Hi.

Pop quiz:

Will a warm/hot (but not too hot to touch) barrel...

A) Decrease muzzle velocity?
B) Increase pressure?
C) Have no discernible effect on either?

Is the answer

A, B, A & B or C?

Cheers

Interesting quiz umshiniwam.

A) Decrease muzzle velocity? ? Not in my rifles.

B) Increase pressure? How would I know? I don’t own a universal receiver. Brass expansion and flat primers don’t tell you much about what your pressure really is.

C) Have no discernible effect on either? Shoot a 17 caliber and report back to me that heat did not make a difference, I’ll be interested to hear about you’re results.

.

Temp of the gun barrel doesn`t have any noticeable effect on the velocity or pressure, but the temprature of the ammo which can absorb heat from the barrel does to small degree.

A couple of my manuals suggest there is ~2 fps change in velocity for each degree F in ammo temprature. Another rule of thumb I`ve heard says velocity increases at around 1% for each 3% of increase in pressure.

I once ran a informal test on temp vs vel in a 6.5x55 with R22 using a RSI Pressure trace and Pact crono using the same load, but from 2 different lots of components (a load of all the same components was shot at both cold and warm temps, and another similar load with new component lots was also shot at very similar high and low temps on different days) 4 different days at 4 different temps ranging over about 60* F in each case.
I found ~5K psi more pressure over these temperature ranges, or ~ a 10% difference, but very minor velocity difference, only around 10 fps as I recall.
Neither of these findings of mine agreed with the "rules of thumb" I`ve been told or read of. Velocity didn`t change by 2 fps/ degree, and the 10 fps didn`t jive with the pressure variation if the 1 to 3 vel/pressure rule is correct.

Thanks for the responses!

Reason I was asking; I have been experiencing some pressure issues in my .270 with higher-end koads, but velocities are low.

Pressure signs are: Bolt lifst easily but extraction of case is sticky and requires some force. Primer is slightly flattened.

I was wondering if this may not be due to the fact that I have typically fired about 12 rounds (in 3-shot groups with a few minutes 'cooling off' between groups) before I fire the heavier loads, and thus teh barrel & chamber are warm.

I was wondering if the warm barrel & chamber might give 'false indication' of pressure.

I guess one way to find out would be to just fire the heavier loads in a cold chamber.

Cheers!

Years ago, when I was really interested in such matters as a well functioning military rifle, I tried an experiment with my heavy barrel Colt AR-15. I fired ten rounds rapid fire, as fast as I could pull the trigger, but still slower than a burst of full auto, and then left a loaded round in the chamber, simulating what would happen in a fire fight. At the end of ten minutes, I fired the round in the chamber.

Fired cartridge case failed to extract, and the extractor pulled through the rim, making extraction by normal means impossible. After knocking the fired case out of the chamber with a cleaning rod, I determined that the case had expanded beyond the limit of its elasticity. The case head had expanded to the point that the primer pocket would no longer hold a primer.

The similarity between my experience and what others had reported in combat was unmistakable.

I've chronographed enough 10 and 20 shot strings over 40 years and measured the psi (M43 Oehler) the last 3 years to know that if shot at a normal cadence there is really no discernable rise in velocity or psi. However, if the barrel is hot and round allowed to stay in the chamber as xausa did then a noticeable rise in velcoty will occur because psi can become extreme. This is why machinguns fire from the open bolt, to prevent cook offs in hot barrels.

Suggest you simply shoot the "sticky" loads from a cold clean barrel to determine if they stick then. If not then those loads are borderline too hot and do increase in psi from the hot barrel.

Larry Gibson

Exert from ; http://www.shootingsoftware.co...essure%20Factors.pdf

4. In this loadvii, Hodgdon’s Varget is highly affected by barrel temperature, with powder
temperature held constant viii. These data were taken at an indoor range, and, as usual, the rifle was
fed single-shot style.
This is a very important finding. Both barrel temperature and powder temperature are
important variables, and they are not the same variable. If you fail to take barrel
temperature into account while doing pressure testing, your test results will be very
significantly affected. As nearly as I can determine, SAAMI does not rigorously control this
variable, though individual testers mightix.
The effect of barrel temperature is around 204 PSI per degree F for the Varget load. Also, the small
sample gathered with 4350, before my thermocouple meter went kerflooey, is consistent with this
result. Since the 4350 sample is small, the uncertainty is high, but the best estimate is 177 PSI per
degree F. If you’re not controlling barrel temperature, you about as well might not bother
controlling powder temperature, either. In the cases investigated, barrel temperature is a
much stronger variable than powder temperature.
I suspect that the mechanism for the effect of barrel temperature on pressure is from the large
thermal mass of the rifle quickly bringing the primer to the same temperature as the chamber, and
that what we are seeing really represents primer temperature. That’s the topic of yet another
experiment, to come.

Some of you may be interested in reading about these experiments .

My experience is similar to xausa's. If you heat up the barrel (read chamber) to really hot, then chamber a cartridge and leave it there a few minutes it will cook as if in a very hot oven, resulting in a nice overpressure situation. If the barrel/chamber is hot, but you chamber and immediately fire, then the effect is lessened. It boils down to how long you leave the cartridge in the superhot chamber.

I forgot to add that open bolt machine guns are designed to minimize the problem of overheating the cartridges left in hot chambers. See link:

http://en.wikipedia.org/wiki/Open_bolt

There could be other factors in play, but one factor clearly mitigates in favor of LOWER pressures from a hot barrel: As steel heats, it expands. Therefore, the diameter of the bore increases as a function of the temperature of the barrel. A hot barrel has a larger bore than a cold barrel. All things being eqaual, a larger bore produces lower pressures.

.

Expansion is just that !. Get a mic go too the range fire 10 shots out of your Rifle mic before an after give us the readings

quote:

Originally posted by ALF:
Does the barrel only expand outward and not inward? Is the expansion unidirectional ?
What gets the warmest, the inside or the outside ?

Yeah, this is at first blush confusing. Expansion is "in all directions" in that the molecules all move away from one another. However, the physics of a circular (or tubular) shape require that for all molecules to move away from one another that the both the outside and inside diameters get larger.

For example, if you want to fit a metal collar onto a pipe of the same outside diameter as the collar's inside diameter, you heat the collar and it grows larger so that you can slip it over the pipe, then as it cools it shrinks to sieze the pipe inside. Or if you're young enough to remember the classic demonstration of heat causing expansion in elementary school, your instructor had an aluminum ball on the end of a rod and an aluminum ring on the end of another rod. The ball was too large to pass through the ring, but when the ring was heated over a bunsen burner it grew large enough for the ball to pass through. In a few seconds of cooling, it would shrink enough that the ball would not pass back through.

The same phenomenon would occur (to some degree) with a rifle in that the hotter the barrel the larger the hole (bore) in it.