338 LM Knowledgeable people please tell me!!!

[UA SOF SNIPE]

What is the question basically:
338 300grin SMK.
The powder weight has been selected and the jumps in speed are minimal; the speed does not change with a difference in the powder weight of 0.3 grains.
The ammunition was tested for thermal stability from +25°C to -15°C; initial velocities were measured at 6 temperatures in this range.
But today, at a temperature of -1, for some reason the vertical corrections began to overestimate; when checking the speeds with a chronograph, the speed crept up by 10 m/s.
And could it be that the cartridges, when fired, may overestimate the value of the initial speed. The cartridges are on the 9th cycle.

 

[Huskydriver]

Wet ammo... Different lot of powder.... Barrel is wearing out... Etc etc... you know.... The usual things you know about if you actually shoot....

 

[UA SOF SNIPE]

Wet ammo... Different lot of powder.... Barrel is wearing out... Etc etc... you know.... The usual things you know about if you actually shoot....

Does the initial velocity change depending on the number of bullets fired from the barrel?
Have you encountered the fact that the number of shots fired from a cartridge case can overestimate the initial velocity?
The initial speed didn’t jump, it just got too high. And for what reason I can’t yet give an answer.
A batch of gunpowder as a working version of events

 

[sinister]

The more rounds fired the further the throat will wear.

The more wear, the more gas blow-by and decreasing velocity (over time).

Your highest velocity will be when the barrel is new. Depending on your precision standard and steel quality you may get between 3,000 and 5,000 rounds for best accuracy. The Finnish Army will change barrels on their TRG-42 once the rifle will no longer hold one minute of angle (roughly 1 Meter at 1 kilometer).

Some powders work inversely with ambient temperature -- the lower the temperature the higher the velocity.

 

[Carolina Hill Billy]

Barrel can also get carbon in throat, sometimes to the point of rings forming in the chamber that are hard to remove.

Honestly, it could be a hundred different things…..

 

[UA SOF SNIPE]

The more rounds fired the further the throat will wear.

The more wear, the more gas blow-by and decreasing velocity (over time).

Your highest velocity will be when the barrel is new. Depending on your precision standard and steel quality you may get between 3,000 and 5,000 rounds for best accuracy. The Finnish Army will change barrels on their TRG-42 once the rifle will no longer hold one minute of angle (roughly 1 Meter at 1 kilometer).

Some powders work inversely with ambient temperature -- the lower the temperature the higher the velocity.

Yes, I understand that the initial velocity of the bullet sag with barrel wear.
The problem is that the proportional speed has increased inversely from the calculated one.
My question is this:
-Couldn’t the number of shot cycles on the cartridges be the reason for everything?

 

[UA SOF SNIPE]

Barrel can also get carbon in throat, sometimes to the point of rings forming in the chamber that are hard to remove.

Honestly, it could be a hundred different things…..

I cleaned the carbon rings and polished the barrel with JP paste and KROIL oil, the speed dropped significantly.

 

[Gunfighter14e2]

The Finnish Army will change barrels on their TRG-42 once the rifle will no longer hold one minute of angle (roughly 1 Meter at 1 kilometer)

1 moa at 1 k meters is 1 meter? I would have thought it was just a shade under 12" ?

 

[UA SOF SNIPE]

1 moa at 1 k meters is 1 meter? I would have thought it was just a shade under 12" ?

1 meter per 1 km is 1 mil.
I am far from the fact that Finnish shooters will shoot with such a spread.

 

[Gunfighter14e2]

1 meter per 1 km is 1 mil.

Was not responding to that part at all.
I don't think 1 moa at 1093 yards (1K meter)is 36"

 

[sinister]

You are correct. Not enough coffee this morning.

With higher velocity your bullets will fly slightly faster with a flatter trajectory. Hitting lower, you might have to add a click or two to compensate.

 

[UA SOF SNIPE]

You are correct. Not enough coffee this morning.

With higher velocity your bullets will fly slightly faster with a flatter trajectory. Hitting lower, you might have to add a click or two to compensate.

Well, this is a simple solution to the problem. The question here is different:
-What caused this problem

 

[sinister]

What powder and charge are you using?

 

[UA SOF SNIPE]

What powder and charge are you using?

