Unexplained shift in velocity

[seymour fish]

Keith, I'm gonna check things out with my borescope. I have had a carbon ring before and many barrels ago! I try to clean after matches or at least between 60-100 rds. When I did have carbon, velocity stayed high and never dropped until an aggressive cleaning was performed.

I have not checked bullet diameter but I did weight sort before loading. I also came to the conclusion that base to ogive was also consistent because I did not have much variance in base to ogive after seating. I had no more than .0005 between all rds. Used.

I would also think that a larger diameter bullet would show a higher pressure reading on the hydro press but the press showed all rounds to be within 10-15 psi. The rounds that had significantly higher or lower psi showed NO correlation on target at 1000 yards.

From most that have chimed in and those brains I Have picked elsewhere... it seems as if something large would be needed to get that much of a shift in velocity. Powder charge would have to be almost 1 full grain higher! Some disturbance in the barrel,etc.

It seems to keep coming back to a 1 grain powder variance if all other sources of variance check out

 

[memilanuk]

If it wasn't for the difference in POI, my first inclination would be that something in the chrono setup changed. Are you certain that nothing else (lighting, atmo, etc.) changed that could explain the change in come-ups?

 

[lawrence97]

If it wasn't for the difference in POI, my first inclination would be that something in the chrono setup changed. Are you certain that nothing else (lighting, atmo, etc.) changed that could explain the change in come-ups?

Monte, all rounds were fired under a 25-30min window. Lighting does not affect the labradar. The vertical shift of poi coincides w velocity.

Powder variance is out of the equation! The Prometheus is as good as it gets and I measured all drops from Prometheus on my digital scale.

At this point I cannot think of what could have caused this. I am going to leave the barrel dirty as it sits and load 15 rds w S&B primer and 15 with br2's.

 

[lawrence97]

Here is what my scores/velocity looked like using same load and seating depth:
S&B primers (sighters):
2831, 2822, 2820 and 2827. My last outing my elevation knob was set to 24.75 but that was with velocities running around 2750. I adjusted down to 22.5.

Br2 primers:
2761, 2760, 2757, 2751 and 2745. These rounds impacted at the bottom of 7 and 8 ring so I adjusted my elevation back up to 24.25.

2766(10),2760(10),2765(10),2755(10),2756(10),2780(9),2762(10),2757(X),2811(high 7),2790(high 9),2766(X),2761(10),2802(high 8),2813(high 8),2769(9 at 9o'clock),2792(high 8),2771(9 at 9o'clock),2831(high 7),2843(high 7),2823(high 8),2824(high 7).

 

[lawrence97]

Well, I borescoped the barrel and I couldn't find any anomalies. Everything looks good for a barrel that hasn't been cleaned in 50-60 rds. No carbon ring, just light carbon in the throat that will come off with wet patches.

I will load up the same and test it this weekend.

 

[Milo 2.0]

Try opening a new jug or pound of H4831sc, I've found the more you open and pour out of a keg, the hotter it gets. Replicate your load with the new powder and test.

 

[Zero333]

I always put 4 layers of saran wrap under the lid of the bottle to keep it as air tight as possible so Humidity and other factors don't affect the powder (as much).

Some of the kernels could of dried up or absorbed humidity changing their burn rate.