Barrel Temperature and Velocity Variation

[Bill2905]

I put together some 223 and 6BR loads and took them to the range with the chronograph this morning. I didn't have a lot of time and went through each rifle pretty quickly. As expected, the barrels got pretty hot. Each rifle produced the lowest SD on the first load with a cold barrel. As the barrels heated up, the SD started to increase. My goal was only to get numbers and I didn't try for any groupings.

Most of us shoot with the knowledge that groups start to open up when the barrel heats up. I always assumed that this is caused by dimensional changes in the barrel as it gets warmer.

Is it commonly assumed that shot to shot velocity differences also become more variable as the barrel gets hotter?

[charlie b]

Depends on the barrel. My Axis (.223) is better when hot. POI change from cold to hot is not much either.

The 12 .308 with stainless is more sensitive. POI change from cold to hot is noticeable. Groups sizes decrease when shooting cast. When shooting jacketed they increase a bit.

PS the change in velocity may be as much from powder temp change than changes in barrel. Especially if your cartridges are sitting in the sun for any length of time.

[Robinhood]

On thing that can be done to mitigate varying velocities like Charlie hinted at is to not leave the cartridge in a hot barrel. Eject the last round fired and hold off on loading the next round into the chamber until the flags drop. Then chamber the next round when you are ready to fire. If the wind picks back up before you shoot, eject that round and set it back in the tray and grab another one. Point being that you cant leave a cartridge in a hot chamber without seeing some velocity increase.

[CFJunkie]

charlie b's and Robinhood's suggestions are good advice. Leaving a round in a hot chamber causes the powder to heat up above ambient temperature. The longer you leave it chambered, the closer to barrel temperature it will be when you finally fire it.

Temperature effects on velocity and its potential for moving the POI depends upon the powder.
A good thing is that "extreme powders" are relatively temperature insensitive compared to normal powders.
H3450 is rated at 4 fps change from 0 to 125 degrees F. and Varget is rated at 8 fps over the same range.
Most of the other extreme powders (H4198, H4831SC, H4895, H1000, Retumbo, H322, Benchmark, IMR4166 Enduron, and IMR4451 Enduron) can change from 8 to 20 fps over the same 125 degree range.
The variations change slightly with caliber - the smaller .223 cases increase the effect of temperature even on the most temperature insensitive powders.

Normal powders range from 50 to 166 fps over the same range depending upon the powder.
Hodgdon has some information their extreme powders at https://www.hodgdon.com/extreme-rifle-powders/

If you shoot under conditions that are conducive to large temperature variations, extreme powders might be worth trying.

[Bill2905]

On thing that can be done to mitigate varying velocities like Charlie hinted at is to not leave the cartridge in a hot barrel. Eject the last round fired and hold off on loading the next round into the chamber until the flags drop. Then chamber the next round when you are ready to fire. If the wind picks back up before you shoot, eject that round and set it back in the tray and grab another one. Point being that you cant leave a cartridge in a hot chamber without seeing some velocity increase.

I was cognizant of not placing a round in the chamber until ready to fire. The one thing charlie b mentions that I did not consider was leaving the box of cartridges in the hot sun. After the line took a break for target changes, I grabbed a round from the box to continue and quickly noticed it was much warmer to the touch.

For me, POI changes due to temperature are probably hard to measure because errors in my shooting technique will contribute much more to the overall variation.

Thanks for your input.

[Bill2905]

If you shoot under conditions that are conducive to large temperature variations, extreme powders might be worth trying.

I was using H322 in both of these cartridges. I recently bought a pound of IMR4895 but after reading up on this topic, I kind of wish I would have chosen something different. I'll use it and see how it goes.

Thanks

[charlie b]

....For me, POI changes due to temperature are probably hard to measure because errors in my shooting technique will contribute much more to the overall variation. ....

This is my problem on many days as well :) Sometimes my cold barrel shots are right on top of the hot barrel shots. Other days with same temp swing they are different. Many other variables at work as well.

If you really want to be picky about it you need to measure your barrel temp and ammo temp when shooting. Log bullet POI on target and compile dope tables based on that (as well as air temp, humidity, pressure and wind). No, I do not :)

[Robinhood]

When shooting comp in the summer months(April through October) with no sunshade in the south, you must cover your ammo box with a towel or something to keep the sun off your rounds. The longer the distance the more a velocity change effects things.

[yobuck]

Of coarse the opposite happens in very cold wether when ammo is left in vehicles for example when overnite temps dip to zero or below.
All of this leads to the question of just how precise is precise data?
And we havent even touched upon the affect various wind conditions have.

[Fuj']

The temp stable powders I use (mainly R-16) have been decent
in this regards, But I still do not let a round sit for a period of time
in a warm barrel. I did a test where I got the barrel up to 200
degrees, and let the round sit for 5 minutes in my Creedmoor.
I gained a good 35 fps. Barrel was down to 160 at the end of 5
minutes when I pulled the trigger. I have in my kit, a laser temp
gun I picked up for cheap at "Horror Freight". It's nice to have.
I use it more though on my buddies suppressor !! Them things
like to get cooking, and with very few rounds.

