Neat little look at cold bore #'s

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AKD

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I was out shooting today with a temp of about -17c. windchill of -23c.

I just wanted to share some chrono #s with you guys to show the difference a cold bore shot makes. After a 3 round group the rifle would sit for a few min aloud to cool back down.
Take note of the spread difference between the first and than the 2&3 shots.


43.5gn
-2666
-2634
-2625

43.8
-2655
-2671
-2663

44
-2734
-2658
-2645

44.3
-2705
-2687
-2685

44.5
-2708
-2739
-2730


Interesting. Kinda makes you re-think 3rd groups when load testing.

*this was varget powder pushing 175smk's*




IMG_0353.jpg

I like pictures. Here is one of the rifles we were shooting this day. .338 timberwolf owned by shprac and my .308 5R with a sightron 10x42 MMD
 
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Hmmm, is it normal for the 1st shot to be the fastest? Most of my testing from my 338 Lapua has led me to believe that the first shot is usually moving the slowest, yet seems to hit high! One of lifes little mysteries.
 
Thats an interesting observation. I wonder if there is any consistency if the same testing was also done at 0c, and 10c? That is enough difference in velocity to affect a POI of any 200+ meters.
 
datatools said:
Hmmm, is it normal for the 1st shot to be the fastest? Most of my testing from my 338 Lapua has led me to believe that the first shot is usually moving the slowest, yet seems to hit high! One of lifes little mysteries.
Click to expand...

barrel harmonics are the biggest effect on POI, not velocity, it may be moving slower, but if the barrel is flexing up when it leaves, it will hit higher. the faster bullets leave while it is still traveling up, so they have less elevation angle.

this will be different for every barrel in every gun
 
As the barrel heats, the bore constricts, leading to the logical acceptance that following shots will be slower. But, apparently that is not consistently the case.

Which charge weight was most accurate?
 
I don't think there are enough data there to generalize about anything. Would also suggest that three shot samples are not particualrly useful, apart from giving an approximate idea about what might be going on.
 
I agree with Tiriaq. If you eliminate load 44, which could be dismissed as a freak, I dont see much of a correlation. Bear in mind the accuracy of the chrony is probably +/- 1 %.
 
44.5 was the best but I discarded the groups. I was trying a new set of scope rings that were a bit to high and it really felt off. I was fighting to get comfortable behind the gun all day. Not my best shooting. I opted to scrap the targets because I did not think they were an accurate use for data.


*this is my main rifle, I shoot very often and the loads here were a random test. My rifle is set up @ 44.8 gn *
 
cosmic said:
I agree with Tiriaq. If you eliminate load 44, which could be dismissed as a freak, I dont see much of a correlation. Bear in mind the accuracy of the chrony is probably +/- 1 %.
Click to expand...

What this is showing is why you might not want to stick with 3rd groups for load testing.
 
tiriaq said:
I don't think there are enough data there to generalize about anything. Would also suggest that three shot samples are not particualrly useful, apart from giving an approximate idea about what might be going on.
Click to expand...

Agree, but one question is the rifle clean before the first shot or dirty?
 
Well - The whole effort can be dismissed as somewhat inconclusive, if you admit that your hold/conditions were sub-optimal. What groups did you get, and at what distance? There are folks that know more about this than I do - velocity spread is not necessarily the ultimate determinant of accuracy.
 
no not at all actually, All data is good data. The main goal with the loads was to compare with a friend from the states who cut his 5R barrel down to 21". For me it worked well to get the max spread from the first round in that cold of temp. later in the summer when its warmer, ill load up the same ammo and see if its any smaller spread on the cold bore shot or not.

Consistent velocity is very important when shooting long range. It is a huge factor in vertical spread.

Look at this ladder test, prime example.

175smk%20LT.jpg


This is at a 600y distance with a difference in velocity of about 200 fps from top to bottom.

