Lands shifting..

[Rutland lad]

So I guess the drop in velocity could be some pressure escaping past the bullet before it has a chance to contact the lands?

Can this happen ? If you’re loaded 100,000thou off the lands (for example) then surely the front of the bullet would engage the lands and create a gas seal well before the rear of the bullet has left the case. So the gases on obdurating are still trapped within the chamber ?
I’m willing to be edumacated here.

 

[Bavarianbrit]

Can this happen ? If you’re loaded 100,000thou off the lands (for example) then surely the front of the bullet would engage the lands and create a gas seal well before the rear of the bullet has left the case. So the gases on obdurating are still trapped within the chamber ?
I’m willing to be edumacated here.

100,000thou is 100 inches.

 

[gixer1]

Can this happen ? If you’re loaded 100,000thou off the lands (for example) then surely the front of the bullet would engage the lands and create a gas seal well before the rear of the bullet has left the case. So the gases on obdurating are still trapped within the chamber ?
I’m willing to be edumacated here.

I would imagine some gas/pressure is going faster than the bullet, the bullet has more mass to get moving.

 

[HandB]

I noticed a bit of a drop off in velocity in my 6.5cm load - nothing wrong with the accuracy, it just slowed down by about 100ft/sec…I had a node at 2,665ft/sec with 140gr ELDM’s and 42.5gr of RS62, SMRP’s and seating at around 2.190” OGIVE which was a .067” jump.

I noticed recently that the velocity had dropped a little, accuracy is still good, so I decided to measure the lands again with an OAL gauge.

The lands are measuring at 2.277”…whereas when new the lands measured at 2.257” (.020” change)

The rifle has fired 1,267 rounds, would this be a normal amount of wear to lose .020”?

My plan is to increase the length to 2.210” and then go in .1gr increment charge increase until i get back to the node and see if it’s at the same velocity.

regards,
Gixer

I would not change anything on your rifle's load since it is maintaining accuracy. I say this because I went down the same rabbithole a couple of years ago with my .243 as I discovered its lands had eroded by a similar amount to your 6.5cm. I laboriously reset all my bullet jumps to compensate for this erosion, so that they were the same jump as when I initially set them, only to find that accuracy went to pot. So I reset all my bullets back to their original OAL and their accuracy was restored. What I realised is that the distance that matters for an accuracy node is not the distance from the bullet start to the rifle's lands, but the distance from the bullet start to the muzzle departure point and that is not affected by erosion of the lands. So this explains why accuracy is still good with your load even though the lands of your rifle have eroded by .020".

 

[gixer1]

I would not change anything on your rifle's load since it is maintaining accuracy. I say this because I went down the same rabbithole a couple of years ago with my .243 as I discovered its lands had eroded by a similar amount to your 6.5cm. I laboriously reset all my bullet jumps to compensate for this erosion, so that they were the same jump as when I initially set them, only to find that accuracy went to pot. So I reset all my bullets back to their original OAL and their accuracy was restored. What I realised is that the distance that matters for an accuracy node is not the distance from the bullet start to the rifle's lands, but the distance from the bullet start to the muzzle departure point and that is not affected by erosion of the lands. So this explains why accuracy is still good with your load even though the lands of your rifle have eroded by .020".

Did you see any drop in velocity? That’s the main reason I’m adjusting load as it’s just a bit too slow now…hopefully by adjusting the length with the addition of charge weight changes I can find a sweet spot and get tha velocity back.

 

[HandB]

Did you see any drop in velocity? That’s the main reason I’m adjusting load as it’s just a bit too slow now…hopefully by adjusting the length with the addition of charge weight changes I can find a sweet spot and get tha velocity back.

Good question - I reckon it did lose some velocity as I recall having to dial a bit more elevation but I don't know for sure as I don't have a chronograph. If you are keen to regain velocity I would recommend increasing powder charges gradually but keep the OAL the same as you normally use, so you are only changing one variable at a time.

 

[Utectok]

I mean we’ve all seen it right??

 

[MarinePMI]

I noticed a bit of a drop off in velocity in my 6.5cm load - nothing wrong with the accuracy, it just slowed down by about 100ft/sec…I had a node at 2,665ft/sec with 140gr ELDM’s and 42.5gr of RS62, SMRP’s and seating at around 2.190” OGIVE which was a .067” jump.

I noticed recently that the velocity had dropped a little, accuracy is still good, so I decided to measure the lands again with an OAL gauge.

