Measuring Overall length and Throat Erosion

[Black Tail Hunter]

When I first got my rifle and started load development I got a Hornady overall length gauge and made my own checker case since they did not offer one and they are simple to make.

My measurements were at around 4.012-4.015 so I loaded roughly .010 off the lands which worked fine.

I now have roughly 150-200 rounds through the gun and I decided to take another measurement to see if it had changed any and my measurement now is 4.002-4.005. This seems wrong to me but maybe my understanding of throat erosion is incorrect. I find that there are a couple possibilities but would like input from people who have more experience than me in this.

First possibility is my initial measurement was for some reason inaccurate and the 4.012-4.015 wasn't the actual dimension.

Second possibility is my understanding of the result of throat erosion, being the bullet protruding further out of the case as it chases the lands down the barrel is incorrect and the case actually goes further into the chamber?

Please weigh in as I am curious as to what the most likely answer is.

 

[bill123]

Your understanding of throat erosion is correct.
Assuming that you are using the right tools and your technique is correct, I have 2 thoughts.
1. The initial measurement was incorrect.
2. Fouling can cause discrepancies. Was the barrel clean when you measured both times?

 

[Black Tail Hunter]

There may be better tools out there than the Hornady OAL gauge and their comparator set but I think they have to be at least close. And I'm careful to not jam the bullet into the lands when I am pushing the rod through the tool, and always take several measurements and work from the average.

Great point, I had not considered fouling. I shot around 30 rounds this weekend and have not yet cleaned my barrel. I suppose there could be .010 to .015 of carbon build up? And yes the bore was clean upon the initial measurement.

 

[bill123]

I've been using the Hornady Gauge for a while and find it very accurate. Good luck!

 

[Black Tail Hunter]

Thanks for your insight. I will thoroughly clean and recheck.

 

[TwoMore]

I use a .250" brass rod that is 38 inches long and chamfered nice that I (carefully) insert into the muzzle end and I use it to just touch the meplat of the bullet and I gently move the bullet back and forth with the hornady gauge push rod and brass rod and I get a real good feel of where the bullet makes contact, it is real easy to jam it when checking, my test case is a formed case out of that chamber with a good known head space number.

I think if you clean it it will make a difference

 

[Rhovee]

Doesn't seem like to much to me. I had about .025 in 250 rounds down my 6.5 WSM. I would shoot (2) 3 shot groups and have to let it cool for a long time. Somethings to consider are: How smooth the chamber job was. How clean the barrel was when you took measurement. Mine was a first cut off a brand new reamer. Not sure how much of a difference that made. My 28 Nosler has .008 in 400 rounds.

 

[Black Tail Hunter]

Well what confused me is that the measurement was getting smaller not bigger. Almost has to be carbon build up. I use a copper cleaning solvent but not a powder solvent. I will try using both. perhaps the copper solvent isn't removing carbon build up. As far as the chamber goes, it looks smooth. I don't see a lot of tool marks etc. That being said I'm no expert on judging the finish quality of a chamber...

 

[tbrice23]

Well what confused me is that the measurement was getting smaller not bigger. Almost has to be carbon build up. I use a copper cleaning solvent but not a powder solvent. I will try using both. perhaps the copper solvent isn't removing carbon build up. As far as the chamber goes, it looks smooth. I don't see a lot of tool marks etc. That being said I'm no expert on judging the finish quality of a chamber...

I've found that, when your lands are sharp on a newly chambered barrel, the bullet will be engraved a few thousandths by the lands when measuring for the first time. Thus a longer measurement.

After a few rounds the sharp edges are rounded enough that the bullet will have a positive stop for subsequent measurements.

Ive experienced this on a few different barrels.
Nothing to worry about, just trust your measurements you're getting now from now on.

 

[Black Tail Hunter]

I've found that, when your lands are sharp on a newly chambered barrel, the bullet will be engraved a few thousandths by the lands when measuring for the first time. Thus a longer measurement.

After a few rounds the sharp edges are rounded enough that the bullet will have a positive stop for subsequent measurements.

Ive experienced this on a few different barrels.
Nothing to worry about, just trust your measurements you're getting now from now on

Very interesting, I would have never thought that. My rifle wasn't new but was low round count, could have played into the scenario. I will clean thoroughly and re measure.

 

[newmexkid]

Here it is http://www.wheeleraccuracy.com/#!videos/p62ad

Watch his videos. You'll probably never use the Hornady tool again.

 

[Roeder]

Please weigh in as I am curious as to what the most likely answer is.

If you are checking your OAL with a bullet from a different lot than you used the first time, your measurement WILL be different and could be shorter or longer. All mass produced bullets vary somewhat as to ogive shape and length from one lot to another. Even though the Hornady OAL tool engages the bullet close to the same area that contacts the throat, it is not exact. A miniscule difference in the ogive curve will make a much larger difference in seating depth. I have seen different lots change the "Jam" length by as much as .050.