Good consistant match made ammo will give you the less spread possible. were talking less than 10fps roughly.
 
rimfiremac said:
As the barrel heats, the bore constricts, leading to the logical acceptance that following shots will be slower. But, apparently that is not consistently the case.

QUOTE]
Click to expand...


Actually, it works the other way around. Both bore and outside diameter increase as the barrel warms. Think of an old time blacksmith heating a iron rim to expand it so it can be fitted over a wagon wheel, then cooling it to shrink it on. Your barrel is a just a bunch of little wagon wheel rims that haven't been sliced up yet.
Click to expand...
 
rimfiremac said:
As the barrel heats, the bore constricts, leading to the logical acceptance that following shots will be slower. But, apparently that is not consistently the case.

Which charge weight was most accurate?
Click to expand...

Have to call you on that one.

Metal expands when heated. It expands out, so both the OD and ID will increase. This is the principle used to seat bearing races into large metal objects. You heat the object so that it expands and you cool the bearing so that it contracts. This will give you the clearance needed to seat the bearing race. As the two parts return to room temperature, the clearance is taken up and you have perfectly mated parts with zero tolerance.

In firearm terms, A constricted COLD bore will increase pressure, which will result in faster muzzle velocities. As the barrel heats up and expands, less pressure is created because the bore is not as tight, and the muzzle velocity will reduce.

The velocities posted are common to what I see shooting in these cold temperatures. Cold bore is fast, warm barrel slower.
 
ryandpahl said:
Have to call you on that one.

Metal expands when heated. It expands out, so both the OD and ID will increase. This is the principle used to seat bearing races into large metal objects. You heat the object so that it expands and you cool the bearing so that it contracts. This will give you the clearance needed to seat the bearing race. As the two parts return to room temperature, the clearance is taken up and you have perfectly mated parts with zero tolerance.

In firearm terms, A constricted COLD bore will increase pressure, which will result in faster muzzle velocities. As the barrel heats up and expands, less pressure is created because the bore is not as tight, and the muzzle velocity will reduce.

The velocities posted are common to what I see shooting in these cold temperatures. Cold bore is fast, warm barrel slower.
Click to expand...

This make sense, but doesnt always seem to be the case. With much of my velocity testing it seems that my first shot is always the slowest. Each following shot gains velocity. Why would this happen? I've also noticed that a clean bore is slower than a dirty bore. With my Lapua, the first shot is always the slowest, but consistently hits high and slightly left.
 
datatools said:
This make sense, but doesnt always seem to be the case. With much of my velocity testing it seems that my first shot is always the slowest. Each following shot gains velocity. Why would this happen? I've also noticed that a clean bore is slower than a dirty bore. With my Lapua, the first shot is always the slowest, but consistently hits high and slightly left.
Click to expand...

I haven't enough firearm experience to say why your results are different. May have something to do with magnum? Larger caliber?

Even my .22LR shows the same results, first shot is fast and high, as the barrel warms up the POI moves down a bit. Mabey its because My barrels are crappier than yours...
 
coefficient of linear expansion is .000012 so with .9 " barrel expansion is roughly .0007" this is figured with a 60c rate of change which is rather high. -20 to 40c. this figure is even less since there is a .30 cal hole roughly. this brings us down to .0004''. if we keep our extreme spread of temp to 30c it is now .0002. this is measuring bored diameter not barrel lengths. 30'' with 60c spread .0216. expansion is not a factor IMO
 
Did you keep the battery warm?

Sorry, gave up chronie testing in the cold cause the chronie and power source 'froze' up.

Also, remember that a chronie (typical ones we can afford to buy) has a 1/2% error in the clock readings. Add to that any issue with cold affecting the power source and electronics and I would think twice about holding the numbers as gospel.

I look at the holes in paper at 200 and 300yds. If the cold bore is ALWAYS out, then you need to worry about that cold bore shot.

If its in the group and the group is small with little vertical, I use the chronie to confirm that I am not being a hero and run a risk of blowing up my rifle. I don't bother crunching numbers anymore.

Jerry
 
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