accurately measure cartridge length due to the slight inconsistencies of some bullets? For example, I have been loading Sierra Gameking 165gr BTHP in my '06 for years and setting my OAL to 3.35" but the caliper measured length varies say 3.30" - 3.35" in the same sitting. I understand that it's really not important as long as at the olgive it is consitent to the lands right?

    I just purchased another box of bullets and set up my seating depth using an existing load measured at 3.35", then when loading the new bullets they actually measured 3.34" OAL. Again, I understand it may not be important as long as the olgive measurement is correct.

    I know there are tools for measuring the length to the olgive and it got me thinking of another way to do it easily and inexpensively. Couldn't I buy an extra seater plug, cut the threads off, "grind" it smooth then use it with calipers to measure the length?

    I somewhat tested it by taking my seater plug out of the die and measuring a buch of handloads from the case head to a flat spot on the seater plug and they all measured exact. When measuring OAL, they varied.

    Isn't this basically the same thing as using a comparitor?
  2. Kevin Thomas

    Kevin Thomas Well-Known Member

    Feb 16, 2009
    Hi Nitro,

    yeah, you sure could. However, there's so many different comparators out there already, at very resonable prices that I doubt it'd be worth your while to make your own. Add to that the fact that a seater plug is contacting the ogive at the extreme forward portion rather than nearer the point at which the leade contacts the bullet, and your measurments become somewhat suspect. There's several out there. My preference is for either the Davidson SDC, or the Sinclair comparator, but that's just me. Easy to use, repeatable with a little experience, measures the ogive at the point which I want to measure, and pretty inexpensive.
  3. 338winmag

    338winmag Well-Known Member

    Jan 9, 2011
    I often build and invent little devices to assist in my practices as well as save some dollars but... somethings for sale are just worth the dollars.

    For your concern:
    Larry Willis's Digital Headspace Gauge
    I use it for both case resizing (accuracy from .0005v to .001) and also bullet seating depth to same accuracy. Sure it is a few dollars but you will never ever look back. Check it out. No more guess work.

    Innovative Technologies - Reloading Equipment
  4. boomtube

    boomtube Well-Known Member

    Oct 8, 2007
    I suspect you have no inconsistant seating depth problem at all. It's much more likely the result of different bullet lengths.

    Buying the proper tool will probably cost little more than a bullet seater stem and the right tool will work MUCH better.
  5. gunner69

    gunner69 Well-Known Member

    Jul 9, 2011
    Worth watching for consistent seating depths.

    [ame=]Seating Depths Berger bullets - YouTube[/ame]

    [ame=]Consistent Bullet Depth Seating - YouTube[/ame]

    MTBULLET Well-Known Member

    Jul 16, 2007
    I machine the seating stem to fit the ogive of the bullet i'm using, much easier.
  7. Bart B

    Bart B Well-Known Member

    Dec 25, 2005
    A few thousandths spread in how far bullets jump into the rifling is not a big issue. I doubt if it's even a small issue. There's other variables in the cartridge that are more important to accuracy. Besides, there's a small spread in bullet's ogives because the pointing die used to shape the ogive doesn't make each one exactly the same shape and size.

    Unless the seating plug's shaped to match and touch only the largest diameter of the bullet's ogive equal to the bore diameter point to the bullet's maximum diameter, there's always going to be a tiny spread in bullet jump to the rifling. That part of the bullet's what engages the rifling. Any point on the bullet's ogive at lesser diameters never touches anything except the atmosphere and target.

    I don't know of anybody making a seating plug with its outside dimension and seater chamber equal to bullet diameter and its wall only 4 thousandths inch thick; the height of the lands in most rifle barrels. This is what's needed if the bullet's rifling contact point has to be exactly the same distance from the case head for each and every round when it's seated. And this will only happen when the case headspace point (shoulder or belt) is also exactly the same distance from the case head on each and every round. With rimless bottleneck cases, the spread in case headspace (head to shoulder) will also be the spread in bullet jump to the rifling..... plus whatever spread there is between case head and bullet obive contact point. Note that rimless bottleneck cases have their shoulder hard against the chamber shoulder when they're fired; belted cases may or may not behave the same.

    If it's important to have exactly the same jump to lands distance, size fired case necks down less then "soft seat" bullets about 10 thousandths longer than rifling contact. Then they'll all have zero jump distance. You'll have to seat them shallower as your barrel wears but that's normal.
  8. Mikecr

    Mikecr Well-Known Member

    Aug 10, 2003
    Bob Green's ogive comparator does what you're inventing here.
    Bob Green New Products
    This is for qualifying ogive radius, prior to basis of all other measures/actions.

    I totally disagree about seating accuracy being a non-issue. Truth is, there is often no larger impact to accuracy than ms-seating.
    It can take a 1/4moa gun to >1/2moa, and a 1/2moa gun to >1moa, and nothing else does this(powder, bullets, primers, not even mis-tune).