[tric3imagery]

Why some ELR competitors use a Magneto Speed chronograph durring competitions. as the barrel heats your shots are changing POI, making adjustments as you shoot. Interesting.

Sent from my SM-G955U using Tapatalk

[Robinhood]

I have in my kit, a laser temp gun I picked up for cheap at "Horror Freight". It's nice to have. I use it more though on my buddies suppressor !! Them things like to get cooking, and with very few rounds.

Seen some sizzled skin from them thingy's. Skin doesn't smell that good when it gets hot. Like you tell them teen boys. Wrap that rascal.

[Texas10]

I put together some 223 and 6BR loads and took them to the range with the chronograph this morning. I didn't have a lot of time and went through each rifle pretty quickly. As expected, the barrels got pretty hot. Each rifle produced the lowest SD on the first load with a cold barrel. As the barrels heated up, the SD started to increase. My goal was only to get numbers and I didn't try for any groupings.

Most of us shoot with the knowledge that groups start to open up when the barrel heats up. I always assumed that this is caused by dimensional changes in the barrel as it gets warmer.

Is it commonly assumed that shot to shot velocity differences also become more variable as the barrel gets hotter?

I was at the range two weeks ago and had the exact same experience you had, with the same calibers. With Magnetospeed attached, I shot just for velocity data, and watched the speed rise with every shot until the barrel came up to temperature, usually by the 8 or 9th round. Obviously this affected the SD and ES.

FWIW: I've removed the ejectors from my bolts and will chamber a round and immediately open the bolt. This leaves the cartridge suspended on the bolt head which I then close when ready to shoot however I've not seen much difference in groups by doing this.

I remember reading a study performed to test the theory that a hot chamber heats up the powder charge. A cartridge modified by running a thermocouple through a drilled out primer pocket and into the powder charge. This was placed in a hot chamber and temperature monitored. There was little change in charge temperature.

This was due to two reasons. First of all the cartridge brass is not an intimate fit to the chamber and is therefore less efficient in conducting heat.
Secondly, the propellant itself is not effective in conducting heat due to kernel to kernel contact is not very efficient. That being said, there is no doubt that heat soaking your ammo will change its behavior. It is a fact that applying heat to a chemical compound will accelerate it's reaction time.

Regarding barrel temp, my experience might be interesting to you. I have made a barrel cooler from a battery operated mattress inflator pump. It moves a lot of air as compared to other similar barrel coolers, and cools down the barrel rapidly, too quickly it seems. If I don't give the barrel several minutes to stabilize temps after removing the barrel cooler, it shoots larger groups. It should be noted that I rarely shoot a factory barrel, and the ones referenced above are high quality aftermarket barrels.

[Bill2905]

I was at the range two weeks ago and had the exact same experience you had, with the same calibers. With Magnetospeed attached, I shot just for velocity data, and watched the speed rise with every shot until the barrel came up to temperature, usually by the 8 or 9th round. Obviously this affected the SD and ES.

I too make it a practice to minimize the time a round being used for velocity data sits in a hot chamber. On my next trip to the range, I plan to get more velocity data and also do some target shooting. I think I will try gathering the velocity data after I finish my target work when the barrel is hot.

[charlie b]

I do both at the same time. Set up chrono and shoot. Record position and velocity of each shot. Sometimes the chrono data actually shows why I get a high or low flier. When shooting cast I sometimes also record the weight of each bullet being shot and add that to the data.

[Bill2905]

I have done both at the same time more often than not. I use the Magnetospeed Sporter and sometimes wonder if the bayonet influences POI. Always looking for an excuse to cover for my occasional lack of skill.

[charlie b]

I never went for the magnetospeed because it attaches to the barrel. That changes the vibration nodes in the barrel so could 'upset' the accuracy of the load. Mine is just one of the simple ones set up in front of the bench. I wish I could justify buying one of the radar systems.

[Robinhood]

Real world experience is always good to read from you Texas10. I would be interested in the test you referred to along with test parameters. Sounds like an interesting read.

When hitting targets at the 600 plus range I see the results of a round sitting in hot chamber on the target. Sometimes it takes a minute or more for the flags to become favorable. Experienced drove me into chambering a round later rather than earlier.

[Bill2905]

I never went for the magnetospeed because it attaches to the barrel. That changes the vibration nodes in the barrel so could 'upset' the accuracy of the load. Mine is just one of the simple ones set up in front of the bench. I wish I could justify buying one of the radar systems.

I used a simple Pro Chrono that sets up on a tripod in front of the bench for 20+ years. When the electronics started acting up, I opted to replace it with the Magnetospeed because it eliminates the logistics of trying to set it up and get the position adjusted at a busy public range with strict rules about where you can/can't be during cease fires. I have been shooting at another range lately where they aren't as fussy and I may get another one of the free standing units.

[charlie b]

I understand that. With a very restricted range I'd probably save my pennies for one of the radar units.

After years in the military I developed an allergic reaction to controlled ranges. :)