Throat erosion and double-based powders.

[Max Heat]

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?

 

[Bart B]

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.

 

[Outlaw6.0]

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.

 

[Max Heat]

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?

 

[Bart B]

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.

 

[Max Heat]

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.

 

[Max Heat]

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.

 

[Bart B]

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 know of a military unit building long range match rifles and a new barrel was fit to one. So they gave the rifle and 30 rounds of .30-.338 ammo to some kid to "break in" the barrel. He came back to the unit and a top shooter took it to the 1000 yard range to check it out. Best 10-shot group was well over 2 feet; the previous barrel shot under 7 inches. They later learned that kid shot all 30 rounds in about 3 minutes. Its throat was quite eroded.

Note that bullets don't move very fast when they enter the rifling; they only go a couple hundred feet per second at that time.

Long bullets wear out barrels the same rate as short ones do. They both wear very little metal off the lands and grooves. Many folks have taken long barrels that have lost accuracy, cut a couple inches off their back end then refit them to the receiver. I've had that done with three barrels and the shorter versions shot just as accurate as the original ones did.

 

[Haagen Dazs]

This is a great discussion. I've got a question if someone can shed some light on the whole barrel life issue:

A 7 WSM shoots a 180gn bullet at about 3000 fps and a 338 norma mag shoots a 300 gn bullet around 2650 fps. They both use about 1.4 gn / sq mm which means they should have about 1500 rounds of barrel life for decent precision (~1/2 MoA).

Shouldn't the 338 have a better barrel life because its shooting a larger bullet slower? Or does barrel life have nothing to do with the interference fit between the rifling and the bearing surface? Also, what is the 'coolest' burning powder that is recognized as a powder that's easy on barrels? (if there is one)...

Thanks

 

[Thomas B.]

Haagan Das,
I contacted Hodgdon powders about 10 years ago on this very subject.They told me that the double base ball powders like H380 burn at lower temperature than single base stick powders. I have unscientifically proven this by shooting 10 shot strings through a 22-250 with both types of powders loaded to close to the same velocity.

My barrel was significantly cooler to the touch with the H380 as opposed to 4895 when starting from cool barrel. It was enough to satisfy my curiosity as to which powder to go with.

 

[Max Heat]

Try Alliant's Reloader series. I've found that they DO generate higher velocities before any overpressure signs appear, than those "H" series (and other) powders. They supposedly only burn 100 degrees hotter, but I think they probably will burn the barrel faster. I'm all about velocity (heat), so I'll still ante up, if that's the price to play! But I plan to let the new barrel cool between every shot, and clean it after every shot, for at least the first 50 rounds.

 

[kc0pph]

I hear H-870 is the powder to use for extreme overbore shooting. I have a 338-378 which is overbore and running around 115gn of the 870 through it., I have played with other powders and only 870 will get them 300 pills flying fast.

Not sure if 7mm is too small of a bore to benefit from 870 but you might give it a try. Thunderbird sells it for 88 per 8lb keg, and another guy GIBRASS (i think) sells it for 46 per 8 lb keg.

 

[Bart B]

I hear H-870 is the powder to use for extreme overbore shooting........ Not sure if 7mm is too small of a bore to benefit from 870 but you might give it a try. Thunderbird sells it for 88 per 8lb keg, and another guy GIBRASS (i think) sells it for 46 per 8 lb keg.

In the late 1960's, I used a .264 Win. Mag. with Norma 139-gr. match bullets atop H870 for long range matches. It shot very accurate but its barrel lasted only 640 rounds.

While all sorts of ball powders have a wide-spread following, I don't know of any top level long range competitor that uses them. Extruded powders have a long history of best accuracy in rifle cartridges.

PS:
That 870 powder I used decades ago was a coarse extruded powder. I thought it was "H" 870 but it may have been something else. My memory is playing tricks again.

 

[kc0pph]

Cant wait to burn the barrel out of this guy... Want a 32-34" tube

Never heard of 870 being a real barrel burner, but it is a ball powder and i do know a lot of people do not like them. Of all the powders i have tried in 338-378 none of them preform like the 870 (It was bought from WalGreens and that tells you how old it is, but still works great).