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Excessive Throat Growth Question

 
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  #1  
Old 02-27-2012, 07:16 PM
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Excessive Throat Growth Question

Not sure how to proper ask but, here goes.

My favorite rig has dropped off in accuracy. I've used the same COAL (Cartridge Base to Ogive) since the barrel was new. Have 900 hot rounds down the old girl.

Guilt edge accuracy has gone.

How much increase in length of throat, from original, is a decent give up and redo point? Or is there something that can be done to squeeze a few more rounds out of her?

As close as I can determine throat has increased 0.047".
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Old 02-27-2012, 07:34 PM
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Re: Excessive Throat Growth Question

i'm having one of my rifles done right now. i'm cutting 1" off minimum is about 1/2 but i'm goin on the safe side.
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Old 02-27-2012, 09:07 PM
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Re: Excessive Throat Growth Question

With about .050" of throat wear at the bullet's ogive contact from where it started when new, that's about normal when accuracy drops off. But the worst part of this is the throat's eroded more than that further down the bore. That's the part that tends to gouge bullets and upset their perfect form so accuracy degrades.

Usually, if one seats bullets .010" longer than throat contact in necks that aren't too tight so the bullet seats back a bit when the bolt's closed, accuracy will still continue to be great. Sometimes such that bullets need be seated .075" further out than when the barrel's new.

What cartridge were you shooting? For some, 900 rounds to loss of gilt-edge accuracy is normal.

And what do you mean by "hot" loads?
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Old 02-27-2012, 09:25 PM
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Re: Excessive Throat Growth Question

Quote:
Originally Posted by Bart B View Post
What cartridge were you shooting? For some, 900 rounds to loss of gilt-edge accuracy is normal.

And what do you mean by "hot" loads?
Thanks for the informative response.

This rig has been ridden hard but not mistreated.

270 Allen Magnum.

Hot = 5 shots before having to retire cases due to no longer holding primer. No longer holding means when the primer falls out after hand seating the case goes in the bucket.

170 bullet at 3400
195 @ 3100
150 @ 3650

The cartridge is designed for a lifetime of taking game. I seemed to have used it for a practice gun with most shots at paper and rocks out to 1400+ yrds.

I'll have much more respect for the next barrel.

I can feel the rough bore ahead of the throat. Reduced the roughness considerably by thorough cleaning. Will give it good dose of Bore Tech Eliminator, seat things a little longer and see if I can milk a little more accuracy.

The shame is is that she held a solid 1/2 MOA for well over 800 rounds. Count how many woofs cudda been done in instead of rocks...
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Old 02-27-2012, 09:55 PM
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Re: Excessive Throat Growth Question

I was reading on Accurate shooter that sometimes you also get a buildup of carbon fouling in the throat area that copper cleaners don't really remove. I also swear by Bore Tech eliminator. They recommended using something more abrasive like JB Brobst to scrub the throat and the first 10 inches of the barrel. Most of the 600yrd to 1000 yrd benchrest shooters do that about every 150 to 200 rounds or when accuracy starts to drop off. Couldn't hurt to give it a try.
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Old 02-27-2012, 11:35 PM
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Re: Excessive Throat Growth Question

Depends on the condition of the bore ahead of the throat. Sometimes a turn (.060) is plenty. In your case, I bet it's cracked and hardened for at least an inch, if not several. Gotta clean 'er up good, take it off, and have a look.

You can chase the lands a little with your bullet seating depth. Might get you a few more rounds.
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Old 02-28-2012, 05:50 AM
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Re: Excessive Throat Growth Question

Quote:
Originally Posted by grit View Post
Depends on the condition of the bore ahead of the throat. Sometimes a turn (.060) is plenty. In your case, I bet it's cracked and hardened for at least an inch, if not several. Gotta clean 'er up good, take it off, and have a look.

You can chase the lands a little with your bullet seating depth. Might get you a few more rounds.
Bunch of good tips here. My gunsmith will not re-chamber a barrel with a firecracked bore, says it is a waste of money. Even if the bore is reasonably clean, all you can expect (with a 6ppc) is maybe another 600 rounds before it is toast.
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