load development input please

[skep_tic1]

Well I did some studying on my main load today and I came up with a question. I've always gotten great accuracy from 175 smk's over 43.5 rl15 using federal brass and winny primers, so it is what I use. Today was the first time I've had the opportunity to check the muzzle velocity of this load through a crony.
Here is what I found.............

1st grp was a 9 shot string (NEW FED BRASS); Avg vel=2547fps, ES=25, SD=8
In the 1st grp there were 2 duplicates and 1 three shot string of matching fps

2nd grp was a 7 shot string (NEW FED BRASS); Avg vel=2542fps, ES=30, SD=9
In the 2nd grp, the first 4 shots all matched fps

At this point I'm becoming pretty impressed with the consistency of my loads. I shot one more string and here is where my questions comes up.

3rd grp was a 5 shot string (FED BRASS FIRED 4X); Avg vel=2616fps, ES=10, SD=5
This string also had a duplicate

How does a load from the same jug of powder and using the same lot of SMK's have a 75fps jump in velocity by going from new brass to old brass?

All three of these groups held less than 1 inch at 200 yards. The group with the HIGHEST Avg velocity had about a 1 inch LOWER impact than the slower groups? I was somewhat stumped by this.

Has anyone else had similar findings?

 

[skep_tic1]

Re: load development input please

All brass was full length sized before loading. OAL was between 2.80 & 2.81 inches.

 

[MALLARD]

Re: load development input please

perhaps your ammo heated up and caused the increase in velocity

 

[skep_tic1]

Re: load development input please

Increasing ammo temps could cause such a thing but I was loading one at a time in the shade with all the ammo sitting right there beside me.

 

[memo43]

Re: load development input please

the differance was in the brass. Do you anneal it. Remember brass grows, or stretches. Might have caused thinner case walls, maybe enough to generate more heat to burn the powder quicker creating higher velocities.
Maybe I'm wrong but that is the only thing I can come up with.

 

[gcoulsey]

Re: load development input please

+1 on what memo said.. X4 fired brass could easily have slightly different behaviour than fresh brass, neck tensions,web of the brass not getting sized in the die blah blah blah..

Sounds like you have a nice load worked up,
But this is one of the reasons to keep your brass in separate lots by how many times it's been fired. Annealing helps keep things a bit more even,
I also get the slight changes you have noticed, but not as much after x4 firings when I start to anneal.

If I move onto a different lot of brass, I just re-check my zero, sometimes have to tweak it .5 MOA or so

 

[skep_tic1]

Re: load development input please

Thanks for the responses. I'm leaning toward the brass as well. The old brass must have had thinner walls from being stretched resized and trimmed. The thick walled new brass therefore had less case capacity increasing case pressure. But, if this was the culprit I would certainly have expected the faster load to have been from the NEW, THICKER walled brass. Plus regardless of anything else, the slower rounds shouldn't have impacted higher at 200 yards. Right? Is this just an anomaly that is defying my common sense?

The only other thing, is that all the older brass had been annealed. The annealing, could've created a softer neck allowing less hold on the bullet, but certainly not any more consistent judging by the crony numbers.

BUT, this still doesn't tell me how higher velocity equaled lower impact. I mean, the angle I was shooting didn't even change. I was aiming at the very same dot on the very same Target........

 

[taseal]

Re: load development input please

Check the cc of virgin brass vs 4x fired brass, I bet there is a change.

My lapua gained 1.5cc from firing it once. They were just neck sized, but I'm sure there are other things going on like the thickness of brass etc.

Check your cc numbers

Eta: I Just read your last post. Did you use the same chrony? (you probably did) you think there was a little grain added? Perhaps the scale wasn't properly showing the right amount and you had few extra grains of powder there? Larger brass caused a better burn ratio?

 

[gcoulsey]

Re: load development input please

Point of impact can often lower with faster rounds,
It's all about barrel harmonics/timing.

Your barrel flexes/whips/vibrates or whatever you want to call it, as the bullet travels along it, a faster bullet will exit the muzzle at a different point during this 'barrel flex/whip/vibration' giving it a slightly different POI than a slower bullet with more barrel time.

Your barrel could be near the top of an upward stroke, so slower bullets exit higher on the stroke.
ALSO faster bullets have less time to be affected by any muzzle lift by recoil etc..