Jump & Velocity - newbie question

[gdbleb]

I've done 2 ladder tests with this load - 6.5 x 47L, Lapua brass, GM205M, H4350, Berger 140 HT

Only difference is test #1 was at 70 degrees and bullet jump was ~.080 while test #2 was at 45 degrees and bullet jump was ~.010.
In most all loads the test #2 load was 50 fps faster than the test #1 load on the Labradar.

Is this within the variability range of the Labradar?
or
Could the much larger jump permit gas "blow by" before bullet engagement in the lands and thus lower pressure & lower MV?
Test #2 cases would have had more capacity so I am somewhat surprised. A common occurance.

Many thanks

 

[LCazador]

I'd bet your #2 load is into the rifling and not .010". Yes it's a common occurrence to think you're off when you're actually in the rifling. I've shot in many matches where we had to help someone clear his barrel of a bullet. And the conversation is always the same " gee I thought I was off the lands".

 

[FeMan]

^^^ This is a strong possibility.

Is it the same lot of powder? If it is a new barrel, the issue could also be your barrel speeding up.

 

[JEFFPPC]

I've done 2 ladder tests with this load - 6.5 x 47L, Lapua brass, GM205M, H4350, Berger 140 HT

Only difference is test #1 was at 70 degrees and bullet jump was ~.080 while test #2 was at 45 degrees and bullet jump was ~.010.
In most all loads the test #2 load was 50 fps faster than the test #1 load on the Labradar.

Is this within the variability range of the Labradar?
or
Could the much larger jump permit gas "blow by" before bullet engagement in the lands and thus lower pressure & lower MV?
Test #2 cases would have had more capacity so I am somewhat surprised. A common occurance.

Many thanks

Forget the velocity. Find the load that shoots back to back very small groups. Let the velocity be what it will be. Don't get side tracked. German brought up a valid point. If this all happened with in the first 100 normal barrel speed up is probably involved. Forget velocity at this point, it doesn't win per se.

 

[cem]

r
Could the much larger jump permit gas "blow by" before bullet engagement in the lands and thus lower pressure & lower MV?

Maybe so. What is certainly true is that over a wide range of rifle cartridges, (NOT .40 S&W) a running start reduces pressures and moving closer to the lands increases pressure. This shows up in rifle cartridges from Weatherby style free bores to short range bench rest jammed into the lands. There has been ample discussion that the free bore effect giving a running start is much greater than the small changes in case volume associated with changes in seating depth. Again I emphasize in most rifle cartridges. For a good advanced layman's discussion of pressure see Brownell's Pressure Factors available as serialized in the Wolfe publications and printed on paper and CD.

"Test #2 cases would have had more capacity " Agreed but not repeat not the dominant effect of the changed bullet jump - over the range of most rifle cartridges. An internet search on this and other boards will find the subject repeated regularly and at great length in as much detail as desired or more.

 

[Straightshooter1]

I've done 2 ladder tests with this load - 6.5 x 47L, Lapua brass, GM205M, H4350, Berger 140 HT

Only difference is test #1 was at 70 degrees and bullet jump was ~.080 while test #2 was at 45 degrees and bullet jump was ~.010.
In most all loads the test #2 load was 50 fps faster than the test #1 load on the Labradar.

Is this within the variability range of the Labradar?
or
Could the much larger jump permit gas "blow by" before bullet engagement in the lands and thus lower pressure & lower MV?
Test #2 cases would have had more capacity so I am somewhat surprised. A common occurance.

Many thanks

That amount of difference in MV does suggests you're closer to the lands than you think or touching it. When you're really close to (like just a couple thousandths), touching or jamming the lands, pressure will spike resulting in more MV. When you're back away from the lands, you don't get such a sever spike in pressure/MV.

You should probably double check your measurement to the lands, if for no other reason than just to be sure where it is. That'll help you not either touch or jam the lands if that's what you want or not. Other than that, don't give "jump" much thought as it's seating depth that's important in finding a good accuracy node. Find a powder charge that works well on paper for you at whatever seating depth you want to start at, then work on seating depth to fine tune your cartridge. Once you find that seating depth that working well, stick with it and forget about jump as your throat erodes and the jump increases.

 

[gdbleb]

Thank you for all the responses.
The barrel is indeed reasonably new. 350 rounds.
Same lot(s) of powder, primers, brass.
I agree that the results on the target trump everything else but its Winter and I find almost as much inner peace in the shop as I do at the range.

I re-tested chambering with my dummy round that has the same COAL / CBTO and that didn't move the bullet at all although it did show 2 wipes on the Sharpie blackening. Bartlein 5R 1 in 8" barrel on a BAT 3L.

