Cold temp caused faster speeds

[bstomper]

Long story short, this fall I did load development for my 7rm found a good accurate load of 68.4gns h1000, 175gn ABLR, federal 215m primers and Federal brass with speeds of 2891 fps at -7 Celsius out of my win m70 stainless. Went out today to get some colder temp speeds for my ballistics app. Put the gun and the bullets outside for several hours to acclimatize to the -26 Celsius temps before shooting and got average speeds of 2923. No pressure signs. Not what I was expecting. Why would my velocity go up in the colder temps.
took the 6.5 creedmoor out as well to once fire the rest of my new hornady brass before I start load development and to finish breaking in the barrel. They were all 41 gns of h4350. Last time I was out at -12 Celsius i dhot 40 rounds at 38 gns and loaded the rest in .2 grain increments all the way up to 43 grains with out any pressure signs. So that's why I had the ones today, -26 Celsius, at 41 for once firing the rest of the new brass. Should be safe since I had seen no pressure signs at 43 gns in warmer weather. Got 15 shot in and the last couple I thought I felt sticky bolt. Shot one more and had definite sign of pressure with sticky bolt and ejector mark. Again, not what I was expecting.
Seems the cold is giving me more pressure. Doesn't make sense to me. Why is this.

 

[bstomper]

I think I figured out where the extra speed came from with my 7 mm. The 2891 FPS was at my initial coal of 3.350". When I did my seating depth testing to tighten up the groups, the best group was at 3.310" so that what I stuck with. I never did check the speed of the new length of 3.310". I'm assuming that with the smaller case volume because of the shorter seated bullet the pressure increases slightly resulting in slightly faster speeds.
still not sure on the creedmoor though

 

[memtb]

I had a similar occurrence using RL 17 in my .375 AI. I do all of my load work on hot days .....preferably 90+ F. The thought being....if I load to max on a cool day, then have a hot weather hunt, I won't have a high pressure issue.

Several years ago (during the winter) there was quite a discussion about the temperature sensitivity of RL 17. So, I performed a test. The daytime highs that day were 4 F. I left the rifle and ammunition out for a couple of hours, to settle in at the ambient temperature. When my shooting was complete, my average velocity was 8 fps faster, with comparable shot velocity spread.... than when the load was developed on a hot day! Go figure! memtb

 

[bstomper]

Yeah it doesn't make sense to me. I could understand the pressure and speed increasing shooting in warmer weather than the load was developed in but not in colder weather.

 

[ShtrRdy]

One possibility I've thought about, and I dont know if it's possible, is that the rifle barrel bore diameter is slightly smaller at colder temps. This would cause higher pressures to push the bullet out of the barrel and therefore higher velocity.

 

[memtb]

That actually crossed my mind as well. Although different metal types, the bullet, also at ambient temperature, should be smaller in diameter! Which would exhibit the greater change.

This could be similar to heating a bearing race, then placing it on the shaft......once cooled, a very tight bearing (zero tolerance) to shaft fit! memtb

 

[bstomper]

Yes the thermal expansion and contraction has crossed my mind also but having said that hunting and shooting has been going on for a long time in a lot colder temperatures than -26 Celsius. That's just a normal winter day in this country.
Only safe option I have is to reduce the powder load and work up again.

 

[willyqbc]

Stomper, how many rounds down the pipe on these barrels? If they are new, the temps may be a red herring and your just seeing the barrel speed up as it "breaks in".
Also, was the first round done with virgin brass, second time out with 1x fired? That can also account for a bump in fps

 

[bstomper]

About 2000 rounds on the 7rm with several times fired brass. I figured out the reason for the extra speed for the 7rm. I shortened the coal from 3.350" to 3.310" during my seating depth testing because it gave me the best groups and I never tested for speed after. So I assume The smaller case volume would give me more pressure and more speed.
But for the creedmoor I am still breaking in the barrel before I do load development. As I mentioned earlier I had loaded 30 rounds at 38gns with new brass and shot them then loaded from 38 up to 43 gns in .5 grain increments to find my max load for load development. Never seen any pressure signs at 43 gns. I thought 41 gns was safe to shoot so I Loaded up the last 50 brand new cases at 41 and went out to shoot. I cleaned my barrel of all copper before going out. Fired off 12 rounds just fine and thought I felt a sticky bolt on shot 13, 14, and 15. I stopped there. The sticky bolt seemed to get progressively worse from shot 12-15. I checked the barrel and it looks fine. I have no idea what happened. Why all of a sudden I am getting pressure signs.

 

[thatguyshm]

Could it be not the barrel temp but the temp of the actual bullet? Could it be a bit more difficult to conform to the lands and grooves of the barrel when the bullet itself is cold and less maliable?

 

[DUSTY NOGGIN]

did you get velocities of the CM at those pressures ???

 

[bstomper]

did you get velocities of the CM at those pressures ???

Unfortunately no.

 

[AudentesFortunaJuvat]

Figured I'd tag on a year later-

I've been shooting my 6.5 CM starting the shooting season back in march of 2020. I was shooting anywhere from 40F(march) to 90F(august) and found my velocity with RL16 was going between about 2710-2740 so about 30FPS through roughly a 50 degree temperature fluctuation, all at the same shooting location.

This past weekend it was about 30F, and my velocity had increased to about 2760, peaking at 2767. I've always understood that pressure would gain with temperature gain so I'm quite thrown by getting pressure signs with my go to load that was solid at 40F-90F, now giving lightly sticky bolt and some fresh ejector marks at 30F.

Specs:
Christensen Action
Proof Carbon 1:8 22" Barrel
6.5 Creedmoor
16x Fired Petersen Brass, sized, charged and seated as usual with 42.2g RL16.
Berger 140VLD Hunting
CCI#450 SRP

No visual signs of concern with the brass, same bullet lot I've been using, same RL16 8lb jug I've used all year, same reloading process, trim length, neck tension, etc. Overall, I haven't changed a thing that I can tell, apart from the brass being on its 16th firing. Loads were stored in 60F garage, transported to shooting location in 70F truck, outside in 30F air for about an hour before first firing.

Thoughts?

 

[BUSTINDOGS]

I have found the same thing. If the whole system is cold ( rifle and ammo) I get higher velocities. I would have to check my notes but with my 7mm SAUM using RL23 and FED215 from 68 degrees to I think it was in the teens this is where I would have to check my notes I had an increase in velocity. If the ammo is cold but the rifle is at 68 coming out of the house I would see a decrease in velocity. My control was the rifle and ammo both at 68. I can shoot in my back yard and living in MN I can test in some pretty cold temps -20 to -30 ambient and up into the high 90s on the other side of things.
I have seen the same thing with other rifles and powders, too.