Hornady OAL Gauge, What am I measuring

[z7]

I just received my Hornady OAL straight gauge (http://ads.midwayusa.com/product/570611/hornady-lock-n-load-overall-length-gauge-bolt-action) and some 178ELD Match bullets

I wanted to find the distance to the lands in my rifle to figure out max oal and distance to the lands. i used it as directed and came up with a 2.200" base to ogive using the hornady bullet comparator (http://www.midwayusa.com/product/23...ad-bullet-comparator-basic-set-with-6-inserts)

this measurement gave me an oal of about 2.830" with the ELD Match kissing the lands.

now my question. I repeated the drill with the 178 BTHP and got different measurement on the comparator. with the ELD Match I hit what felt like a solid wall. with the BTHP there was slight initial resistance and then a solid wall. I assume it is difference bullet shapes. my measurements were either shorter or longer when measured on the Ogive. 2.185 (initial) or 2.225 (solid wall) but not the consisten 2.200" with the ELD Match.

Am i measuring the distance to the lands with a specific bullet or will the length translate between bullets? Do I need to measure the length of "kissing the lands" with each bullet when measuring with the bullet comparator?

 

[Varminterror]

The comparator mouth is a sharp 90• bore at it's set diameter. Your leade is not. The angles/curvatures of the ogives of different bullets where they meet the angled leade of your bore are not the same, so the same reference diameter will not apply. It's close, but not close enough when guys are talking about kissing or jumping a specific 10thousandths.

It is NOT measuring the actual distance to the lands, it is only measuring the distance to a reference position on the bullet, which in practice, is seated such that it is touching the lands. So if you use the same exact bushing on the same brand and model of bullet, assuming uniform ogives, you'll get the same reference measurement for the same jump or kiss position.

Another failing of the system: it's really only useful for determining distance from the case shoulder to the ogive of the bullet, because the position of the case base is arbitrary. When you use the modified case, the case base may not actually accurately represent the true bolt face - meaning the headspace length for your modified case may not actually match up to the headspace of your chamber and bolt.

Since it's a Saturday, and I've been seeing these questions for way too long (back before it said Hornady on the box for these tools), maybe I'll try to work up a pictorial to explain why these don't work as the instructions claim, AND how to use them appropriately. I'm working up a new load for my .30-06 and the 178 ELD-x anyway...

 

[Bart B.]

Hornady's claim:

The Hornady Lock-N-Load Overall Length Gauge is one of the best kept secrets to accurate reloading. This tool allows reloaders to quickly and easily determine the "proper" bullet seating depth in their firearm by accurately measuring their rifles freebore.

Reality is, the .308 Win freebore dimension is measured from the chamber mouth to where the rifling starts. SAAMI specs show it in this drawing:



Freebore is the red marked dimension between chamber mouth and start of the rifling. It's .090" long starting 2.0488" forward of the bolt face.

Throat/Leade is the green marked dimension from the front end of the freebore point where rifling starts angling down to bore diameter .1637" to bore diameter.

Your bullet will touch the lands somewhere in the green section depending on its diameter and that of the throat/leade at its 1.75 degree angle. But only if your barrel's dimensions are exact SAAMI spec. Note the tolerances.

OAL will have a small spread due to bullet dimensional spreads. What counts is the dimension from shoulder to where the bullet touches the rifling. Case shouldler's hard against the chamber shoulder when the round fires. The case head can be a few to several thousandths off the bolt face when the round fires.

 

[Varminterror]

Another failing of the system: it's really only useful for determining distance from the case shoulder to the ogive of the bullet...

What counts is the dimension from shoulder to where the bullet touches the rifling.

It's sad it's so obvious once a guy sits down and thinks about what the tool is actually doing, but the instructions for the comparators don't point that out - AND - they really should be selling them as kits including the headspace bushings AND bullet bushings since you need both to get any meaningful info from them.

 

[Bart B.]

