Bullet jump and throat wear

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RBriscoe
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Bullet jump and throat wear

Post by RBriscoe »

Here’s an interesting article about bullet jump affecting the consistency of point of impact as the throat wears.

Many of us have been seating bullets well off the lands for a variety of reasons, sometimes as simple as magazine length restrictions.

The article refers to a rather exhaustive study (50 rifles and 2,500 rounds expended) conducted by an industry company which suggests we may not be seating far enough off the lands.

https://precisionrifleblog.com/2020/04/ ... ment-tips/
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Merlin
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Re: Bullet jump and throat wear

Post by Merlin »

Interesting in an academic sense.... Might be more of a concern if I was shooting a barrel burner pushed hard for F Class or Long Range BR. With regard to my HP Silhouette rifle - With the wobble in my hold to worry that my POI changes every time I pull the trigger due to throat erosion seems to be low in my list of priorities.
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RBriscoe
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Re: Bullet jump and throat wear

Post by RBriscoe »

Merlin,

The point of the article is to establish a seating depth that gives a predictable result as the throat wears, not just across a two day event, but over time. This will not necessarily be “the smallest group”, but one which is consistent and a point of impact that does not significantly change with wear. We have enough atmospheric changes to deal with as it is.

This may not be for everyone, but is similar to a ladder test with powder charges where one seeks a powder charge that yields a consistent result even with a fluctuation of a few tenths of a grain in the powder charge. This is not to be confused with the F-Class practice of weighing powder charges down to the individual grain of powder.
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Merlin
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Re: Bullet jump and throat wear

Post by Merlin »

I'll read it again and if I'm smart enough to figure the proper distance I might try it. I do the ladder test to find the "node" so with your explanation i kinda get it.... If someone writes a How to Determine Proper Bullet Jump for Dummies/Merlin article.........
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malinois
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Re: Bullet jump and throat wear

Post by malinois »

Check out Erick Cortina and his work with F-class rifles regarding bullet jump...Needless to say after trying it out... I no longer chase the lands nor do I care where they are... I also only full length size as well...
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Re: Bullet jump and throat wear

Post by Snake »

I can only comment from experience and conversations with various bench rest gurus. Seating the bullet at the lands or just off .001-.005 prevents the bullet tipping before it enters the lands and so entering 'crooked'. The tipping occurs because of uneven release from the case neck...the neck does not always release evenly and this uneven release is related to the seating tension and the brass age and uniformity. So the article may suggest a longer bullet jump empirically relates to more consistent group sizes....it begs the question....are 10 5" 100yd groups better than 3 1" groups at 300? What is the criteria for quantifying consistency.....for example are 5 4" groups deemed "consistent" but 4 4" aren't?
Like women each rifle has its like and dislikes. Chasing lands changes the pressure curve in the cartridge ...the further out the bullet goes to approach the lands the less in extends into the case and ergo less case pressure and a changed ignition. If you chase the lands you have to increase the charge to keep the same pressure of the formerly 'good' load. As an anecdote...years ago I chased the lands without increasing the charge. Eventually the cases started showing soot back on the shoulder area and the primers started backing out! Low pressure would not expand the brass enough to seal out the back pressure/soot and drive the case rearward against the bolt face with sufficient force to keep the primer from backing. The question then developed...what increase in powder would get the old JU-JU back? Thus another mystery for the erstwhile tinkerer
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