I was shooting a few .223 loads today that I made up with IMR4198 under a 40-gr. VMAX, and the first cartridge case was not even warm when I caught it right off the ejector. I'm used to fired cartridges being at least warm if not kind of hot coming right out of the chamber. It was blowing about 30 mph, so I don't know about accuracy, but drop was about normal --> right on @ ~300 yards.

Never used IMR4198 in a .223 before. I normally use WW748 or VV N133. In fact, this may be the first container of IMR4198 I've ever had? Is it just a super-cool-burning powder? It also smells stronger than most of my other powders.

EDIT: I should edit to say that NONE of the 5 cartridges I fired in the string was hot. The last one was the warmest and it was only a little warm.

Despite no replies, I have a theory (OK, a hypothesis) to put out there: The IMR powder is single-base, whereas 748 and N133 (I think?) are both double-base. The nitroglycerine in the double-base powders should add a tremendous amount of heat to the combustion process, as compared to nitrocellulose alone. Is this element alone enough to explain my observation?

I know the PMC Bronze line of .223 will burn you if you catch it right out of the ejection port, but none of my other reloads (.25-06, .270, .280, .350 Rem, .30-06, .44 Mag, .357 Mag, .40 S&W, .45 ACP, .45 Colt... you get the picture) are that hot.

Almost all the heat produced by the powder "passes through" the brass into the gun's chamber. It takes only a second or so to do this. A spent case ejected immediately from a semi-auto will be hot enough to burn skin, but if only one second elapses from firing to ejection, the case will be only warm.

It is true that some powders do in fact burn cooler, and single-base powders are among them. But the difference is not great, and certainly not great enough to make much of a difference in the heat transferred to the brass and thence the chamber.

Interesting. I believe that I shall try some timings and such (if I remember to!) to see how much difference I can find. I'm more curious about this now. Thanks!

N133 is single base, like IMR4198. Only Vihtavuori's N5xx series high energy powders are double-base. 748 was advertised by Winchester for years as being cooler burning at its pressure peak, and they claimed extended barrel life from it. Even though it is a double-base ball powder, it apparently has enough burning rate inhibitors to keep temperature at peak pressure down, then let the rest of the energy be extracted by sustaining pressure longer during the bullets travel in the bore.

IMR4198, with a burning rate factor, Ba, of about 0.90 is significantly faster than N133 at 0.73, or 748 at 0.56. Since heat transfer depends on temperature difference and pressure and time, the fact slower powders maintain pressure and temperature longer means they have more opportunity to transfer heat to the chamber for a given peak pressure. Case brass has a thermal conductivity of 120 W/mK compared to about 35 W/mK for 4140 steel and about 25 W/mK for 416 stainless. This means heat moves more slowly through the barrel steel than through the brass. The steel then tends, especially with rapid fire, to accumulate and increase in temperature near the surface of the chamber. The brass, with its high conductivity and low total web mass tends to equalize its temperature with that of the surface of the chamber, and an extracted case to reflect the surface temperature of the chamber it comes from.

I guess that also explains why brass from my Garand is hot enough to cook an egg, 20 minutes after I shot it. Everybody says the Greek M2 is loaded pretty hot... ;)

I thought the temperature of spent cases was almost entirely heat it picked up from the chamber. Thus, the first few cases might not be hot, but after you've fired, say, 50, they'll be hot enough to burn. The heat of the chamber being caused by smashing and then grinding a piece of metal through the bore of the barrel, creating massive amounts of friction.

N133 is single base, like IMR4198.
OK, nevermind that theory... :)

IMR4198, with a burning rate factor, Ba, of about 0.90 is significantly faster than N133 at 0.73, or 748 at 0.56. Since heat transfer depends on temperature difference and pressure and time, the fact slower powders maintain pressure and temperature longer means they have more opportunity to transfer heat to the chamber for a given peak pressure.

I agree, but this would predict that 4198-loaded brass should remain hotter than 748 upon extraction, since it should have had a reduced opportunity to transfer heat into the steel. This is the opposite of my observations. Still, my observations were probably based mainly on things I hadn't properly accounted for.

I was actually looking for some k values and other things last night to do some calcs myself of heat transfer rates and other things, but that didn't end up being that helpful. I should never have given up my Midwest Plan Service books...
I'm still going to do some more controlled testing with my IR thermometer one of these weekends.

Thanks, all.