Wanna see a big range of bullet contact point to case head dimension across a given cartridge with a couple dozen different bullets of all types, weights and makes all seated to the same exact OAL? Measure yours, then take a sedative to calm down the excitement from learning what some folks never will

I've measured 30 caliber bullet diameters from .3070" to .3092". Bore diameters from .2980" to .3020". Groove diameters from .3065" to .3010".

Note the 308 Win erodes the throat down the bore about .001" for every 35 to 40 rounds fired. Seating bullets shallower to compensate for barrel wear is called "chasing the lands." A waste of time unless one can discern the 1/8 to 1/4 MOA accuracy difference over a 308's match grade barrel life of 3000 rounds.

 

[z7]

thanks for the replies. From the responses I gather:
1. I am measuring the length from the shoulder of the hornady modified case to an arbitrary yet consistent point on the ogive of the bullet with the bullet touching the lands.
2. this measurement will change depending on bullet design
3. it is not an accurate representation of my hand loads that are fire formed to my chamber with the shoulder's bumped back .002"
4. It should afford me some idea, however useful, of my length required to "jam" the bullet into the lands

 

[sugarmaker]

I bought a 5/16-33 tap and 9/32 drill. For a ref case matched to a chamber, I fire and redding body size such taht I don't bump the shoulder but the case chambers freely. I sometimes size the neck a smidge but this is tricky to get just right. Drill and tap the case, and use it for that rifle only. With hand tools and bottleneck cases, a good way to tap is to take the fired case before it is sized, degrease it, remove spindle and lock ting from sizer die, degrease it also, screw die into press upside down, put the lock ring on top and tighten. Tap it into the sizer LIGHTLY and drill/tap. When done unscrew the dir and tap out the case. The comparatot does not measure the actual base to land contact point, but it does tell me what measurement I need to be at when said contact occurs with the particular bullet and gun in question. The measurement does not transfer to other bullet styles as others have noted. When taking the measurement, I rotate the case clockwise a turn while pushing lightly to seat the shoulder, and be careful to use the same pressure on the bullet rod (maybe 4 ounces) and average 5 readings. Using the shoulder gauge I measure the amount of setback the die creates and subtract from the prior reading to get the desired comparator reading for loaded rounds

 

[243winxb]

http://www.brownells.com/.aspx/lid=...g_Success___Minimum_Headspace_and_Maximum_COL

Gauging Success - Minimum Headspace and Maximum COL.

 

[243winxb]

Find OAL - Old cheap guy method. Before fancy tools & the interweb.

Take fired brass from your rifle.
Pinch neck with pliers so its oval by a few thousandth.** Enought to hold bullet snug. Insert bullet a tiny bit.
Place in chamber, close bolt. Remove, with finger on case side , to keep bullet from hitting receiver.
Measure from case head to bullet tip, repeat, 3 to 5 times. Should get a couple OAL's that are close to the same measurement. This is maximum OAL for that bullet.
** tight neck, cut a slit in the neck.

 

[Bart B.]

The link in post #8 is confusing to people using SAAMI terminology for case and chamber parts.

It uses "headspace" incorrectly for the difference between chamber headspace and case headspace. That difference, the space between bolt face and case head, is correctly termed "head clearance."

It also calls the chamber section between mouth and rifling origin "throat" when it's correctly called "freebore."

These errors aside, it's a decent explanation of what happens.

 

[243winxb]

No freebore in a 243win chamber.

 

[Bart B.]

Right.

Also, no freebore in a SAAMI spec 30-06 chamber.

Rifling for both starts at the chamber mouth.

 

[Varminterror]

thanks for the replies. From the responses I gather:
1. I am measuring the length from the shoulder of the hornady modified case to an arbitrary yet consistent point on the ogive of the bullet with the bullet touching the lands.

Yes.

