Friday, March 15, 2013

The poor mans bullet seating gauge.....

With a new rifle comes a new load workup, and more details on that process later.

What I want to discuss here is a way to determine the distance from the camber mouth to the beginning of the lands, or what is commonly termed the 'Throat'.    For hand loaders seeking maximum accuracy, this can be an important measurement.

The idea is the distance a bullet moves forward from the case before it engages the rifling can greatly effect accuracy.  This I know to be true.  If the handloader can find the 'sweet spot' for his best load, usually something like .005-.010" from the lands, then peak accuracy is one step closer.

To measure that distance, special tools can be purchased that are designed to perfectly gauge the 'freebore', or distance from case mouth to rifling.  Being a frugal (read that as CHEAP) old fart, Carteach prefers using a little ingenuity to achieve the same results.

Here, we see a full length sized un-primed 30-06 case that's been sliced with a diamond wheel on a Foredom tool.   Slicing the case almost to the shoulder seems to work well, although I've seen good results from slicing a little farther, about halfway down the shoulder.

The idea is to have the case neck pressure relieved just enough that a bullet will slide into the case, but still be tight enough it won't shift or fall out on it's own.

What we'll do is hand start a bullet into the case mouth, and then carefully... but firmly.... chamber the round in the rifle.  It's important the bolt not be slammed home, but be moved slowly into full battery.  Treating the operation roughly will result in false readings.  

In a bolt action, the bolt must be fully cammed into lockup.  With a pump or semi-auto, assuring full lockup is a little more difficult.  Since most rifles will not fire out of battery, if a dry fire gives a satisfying click, the bolt is usually closed all the way.  It's critical beyond my ability to stress.... UN-PRIMED cases are used for this!  If a live primer is in the case, it can propel the bullet far enough into the bore to create a deadly dangerous situation.... a blocked bore.

Carefully open the bolt, and  draw it backwards slowly.  Try not to let the ejector kick the case out, but preferably lay the rifle over and drop the case into your hand.  The bullet should have contacted the rifling, and been pushed back into the sliced case neck.... where it should have stayed.

There is something to be said for repeating the entire trial multiple times.  If the final measurement is the same every time, it's probably being done right.

Regarding the resulting measurement..... how is it done?    Well, a simple vernier caliper will give a total overall length for that case, with that particular bullet.  You DO own a good vernier caliper, RIGHT?

Using that measurement, the overall cartridge length can be set up with the desired bullet-to-land spacing.  Simply set the seating die to seat the bullet at maximum length, and dial in the a few more thousandths depth at a time till you reach the length you wish to try.

It's rather important not to set the bullet to seat right at the rifling with a field rifle.  If done so, a loaded round in the chamber may have it's bullet pulled simply by camming the bolt open... leaving the bullet stuck in the rifling.  Bench rest shooters often seat right at the lands, or a few thousandths in, but that's a different game.

Now, back to measuring the overall length of the 'gauge' cartridge we just invented.....  it must be understood..... if measured as found, the resulting length will be for THAT bullet design, and no other.  Different bullets come with different ogives, or curves, and will impact the rifling at different spots along their length.  A spitzer bullet will not hit the lands at the same place a round nose bullet does.

The answer here is a widget called a 'comparator'.  It's a big hex-nut looking affair with six precise holes drilled in it that mimic bore dimensions.

Including a comparator to the measuring process gives a number that can be used with any bullet the handloader wishes, as long as the comparator is always used in measuring overall length during seating die setup.

In a field rifle... hunting or match.... there is another factor that must be considered.  In the case of The Fat Man's 30-06 Mauser... it overrides the accuracy potential of seating to the lands.  That factor is magazine length.

On a rifle with a long throat (as I have here),  the longest cartridge that will fit the magazine will not even come close to the rifling when chambered.  That's just the way it is, and must be accepted as a limiting factor.   In the case of this rifle, it effectively nullifies the idea of seating to the lands, as this is clearly a hunting rifle and the magazine will be used.

High power match shooters often have two completely different overall cartridge length loadings in play in the same match.... a short one that fits the magazine for rapid fire, and a longer one that will be loaded singly for the slow fire stages.

Okay.... that's the Carteach cheat for measuring case mouth-to-rifling.  Any questions or concerns?


Old NFO said...

And in your case you have to fit TWO rifles...

ASM826 said...

That's an interesting and easy way to get a maximum OAL. Next time I get into this, I'm giving your method a try.

Anonymous said...

Great Tip! i have a friend that just spent a lot of money on gauges for multiple different calibers. This is a really good tip.