An A-Ha Moment?

[bkondeff]

I have been doing load development with a Sako 75 in 7mm Mag and some 168 Bergers. I found a load that seemed to work, ran some new ones of the same and went out to shoot. What seemed like .7moa at 200 yard loads started shooting 1.2-1.4 moa. Very frustrating. I realize that these shots where now hitting the 30-40 shots since cleaning range. BTW, the velocity at this load was right about 3000 fps.

I did a thorough barrel cleaning, grabbed the same loads and went back out. My groups were back at .7 moa at 200 and seemed to be closer to .5 or under at 450-550, but since I was shooting at wet dirt mounds I can't confirm anything more than approx 2-3" from point of sight(1" rock). The odd thing is that the last groups seemed to be at an ave of 2950, again with a fairly low SD.

That was last night, now other than the low sunlight conditions possibly affecting the chrono, I wonder why the velocity would drop. I was reading a post about using the Tubbs bullets to lap a barrel and how the velocity dropped about 200fps. The answers seem to lead to a much cleaner barrel creating less friction.

So could simply cleaning my barrel after 40 or so rounds of 72gr of Retumbo, create a drop in velocity?

This would help confirm how much of a difference my gun seems to have with a clean barrel. Some of my best groups seem to come from a Cold and newly Cleaned barrel.

 

[Varmint Hunter]

I have been doing load development with a Sako 75 in 7mm Mag and some 168 Bergers. I found a load that seemed to work, ran some new ones of the same and went out to shoot. What seemed like .7moa at 200 yard loads started shooting 1.2-1.4 moa. Very frustrating. I realize that these shots where now hitting the 30-40 shots since cleaning range. BTW, the velocity at this load was right about 3000 fps.

I did a thorough barrel cleaning, grabbed the same loads and went back out. My groups were back at .7 moa at 200 and seemed to be closer to .5 or under at 450-550, but since I was shooting at wet dirt mounds I can't confirm anything more than approx 2-3" from point of sight(1" rock). The odd thing is that the last groups seemed to be at an ave of 2950, again with a fairly low SD.

That was last night, now other than the low sunlight conditions possibly affecting the chrono, I wonder why the velocity would drop. I was reading a post about using the Tubbs bullets to lap a barrel and how the velocity dropped about 200fps. The answers seem to lead to a much cleaner barrel creating less friction.

So could simply cleaning my barrel after 40 or so rounds of 72gr of Retumbo, create a drop in velocity?

This would help confirm how much of a difference my gun seems to have with a clean barrel. Some of my best groups seem to come from a Cold and newly Cleaned barrel.

In my experience, a clean barrel will always shoot lower velocities - at least for 2-3 rounds. After that the velocity will remain fairly stable. I have also noticed that some rifles will give a moderately higher/lower velocity reading when the same load is shot on different days. Temperature could have an affect on velocity but I also believe that something else may be at play that results in the shifted readings.

 

[bkondeff]

Thanks for the response.