If you use the bullet comparator AND the headspace comparator bushings, both, then you have the length between the headspace bushing datum line and the comparator bushing datum line. Neither line from either bushing actually corresponds to an actual contact point in your firearm, in any true way. But assuming you have a consistent bullet ogive profile, and have consistent shoulder angle, then this reference measurement of the two datum lines will be relevant.

So you know the cartridge fits in the actual space, but not knowing the actual space dimensions or angles, you can pick reference points on the reference cartridge, and use them as a repeatable and useful measurement.

2. this measurement will change depending on bullet design

Yup. Different ogive angles/curvature will change the distance between the reference datum line in the bushing and the actual contact point datum line in the angular leade. Different bullets will make contact at different points (diameters) in the leade as well, all based on ogive profile.

3. it is not an accurate representation of my hand loads that are fire formed to my chamber with the shoulder's bumped back .002"

If you use both the headspace and bullet comparators, then yes, the measurement will be relevant, as you should always seat your bullets as if your shoulder is held dead forward against your chamber. Your COAL measured on the modified case is completely irrelevant, UNLESS you determine a correction offset to your actual cartridge cases. In other words, measure the headspace of the modified case, measure the headspace of your actual cases, subtract to find the difference, then add or subtract that difference from your base to ogive or COAL lengths determined using the modified case.

4. It should afford me some idea, however useful, of my length required to "jam" the bullet into the lands

Yes - when used appropriately, the tools are incredibly useful, and also faster and more accurate than most other methods. I often use multiple methods, especially in my polygonal or caudle rifled barrels, to determine and confirm my seating depths.

Trust me, if I felt the tool were not useful, I would have responded to your original post very differently.

As the instructions read, they are junk. Used correctly, they're very useful.

 

[z7]

varmminterror thanks again, very helpful response.

I did just discover that my 175 SMK load in winchester brass is jamming bullets in by about .06"!

using the modified hornady case I was touching the lands at 2.16"

my Base to Ogive measurement on my loaded ammo is 2.22" with a SMK seated right at 2.800" OAL

as per headspace, using the L.E. wilson headspace gauge I measure the hornady modified case at 2.002/2.003 headspace (gauge is 2.000")
My resized brass (.002" bump) measures right at 2.000/2.001" just enough to catch a razor on the case but not hardly enough to measure. this is enough info for me to assume my headspace is similar enough to not account for a .06" difference in base to ogive measurement.

Now a follow on question. This load is accurate (See attached picture) and doesn't have any visible signs of pressure and is just starting to compress the varget. should I seat the bullets deeper and assume that there is high pressure problems that I am not detecting? For reference it is 45g of varget, winchester brass, cci 200 and a 175 smk seated at 2.800" ***THIS IS A PUBLISHED MAX LOAD***

 

[Varminterror]

Smoke a bullet and confirm your contact. 6thou will give a nice rifling impression, it won't be able to hide.

 

[z7]

I did the sharpie method and got a nice little ring/smudge all the way around. I was surprise to say the least. I am going to clean the rifle tomorrow and retest to ensure there isn't any excess carbon making this seem significantly shorter. it isn't a problem, I just may need to back off the powder a little and seat deeper.
of note, I have fired about 200 shots without cleaning. rifle is a hammer, but carbon may be a little built up throwing my measurements. I will find out and let you all know

 

[Bart B.]

I've used 46 grains of Varget under 155's in thin walld 1500-grain WCC58 match brass getting long case life and wonderful accuracy. So I think pressure is OK.

If you use 45 graiins under 175's, that may be good. Sierra's data says 41.7 grains of Varget is max with their 175, but they're usually on the mild side for pressure. If you get at least 10 to 15 reloads per case with that load, it's probably close to SAAMI max average pressure and safe to use.

I wouldn't change anything as those groups show excellent accuracy. They're probably smaller in reality as your own imperfect shooting skills open test groups up; mine sure do. Something us mortals need to remember about shooting groups as we hold rifles against our bodies as they rest atop something on a bench.