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Throat erosion and double-based powders.

 
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  #15  
Old 03-28-2012, 01:20 PM
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Re: Throat erosion and double-based powders.

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Originally Posted by Bart B View Post
My calculations for barrel life with a 7mm barrel burning 85 grains of powder is rather short. With bore capacity at 38.5 grains, 85 grains is about 220% overbore. Barrel life will be about 600 to 700 rounds.
I'm OK with that number, if that's what it is. Does stainless fair any better?

Does how far into the throat the bullet is make any difference?

Where does the 220 stand, in terms of percent overbore, compared to the 7RUM?
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  #16  
Old 03-28-2012, 01:39 PM
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Re: Throat erosion and double-based powders.

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Originally Posted by Max Heat View Post
I'm OK with that number, if that's what it is. Does stainless fair any better?

Does how far into the throat the bullet is make any difference?

Where does the 220 stand, in terms of percent overbore, compared to the 7RUM?
Stainless barrels last about 10 to 20 percent longer than chrome moly ones.

After the throat/leade has advanced about 1/10th inch, accuracy will get worse.

Here's a chart I made using my formulas based on actual life of barrels from a bunch of folks. For example, if your 7mm cartridge burns 55 grains of powder, it'll get about 1531 rounds of life if it starts out shooting 1/4 MOA at 100 yards compared to a 7-08 cartridge burning 39 grains. If the rifle's a 3/4 MOA shooter, it'll get a bit over 3000 rounds of accuracy at that level.

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  #17  
Old 03-28-2012, 03:28 PM
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Re: Throat erosion and double-based powders.

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Originally Posted by Bart B View Post
I was not wrong. Nor did I make an error. No mistake was made, either. Didn't even goof; not one tiny smidgen. But I sure was weak in being right................................ Shame, shame on me for mixing up the data on Hodgdon's web site. I'm probably the only person on this planet who's done that.

Kudos to your for noticing. Excuse me while I find a rock to slither under..........


Easy Bart. I intended no animosity. Just ensuring I didn't have a wire crossed somewhere.
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  #18  
Old 03-28-2012, 06:46 PM
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Re: Throat erosion and double-based powders.

Bart, it looks like your figures would put the 220 below 200% overbore then.

But I'm not understanding how you are figuring "bore capacity" by the caliber (diameter) alone. Length has to be figured in, to come up with a charge weight that would fit into that space. What are you going by for that?
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  #19  
Old 03-28-2012, 07:52 PM
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Re: Throat erosion and double-based powders.

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Originally Posted by Max Heat View Post
But I'm not understanding how you are figuring "bore capacity" by the caliber (diameter) alone. Length has to be figured in, to come up with a charge weight that would fit into that space. What are you going by for that?
I (and lots of others) figure there's only so much powder you can burn through a hole (the throat of the chamber) in a millisecond and not erode it too fast such that it unbalances bullets by increased gas jetting around the bullet and its rough surface damaging the bullet so it doesn't shoot very accurate.

After comparing to top accuracy shooters got for barrel life with different cartridges from 22 up through 30 caliber, the ones burning 1 grain of powder for each square millimeter of the bore's cross sectional area got about 3000 rounds of best accuracy. Cartridges that burned 40 percent more powder got half the barrel life and those burning twice as much got 1/4th the barrel life. Empiracally speaking, its an invese square root law. Short fat cartridges seem to have about the same barrel life for a given charge weight and caliber as long skinny ones.

This isn't an exact science nor simple grade school math issue. My formulas only give approximate numbers because the human doing the shooting plays an important part. Some duffer will claim his .30-06 deer rifle barrel's got 8642 rounds through it and still shoots no worse than about 2 MOA at a hundred yards. But top high power competitors rebarreled their sub MOA at 100 yards .30-06 match rifles in the 1960's at 2500 rounds.
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  #20  
Old 03-29-2012, 08:06 AM
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Re: Throat erosion and double-based powders.

OK, I understand now. Thanks. But what about the post about someone seeing significant erosion after only 70 rounds? He claimed to be using full-weight bullets. So I'm thinking THAT just might significantly accelerate the erosion, due to the higher MOA (moment of inertia), in getting the bullet going, down the barrel (and spinning it up, as well), when there is a large (overbore) charge behind it. And what about the longer rounds wearing down the rifling faster?

I was thinking blowtorch-like thoughts, but is it possible that some of the burning charge might actually reach a plasmatic state? Now I'm thinking plasma-cutter-like thoughts!

I got some load data for 120gr, using Reloader 25: 3950fps using 96.5gr.

Being that I haven't shot a RUM as of yet, I'll admit that amount of charge seems scary!

I want to start out on the right path, powder-wise. 25 is the slowest of the relaoder series powders (which others just can't match, performance wise), designed for the ultra magnums that are shooting the heavy rounds. So I'm thinking 22 could be the best powder for ultra magnums shooting some of the lightest rounds that an ultra mag can shoot. I'd like to see the velocity listed above achieved with 5gr or more smaller of a charge.
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  #21  
Old 03-29-2012, 09:24 AM
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Re: Throat erosion and double-based powders.

Change-up on that, it looks like. The guy using the load that I listed DID try 22, but the groups widened out to 1.5", while velocity was still in the 3800's, at just over 90 gr.

So it looks like reloader 25 is what it will be. I guess it should also work fine if I ever decide to use heavy bullets, like for BR1K shooting. But for hunting, the 120BTs have always delivered, on whitetail deer. With the RUM, I hope to be able to vaporize groundhogs at up to 500yds.
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