6.5 mm PRC - cold bore shot

[riflemonkey]

Just thought you might find this interesting...

Went to the range with a box of Norma Match 143 gr ammo. Used the Garmin Xero, turned it on and aimed the target, started a session and took muzzle velocity data as I shot my custom made 6.5 PRC rifle. Look at the velocity versus shot plot below. Can you see the cold bore effect here ?!? The data gave 175 ft/sec deviation from the lot mean of 2983 ft/sec. I waited about 60 seconds between all shots including the first and second... The cold bore effect is real. Again, just thought you might find this interesting...



riflemonkey
monkeying in TX...

 

[boltfluter]

Just thought you might find this interesting...

Went to the range with a box of Norma Match 143 gr ammo. Used the Garmin Xero, turned it on and aimed the target, started a session and took muzzle velocity data as I shot my custom made 6.5 PRC rifle. Look at the velocity versus shot plot below. Can you see the cold bore effect here ?!? The data gave 175 ft/sec deviation from the lot mean of 2983 ft/sec. I waited about 60 seconds between all shots including the first and second... The cold bore effect is real. Again, just thought you might find this interesting...

View attachment 1563821

riflemonkey
monkeying in TX...

Not sure what point you are trying to make?
Paul

 

[Mike Casselton]

Clean barrel?
Dirty barrel?
Light lube in the bore from cleaning?

Lots of stuff will affect the velocity of the first shot.

 

[riflemonkey]

When I made the first, "cold bore", shot and analyzed the results I just found it interested that, as could be expected, the first shot was significantly slower than the rest. I would not have guessed it would be as slow in ft/sec as it was. Thought I would pass the data along for grins...

However, I did not say much about why it was so slow - it was a "cold bore" shot. But Mike Casselton had a good comment above, there are many reasons in addition to just the temperature of the barrel in contact with the next brass case that would cause the first shot to be slower - for example initial barrel condition...

Anyway, glad that my data sparked a conversation or two in the forum... Thanks for the above comments.

(So many variables in shooting science - keeps it interesting! imho)

-riflemonkey

 

[Wild Bill IV]

When I made the first, "cold bore", shot and analyzed the results I just found it interested that, as could be expected, the first shot was significantly slower than the rest. I would not have guessed it would be as slow in ft/sec as it was. Thought I would pass the data along for grins...

However, I did not say much about why it was so slow - it was a "cold bore" shot. But Mike Casselton had a good comment above, there are many reasons in addition to just the temperature of the barrel in contact with the next brass case that would cause the first shot to be slower - for example initial barrel condition...

Anyway, glad that my data sparked a conversation or two in the forum... Thanks for the above comments.

(So many variables in shooting science - keeps it interesting! imho)

-riflemonkey

riflemonkey:
I too shoot the 6.5 PRC!!
I have never had a spread of more than 50f/s!!! I use my Labradar for every shot at the Range!!! I am a LR hunter and practice 1st shoot kills, cold barrel shooter!! During your one minute cool down, if you have a new loaded round in the chamber during cool down time, you are lightly COOKING that round!!! Increasing the powder temperature can dramatically increase the pressure!!! That is what I'm seeing in you data!! In chemistry, to spread up a chemical reaction, you slowly heat it up!!! Same is true with propellants since it is a physical/chemical reaction!!!

In my load development, I balance or weigh my final loads by (10) cold fires at 20° barrel spread temperature, and (10) 15 second intervals!!! Generally, my 3 shot foulers are faster hitting high with more spread on the target!! The last fouler settles in!!! Been doing this for plus 40 years with all, but one, big game one shot kills from 225+ to 600+ yard shots!!!

The one exception was a 117SGK shot from my custom 25-06 (26" Shilens air gauged light target profile bbl, Fagen AAA fancy walnut thumbhole stock, Springfield 03-A3 action) hit a rib dead on at ~325 and it grenaded blowing out the right lung on a very large 11 point typical whitetail buck!! Got another shot at 575 left side full profile shot!!! Smacked home with another heart shot!! He went down hard!! He popped up his head and I loaded another round!!! Gave him the count of 5 to die before slowly squeezing the Timney trigger!! Just started the squeeze and the head dropped!!! In cleaning the buck, one left and one right rib bones had, near 1-1/4" blow out holes!!!! I only use that 25-06 now on does, antelope, and varmint!!

