Home Forums Hunting & Shooting Ask The Gunwriters Is pressure related to throat erosion?

This is another "just curious" question.

Let's say I'm shooting a 7 Rem Mag. I have a high pressure load and a low pressure load, using the same grains of (different)powder. If I'm shooting a lower pressure load, but with the same grains of powder at the same speed, would throat erosion be less? I'm guessing it would.

I've heard that slower powders are more damaging. So if I shoot a reduced load with a really fast powder several hundred feet slower(Blue Dot)in a 7 Mag, throat erosion is no longer a concern, right?

Last year I sold my two 7 Mags. I bought a 7-08 instead. I was struggling to get my ES down where I wanted it with the magnum. And I thought since I mainly do target shooting, there was no sense it the extra recoil and shorter barrel life.

I'm really liking the 7-08, I was just wondering if 47 grains of H4350, at high pressure in the 7-08, saves more barrel life than 59 grains in the 7 Mag at lower pressure.

Thanks

That would be an interesting powder which gives you lower pressure but the same speed for the same charge weight. In principle, it should erode less. Lower pressure means lower temperatures, and temperature is needed to soften the steel.

There are several variables with erosion. The temperature of the gases is one, as is the density of the gases (again mostly related to pressure). The grain shape probably has an effect (some of the grains impinge on the softened steel), and the shape of the case can affect how well the flow is directed onto the throat (I think it was Barsness that noted the .243 Win is known for this). The composition of the powder determines the heat at the surface of the burning grains.

Sorry, I know some of the variables, but not how they all come together. I think pressure is your best indicator, though.

According to the late Homer Powley, sometimes called the father of American ballistics, throat erosion is caused by flame cutting.

Flame cutting is directly proportional to temperature.

Temperature is directly proportional to pressure in a gas. So lower pressure means less throat erosion.

Slower burning powders cause less throat erosion. The reason is that the pressure curve rises and falls over a greater length of the barrel, as opposed to peaking at the throat. In other words, for throat erosion, we are worried about peak pressure.

You will also notice that your barrel gets hotter right by the throat than it does farther down the barrel. That's where peak pressure occurs.

Sorry, I wasn't clear. For example, 65 grains of IMR 7828 (at about 60K) and 65 grains H4831 (at about 50K) give about the same speed. I am asking if the 4350 would than have less throat erosion because of the lover pressure.
If it would, I was wondering how that would equate to a low pressure 7 mag load vs a high pressure 7-08 load regarding barrel life.
That's interesting about the shape of the case affecting barrel life.

Indy, thanks for the response. You posted while I was typing my response above.
If I'm understanding the importance of pressure, does that mean a low pressure 7 mag barrel will last as long as a high pressure 7-08 load?
Or would it equal out because the mag has more powder?

Someone on here once posted (maybe JB ) a chart showing witch powders caused the most throat erosion. They were all the slowest powders. Regardless of pressure, I believe it was the latent heat and erosion from the powder basically sand blasting the throat that did most of the damage. I do recall that H870 was top or second on the list. I also believe your example above with the H4831 and IMR7828 would be reversed in the pressure caused by a 65 grain charge in the same case with the same bullet.

I found a source of military experiments with bore erosion a year or two ago, and heat is indeed apparently the biggest factor in erosion, and it's worse with slower-burning powders because the duration of the heat is longer, due to more gas being created. Interestingly, they didn't find all that much difference between single-based and double-based powders, though double-based powders did erode throat somewhat faster. They also found a definite relationship between shoulder angle and neck length and throat erosion.

However, they didn't consider pressure, or direct erosion from the powder granules or the bullet's friction, nearly as big a factor as heat. Or at least that's what I recall. It was long article with a lot of interesting points, and I summarized it in an article for GUNS magazine, but still probably have the link for it somewhere. I'll see if I can find it.

That sounds logical. Powley did not discuss abrasion from powder granules, probably because he had no way to measure it. He was a very literal and mathematical person who discussed things he knew about.

For instance, he concluded (correctly) that the highest BC at the highest velocity would make the best 1000 yard rifle due to less wind resistance. So he necked down a .300 Weatherby to 6.5mm and fired boat tail bullets in it. It failed to capture the Wimbledon Cup because (a) he picked the wrong shooter to shoot it, (b) he forgot about how all the leaping, torquing, and blasting stuff would degrade accuracy, and (c) in those days there was not much of a choice in 6.5mm match bullets as compared with .30 caliber.

I'd guess that pressure is related to throat erosion - in fact I'm sure of it. It seems pretty obvious in extreme examples.

I'd suspect that pressure is a vague term embracing both peak pressure and area under the curve and including such things as a secondary peak and other odd observations.

I'd also suspect that pressure in a gas loosely defined is related to temperature - e.g. diesel effect - and so be hard to distinguish. For another complication molecular weight of the gas is a factor - higher velocities with a lighter gas just might act like lower velocities with a heavier gas. Powder gases will vary in chemical composition and so in molecular weight. In fact I think there are many collinear factors with pressure only one of many.

There's a nice military study out of Australia that uses gas velocity at the base of the projectile - which I think might be related to pressure - as one of several factors in predicting erosion.

Some accuracy shooters like an Excel program Mike wrote inter alia "According to Ken Howell, I had to account for pressure." Notice this formula makes no allowance for neck length for what that might be worth.

Add to that


Given known issues with the 7mm Remington Magnum (and maybe the .243) with respect to variation in pressures as measured in various ballistic labs I'd expect using the 7mm Remington Mag to confound collinear factors more than some other cartridges and so I'd think about using a WSM or something else.