Barrel temperature causing more pressure

R

Rick4070

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I am reading a new, non-fiction book by Brandon Webb, a former Navy SEAL, titled "Among Heroes."

Amongst other things. he was tasked with the responsibility of helping completely revamp and transform the entire SEAL sniper force.

He writes: "The fact that that a metal rifle barrel expands as it heats up translates into increased pressure on the round as it passes through, which in turn means higher muzzle velocity and an altered arc of trajectory... And goes on to said an instructor said: "The adjustment you made for elevation this morning may have worked this morning, but now it's a good twenty degrees warmer, and you your round is going to have a proportionately flatter arc, so to compensate you need to adjust your elevation down...

Now, not being a ballistics expert, just a machinist, I would think that if a barrel expands from heat, the bore as well as the exterior of the barrel would get larger, not the exterior get larger and the bore smaller....

A temperature increase in the powder inside the cartridge causing more pressure, I can understand.

I'm kind of confused, can anybody comment on the effects of temperature ON THE BARREL ONLY affecting pressures and hence velocities???
 
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Go back to first year physics/chemestry...
Ball and ring at room temp,
Ball goes thru ring,
Heat up ball and it does not go thru,
Heat up ring and the heated ball goes thru.

While heat does affect shooting, i do not think this explanation is correct
for the affect that was seen.
 
From my understanding, metals expand in all directions uniformly with heat. If we take a barrel cross section with a 1.000 OD and a .300 ID that leaves a wall thickness of .350. The circumference of a .300 bore is .942. The diameter of the ID and OD will increase in proportion to their respective circumferences as heat builds (.0000028 per lineal inch per degree F(thermal coefficient of expansion in steel)). When bore circumference is smaller than wall thickness the bore will shrink as heat is supplied. The bore would have to be less than .110 in diameter before it would become smaller with heat. Larger than .110 diameter, it will grow in its diameter with heat. I think we need a PHD mathematician to confirm this.
Gort
 
When assembling artillery barrels , the outer bands, hoops and sleeves are heated to expand them and then slid over the inner parts - when they cool they shrink down tight. Been doing it this way since the civil war. The inner diameter expands with heat
 
The short version is that the OP has imagined it incorrectly. Following posters got it right. The ID gets bigger when the barrel heats.
 
Boyd, I'm pretty sure I said that the ID of the bore gets larger with heat, and I agree with you and the other posters.

I said: "I would think that if a barrel expands from heat, the bore as well as the exterior of the barrel would get larger,"

I have machined bearing fits where the bore has to be measured at an ambient temperature of say, 72 deg. F., otherwise, if the material temp is over that, and measured for a slide fit, when cooled, the bore is too small, of course.

I have also heated up bushing housings, and dropped a bushing right into them, and if you aren't fast enough, the bushing sticks when the bore cools slightly.

Then of course, there is the wheel wright heating up a steel or iron wheel rim, dropping it over the wooden wheel and shrinking it onto the wood by quenching it with water.

All of these things tell me that the author of the book is wrong when he says that a hot barrel increases pressure and velocity due to expansion.

I think the author of the book thinks that the OD gets bigger, and the bore ID gets smaller...

Putting a cartridge in a hot chamber, and a pressure increase due to the powder charge heating up, I can see....
 
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No worries!!
I have a tendency to be kind of wordy sometimes.
 
The bullet also expands with temperature, which offsets the effect of the increased bore diameter. Both copper and lead have higher coefficients of thermal expansion than steel. Increased barrel temperature also decreases heat losses from the combustion gases, which leaves more energy to push the bullet to higher velocity.
 
what about the changes in coefficients of friction as the temp goes up ??

mks said:
The bullet also expands with temperature, which offsets the effect of the increased bore diameter. Both copper and lead have higher coefficients of thermal expansion than steel. Increased barrel temperature also decreases heat losses from the combustion gases, which leaves more energy to push the bullet to higher velocity.
Click to expand...
 
I once had my own theory that the faster you shoot, the more likely the bullet will ride down the barrel in the same condition, before the barrel started to go astray and spray rounds in a shot gun pattern.

I was wrong,, I shoot F-Bench 25 round strings in 30 minutes, let barrel's cool and repeat for 40 rounds for score. Last 2 match's I slow fired using all the 30 minutes. on both relays. This resulted in the highest scores I have ever seen..2 clean match's on back to back day's with x counts low 30's for 40 round match. One match shot in Clyde N.Y. and next day was at Towanda P.A. both 300 yards.

Regards,
Dan Wilson
 
Heating of a chambered round in a hot barrel has cost me some high 7s in 1000 yd F matches. Now I don't chamber a round until the target comes up with the marker. Slow pit service! :mad:
 
I would venture a hot chamber caused the issue, vice a hot bbl.

xs hedspace said:
Heating of a chambered round in a hot barrel has cost me some high 7s in 1000 yd F matches. Now I don't chamber a round until the target comes up with the marker. Slow pit service! :mad:
Click to expand...
 
I suspect the velocities will increase due to additional (heat) energy being transmitted from the enclosing chamber to the propellant. A load at temp x will contain a specific total energy. A load at temp x + y will contain more total energy, to the amount of temp increase roughly equal to y. It's basic physics.

Take a hot barrel, load and shoot it once as quickly as possible. Then load it and wait long enough for the case and propellant to attain chamber temp. Fire it and note any difference in POI. The difference could be in any vector; a faster load will not necessarily shoot higher.

When I shoot for score, I rest the case on the mag follower and rim of the chamber and set up my shot. I then close the bolt and finalize my shot prep, releasing the shot as soon as the little imaginary light bulb over my head goes bright.

Hi, Dan.

Greg Langelius
 
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