This is why I purchased a Browning X-Bolt LR Max (1:7 twist) 6.5PRC so I can extend my range to 800, 900 antelope/small deer, 1000 plus for varmint with 140 grainers on up!!!

 

[drop_point]

Have you been able to reproduce this multiple times?

 

[riflemonkey]

riflemonkey:
I too shoot the 6.5 PRC!!
I have never had a spread of more than 50f/s!!! I use my Labradar for every shot at the Range!!! I am a LR hunter and practice 1st shoot kills, cold barrel shooter!! During your one minute cool down, if you have a new loaded round in the chamber during cool down time, you are lightly COOKING that round!!! Increasing the powder temperature can dramatically increase the pressure!!! That is what I'm seeing in you data!! In chemistry, to spread up a chemical reaction, you slowly heat it up!!! Same is true with propellants since it is a physical/chemical reaction!!!

In my load development, I balance or weigh my final loads by (10) cold fires at 20° barrel spread temperature, and (10) 15 second intervals!!! Generally, my 3 shot foulers are faster hitting high with more spread on the target!! The last fouler settles in!!! Been doing this for plus 40 years with all, but one, big game one shot kills from 225+ to 600+ yard shots!!!

The one exception was a 117SGK shot from my custom 25-06 (26" Shilens air gauged light target profile bbl, Fagen AAA fancy walnut thumbhole stock, Springfield 03-A3 action) hit a rib dead on at ~325 and it grenaded blowing out the right lung on a very large 11 point typical whitetail buck!! Got another shot at 575 left side full profile shot!!! Smacked home with another heart shot!! He went down hard!! He popped up his head and I loaded another round!!! Gave him the count of 5 to die before slowly squeezing the Timney trigger!! Just started the squeeze and the head dropped!!! In cleaning the buck, one left and one right rib bones had, near 1-1/4" blow out holes!!!! I only use that 25-06 now on does, antelope, and varmint!!

This is why I purchased a Browning X-Bolt LR Max (1:7 twist) 6.5PRC so I can extend my range to 800, 900 antelope/small deer, 1000 plus for varmint with 140 grainers on up!!!

Good possible explanation, but imho that we are seeing the effects of barrel warm up on the chemical reaction rates of each successive shot. I say this because I never chamber the round until right before I fire. So for the results presented - I shot a round, waited for a minute - then chambered the round from the magazine and immediately fired again. Rinsed and repeated for the rest... Thanks for the response...

-riflemonkey

 

[Wild Bill IV]

Good possible explanation, but imho that we are seeing the effects of barrel warm up on the chemical reaction rates of each successive shot. I say this because I never chamber the round until right before I fire. So for the results presented - I shot a round, waited for a minute - then chambered the round from the magazine and immediately fired again. Rinsed and repeated for the rest... Thanks for the response...

-riflemonkey

riflemoney:
I single shot loads!!! Never loading rounds from the magazine! With your loaded rounds in the magazine, you maybe transferring heat!!! Do you leave the bolt open for that I min interval???
Steel has a thermal conductivity of 17BTU/(hr x ft x °F)!!
Brass has a thermal conductivity of 64BTU/(hr x ft x °F)!!
From thermodynamics, good absorbers of heat are good emitters, and poor absorbers of heat are poor emitters!! After firing a round, the quicker you get that brass out of the case, the less heat is transfer to the chamber and the bolt!!! The longer that fired case stays in the chamfer, the heat in the brass creates a gradient or zone on the chamber surface!!! The thickness of that zone is a function of the temperature (heat energy measuring variable) and time!!! And, every additional round keeps building or adding heat to the steel!!! The hottest areas of a rifle is the chamber and part way down the bbl!! Roughly 8 to 16 inches of the first part of rifling using medium burn rate rifle powders!!!! With you leaving the bolt open for a minute, the hot zone is further down and away from chamber!!!