12fv build w/ pics

[DesertDug]

So are you not concerned with a "jump to lands" measurement, because it's the exit time that dictates?

[CFJunkie]

I have already determined that some bullets like to be seated out further than others.
I have over 6,000 rounds through 6.5mm Creedmoors with about 10 different bullets, all measured and documented so I have a pretty good database of what works for my rifles and powder-bullet combinations.

I do begin choosing the charge for each powder-bullet combination based upon the bullet's 'preferred' seating depth. Based upon the trim length variations, I make the adjustment around that depth by making slight adjustments in seating depth.
The seating depth adjustment rarely is more than 0.008 to 0.010 to tune to the precise exit time once I have chosen the powder charge to get within the exit time range for the preferred seating depth. Most times it is only a few thousandths.

When I started testing the theory several years ago, I would set up a set of loads with their exit times 0.005 msec. apart so I could document the difference in group size average.

As you already know, a 0.1 grain change in powder charge causes about 0.005 to 0.006 msec. in exit time depending upon the powder, so it was a convenient method of creating loads.
Turns out that the increments were pretty small and my shooter variations often masked the differences at those tiny increments.

I quickly concluded that the differences in exit times really showed obvious results when they were 0.020 msec. or more off the exit time goal and the biggest differences were apparent when the shock wave got closer to the muzzle.

[CFJunkie]

DesertDug asked about whether I loaded for jump for this rifle in post #47. I had a good idea of where the various bullets would perform based upon my results with my other two Savage 6.5mm Creedmoor rifles.
When I bought the 12 FV 6.5mm Creedmoor ‘bargain rifle’ for $320, I didn’t even bother shooting the Sierra 140 gr SMK #1740 bullets because they shot so poorly in my other two rifles and I didn’t bother with Berger’s 140 gr Long Distance #26409 and Hybrid #26414 because they didn’t perform much differently than the 140 gr Berger #26401 Match bullets. I still had 200 of them and didn’t want to spend another $60 a box to buy more of the LD and Hybrids hoping for a miracle.
For the 12 FV 6.5mm Creedmoor results since inception, I did an analysis of the effects of seating depths for all the groups shot with the 35 in.-lbs. torque settings so the results would not be skewed by the poor performance at 45 in.-lbs.
The results are shown in O.A.L. measurements because the ogive to tip measurement varies for each bullet type.
The 130 gr Hornady ELD-M #26177 data was limited by having only one half-box of bullets left when I started data gathering with the 12 FV 6.5mm Creedmoor rifle.
The column headings - except for the last two columns - are in increments of thousandths away from SAAMI recommended O.A.L. of 1.910.
Normally I like to keep the jump around 0.015 to 0.020 to minimize the chance of getting the bullet touching the rifling. (The poly tips on the TMKs and ELD-M bullets add to the bullet overall length but the ogive determines the jump.)
“Avg Diff” is calculated difference in group average at each depth range by the using the average minus the best load average at the seating depth (underlined).
Group Averages by Bullet by O.A.L. Range - 35 in. Lbs. Torque
-----------------------------------5-10 11-20 21-30 31-40 41-50 51-60 Avg Diff. Jump for best load avg.
Hornady ELD-M # 26177 130 - 0.374 - 0.024
Sierra TMK #7430 --------130 ----------------------------- 0.393 0.341 0.024 0.005
Berger Match # 26401--- 140 - 0.378 0.386 ----------------------------0.026 0.056
Hornady ELD-M # 26331 140 - 0.404 0.415 -------------0.401 --------0.008 0.024
Sierra SMK #1742 -------142 - 0.370 0.379 -------------0.357 --------0.013 0.014
Hornady ELD-M # 26333 147 - 0.387 0.337 --------------------------- 0.042 0.024

As you can see, the 130 TMKs like to be seated out so the jump for the best load average was shorter than I normally try to achieve.
The 140 gr Berger Match bullets are baffling. They seem to shoot better with a longer jump than expected.
(I even remeasured those bullets to see if I made an error in measuring the bullet depth into the chamber on this rifle. I didn’t.
Then I checked the best load for that bullet that I shot in my 12 LRP and found that it also had a long jump, so I have concluded that it is the bullet not my measurements.)

[DesertDug]

I was able to measure OG of chamber distance and the load I had shot which was OAL 2.821" measures a jump to lands of 0.048".

I was thinking this is a large jump. If I want to load some up closer to the lands, say 0.038", 0.028" and 0.018' to lands and I have all my cases already trimmed to 1.190". Can I adjust the load to remain in correct exit time by adjusting the powder weight? Do I add .2 grains of powder for additional 0.010" in OAL?

[CFJunkie]

That's the way you do it. However, a 0.1 change in powder charge for H4831 SC will usually change velocity by about 6+ fps equivalent to a seating depth increase of 0.016. It is going to be hard to adjust by 0.010 jump by changing powder charges by 0.1 if the trim length is the same.

A 0.1 grain increase in powder charge for H4350 will change velocity by about 6+ fps. and will require a seating depth increase of 0.017.

If your still using H4831 SC powder, I suggest you try 2.837 (a jump of 0.032) with an additional 0.1 grain load and 2.855 (a jump of 0.014) with adding another 0.2 grain to your original load. The next increment would put you into the rifling and I wouldn't attempt that. You could always simply adjust the seating depth leaving the charge the same and sacrifice a few fps if you want to see if you get any added benefit of using a few thousandths shorter jump.

If you are now using H4350, then you need to adjust to make 0.017 increases in O.A.L. for each 0.1 grain increase in charge.

[DesertDug]

great thanks, will adjust setting depth and load charge to see how it effect groups. Still using H4831sc for the time being.