I don't know exactly what kind of powder it is.
Essentially, I take military ammunition and simply rebuild it according to a recipe that gives me maximum accuracy and minimum velocity dispersion.
The weight of the powder charge is 89.3 grains for a 300SMK bullet.
At the exit I have 813 m/s at -1°C, a spread of initial speeds of 4 m/s with 10 shots and a group at 100 meters of 0.6-0.7 MOA

 

[Evintos]

Yes, I understand that the initial velocity of the bullet sag with barrel wear.
The problem is that the proportional speed has increased inversely from the calculated one.
My question is this:
-Couldn’t the number of shot cycles on the cartridges be the reason for everything?

If it's a brand new barrel, often times your velocity increases within the first 100-200rds before stabilizing. After which as your barrel starts to reach the end of its service life, the barrel velocity decreases.

 

[UA SOF SNIPE]

If it's a brand new barrel, often times your velocity increases within the first 100-200rds before stabilizing. After which as your barrel starts to reach the end of its service life, the barrel velocity decreases.

No, the barrel is not new, it has about 1700 rounds on it.

 

[6.5SH]

Perhaps checking the H2O capacity of the brass after resizing at different number of firings may show that it is no longer as malleable at the ninth reload.

 

[UA SOF SNIPE]

Perhaps checking the H2O capacity of the brass after resizing at different number of firings may show that it is no longer as malleable at the ninth reload.

I do anealing every recharge cycle

 

[6.5SH]

I do anealing every recharge cycle

Only the neck shoulder junction though. If the 9x fired cases are now holding more H2O Vs say 6x the velocity should be lower for a given charge.

 

[UA SOF SNIPE]

that’s the question: the speed has increased

 

[6.5SH]

that’s the question: the speed has increased

Misunderstood what you asked. The usual culprits will be temperature sensitivity as @sinister stated, some powders will increase velocity as temp decreases or lower humidity resulting in a faster burn rate.

 

[UA SOF SNIPE]

Misunderstood what you asked. The usual culprits will be temperature sensitivity as @sinister stated, some powders will increase velocity as temp decreases or lower humidity resulting in a faster burn rate.

In practice, I have not seen that gunpowder could increase the initial velocity when the temperature drops. Yes, if the ammunition has not cooled down to ambient temperature, this is possible

 

[Evintos]

In practice, I have not seen that gunpowder could increase the initial velocity when the temperature drops. Yes, if the ammunition has not cooled down to ambient temperature, this is possible

AR-Comp (made by Bofors) actually has the characteristic of increased velocity at colder temp.

Only thing I can think of is different powder on your latest batch or more than 89.3gr from scale calibration drift.

 

[UA SOF SNIPE]

AR-Comp (made by Bofors) actually has the characteristic of increased velocity at colder temp.

Only thing I can think of is different powder on your latest batch or more than 89.3gr from scale calibration drift.

I think the batch of powder has changed

 

[Thump_rrr]

0.3 grains is too small an increment for
a bullet that uses 90 grains of powder or there abouts.

I shoot 285 ELD-M’s in mine and I use 1.5gr increments.
The gong on the bottom was shot using 92.3gr of H-1000. That’s 3 rounds at 425y.

 

[UA SOF SNIPE]

0.3 grains is too small an increment for
a bullet that uses 90 grains of powder or there abouts.

I shoot 285 ELD-M’s in mine and I use 1.5gr increments.
The gong on the bottom was shot using 92.3gr of H-1000. That’s 3 rounds at 425y.

My maximum spread in initial bullet speed does not exceed 5 m/s.
this is enough to hit a target of 1 MOA at 1000m.
Well, the gunpowder that I use will not have the same value as yours.

 

[Thump_rrr]

My maximum spread in initial bullet speed does not exceed 5 m/s.
this is enough to hit a target of 1 MOA at 1000m.
Well, the gunpowder that I use will not have the same value as yours.

5 m/s is 16.4 fps. My spread is below that also.
If you notice the 87gr, 88.5gr, and 90gr groups have very little POI shift.

Have the conditions in your reloading room changed as far as humidity goes?

Im assuming that your scale calibration is confirmed with check weights.

 

[UA SOF SNIPE]

5 m/s is 16.4 fps. My spread is below that also.
If you notice the 87gr, 88.5gr, and 90gr groups have very little POI shift.

Have the conditions in your reloading room changed as far as humidity goes?

Im assuming that your scale calibration is confirmed with check weights.

I always calibrate the scales in accordance with the instructions. Yes, the humidity can change; I cannot always reload ammunition in one place.