My rounds were COAL: 2.825 , CBTO: 2.065

Hornaday LNL measurements are:
LNL "gentle push" (stop at first sign of resistance)
COAL / CBTO / diff
2.681 / 1.937 / 0.744
2.679 / 1.931 / 0.748

LNL "hard push"
COAL / CBTO / diff
2.832 / 2.085 / 0.747
2.833 / 2.083 / 0.750
2.832 / 2.086 / 0.746
2.836 / 2.087 / 0.749

So if my "hard push" is "jam" I suppose I should back them off more.

 

[MikeMcCasland]

Hard push on an LNL is absolutely jammed. By .005 or more.

You will get higher velocity if you are jammed.

Everything else left equal, you will see lower velocity (to a point) as you start backing out of the lands.

Application dependent, it may not be much. 15-30fps difference between .020 into the lands, and .020 off. Even then, .020 into the lands really isn't that far, because you're usually further seating the bullet to an unknown depth with the closing of the bolt.

 

[Ned Ludd]

Thank you for all the responses.
The barrel is indeed reasonably new. 350 rounds.
Same lot(s) of powder, primers, brass.
I agree that the results on the target trump everything else but its Winter and I find almost as much inner peace in the shop as I do at the range.

I re-tested chambering with my dummy round that has the same COAL / CBTO and that didn't move the bullet at all although it did show 2 wipes on the Sharpie blackening. Bartlein 5R 1 in 8" barrel on a BAT 3L.

My rounds were COAL: 2.825 , CBTO: 2.065

Hornaday LNL measurements are:
LNL "gentle push" (stop at first sign of resistance)
COAL / CBTO / diff
2.681 / 1.937 / 0.744
2.679 / 1.931 / 0.748

LNL "hard push"
COAL / CBTO / diff
2.832 / 2.085 / 0.747
2.833 / 2.083 / 0.750
2.832 / 2.086 / 0.746
2.836 / 2.087 / 0.749

So if my "hard push" is "jam" I suppose I should back them off more.

Practice finding "touching" (i.e. bullet just barely touches lands) using the same bullet over and over again, until you can consistently obtain repeated measurements within .001". The problem with using "jam" is that "jam" is not defined. Certainly, there are many that successfully use the OAL gauge and push really hard on the bullet pushrod to find what they refer to as "jam", or even "hard jam", but that is really not a well-defined measurement. I think for someone just learning the ropes, finding "touching" with the OAL gauge is the better way to go. However, it takes a little practice. I hold the OAL gauge body at the back of the set screw cylinder/assembly with my left fingers to keep it pushed solidly into the chamber, then ever so gently push the bullet pushrod with my right thumb until the bullet just barely kisses the lands before [very] gently tightening the set screw (the pushrod is plastic, so over-tightening the set screw may leave a dent in it that may affect future measurements. I use a cleaning rod insert from the muzzle to very gently push the whole assembly back out of the chamber to take measurements with calipers.

Velocity variance over the range of a seating depth test is not uncommon, but the velocity variance from one seating depth to the next is typically small; i.e. less than the ES for a 5-shot group at either seating depth. A difference of 50 fps is a little larger than I would normally expect, but there are potential explanations for it. For example, your predicted .010" jump may have unintentionally seated the bullet into the lands. Alternatively, you may have lapped the barrel and caused it to speed up a bit via the first outing and cleaning. It may not be so simple to definitively quantify exactly what caused the velocity variance, so I'm not certain other than continuing good record keeping that I would try specifically to understand it. However, learning to take consistent measurements with the Hornady OAL gauge will be of significant benefit over the long haul. My suggestion would be to focus on that as you continue load development. Perhaps you will figure out what caused the velocity difference as you continue, perhaps not. But learning to obtain consistent measurements at "touching" using the Hornady OAL gauge, or whatever alternative method you choose to employ will not be wasted effort.

 

[WEL]

What I find helpful with the LNL us to put a dowel or coated rod down the muzzle and push back on the bullet while you seat it with the tool. With practice you can fell just the slightest resistance where the bullet is in the lands.

 

[JFrank]

Nothing wrong with jamming a bullet, MV doesn’t run wild and I’ve not seen pressure spikes from as much as .025 into the lands. Barrels speed up, chronograph numbers can vary from day to day, set up to set up. Leave your chronograph at home and read the target.

 

[gdbleb]

Thank you all.
I was in need of a definition of "jam" or at least something within .005 of "jam".

I am reasonably consistent with the straight version of the LNL tool but this rifle is an Elisio chassis and I had to buy the bent version. I'm not very good with it so that explains the variability you see in my numbers. Winter practice.
I have a delrin rod for pushing back.