The lands are measuring at 2.277”…whereas when new the lands measured at 2.257” (.020” change)

The rifle has fired 1,267 rounds, would this be a normal amount of wear to lose .020”?

My plan is to increase the length to 2.210” and then go in .1gr increment charge increase until i get back to the node and see if it’s at the same velocity.

regards,
Gixer

Sounds reasonable to me. So long as accuracy is good, I wouldn't worry too much.

As to changing the COAL, I'd recommend just finding a node (Satterlee method) and not worry too much about changing COAL; just keep the load the same, and adjust the velocity to your ballistic data as the barrel wears. I literally have not changed COAL in my match rifle over the last two barrels. The deltas in accuracy as the barrels wore out wasn't worth the time and effort to reset COAL. My last two barrels went about 2900-3100 rds before they started throwing flyers at distance. My loads were pushing a 140gr bullet at 2800fps-2850fps. Now I just run 130's at the same speed.

Your load appears to be somewhat mild, so you probably have a good amount of barrel life left.

ETA: It is generally considered a "sweet spot" for the 6.5 CM with 140's/130's, to run velocities around the 2850fps mark.

 

[Rutland lad]

100,000thou is 100 inches.

Big barrel. Like the one Saddam had made …….

 

[Rutland lad]

I would imagine some gas/pressure is going faster than the bullet, the bullet has more mass to get moving.

I understand that - but what I’m saying is the bullet effectively ‘plugs’ the barrel so everything behind it is just trapped with only one escape. That’s down the barrel behind the bullet.
That’s how I see it in my minds eye, but I’m not a gunsmith or engineer. Just curious.

 

[Utectok]

If it was me and I wanted another 100fps MV I’d stick more powder in about 60fps for a grain in my powder but obviously yours will be different

 

[gixer1]

Sounds reasonable to me. So long as accuracy is good, I wouldn't worry too much.

As to changing the COAL, I'd recommend just finding a node (Satterlee method) and not worry too much about changing COAL; just keep the load the same, and adjust the velocity to your ballistic data as the barrel wears. I literally have not changed COAL in my match rifle over the last two barrels. The deltas in accuracy as the barrels wore out wasn't worth the time and effort to reset COAL. My last two barrels went about 2900-3100 rds before they started throwing flyers at distance. My loads were pushing a 140gr bullet at 2800fps-2850fps. Now I just run 130's at the same speed.

Your load appears to be somewhat mild, so you probably have a good amount of barrel life left.

ETA: It is generally considered a "sweet spot" for the 6.5 CM with 140's/130's, to run velocities around the 2850fps mark.

I was having pretty good results at a modest 2,665, very nice to shoot, was hitting consistently at extended ranges and then noticed a change and having to dial a bit more in…and hence FEO’s covered the velocity change.

My thinking on increasing the OAL is to me - it was at .067” jump and now would be .087” so surely would be smacking the lands harder than a smaller jump…thereby speeding up the erosion process…

It’s not a big deal to try it either way, and I’ll likely be due a barrel in about 800ish rounds. Although I have heard of TAC’s still being accurate at 3000 rounds with modest velocities.

Regards,
Gixer

 

[MarinePMI]

I was having pretty good results at a modest 2,665, very nice to shoot, was hitting consistently at extended ranges and then noticed a change and having to dial a bit more in…and hence FEO’s covered the velocity change.

My thinking on increasing the OAL is to me - it was at .067” jump and now would be .087” so surely would be smacking the lands harder than a smaller jump…thereby speeding up the erosion process…

It’s not a big deal to try it either way, and I’ll likely be due a barrel in about 800ish rounds. Although I have heard of TAC’s still being accurate at 3000 rounds with modest velocities.

Regards,
Gixer

Just my personal experience, so take it with a grain of salt, but I ran a load for 6.5CM at 2650fps, due to the last powder shortage. I had to use AA-4350, which runs a lot slower than H-4350.

At any rate, that load was so finicky, I finally gave up. Scott Satterlee had a theory, which has mostly held true, that a given case volume and bullet weight has a sweet spot, velocity wise. And that the 6.5CM was at 2850fps. Didn't matter what powder or bullet, everything just seemed to shoot reasonably well at that velocity node with a 140gr bullet.

That seemed to mirror what I was seeing when running that 2650fps load. It would shoot fine in calm winds, at distance. But as wind increased, I started to notice some erratic accuracies. I still can't explain why, but after moving to a powder that kept velocities up in the 2850'ish range, the flyers went away. H-4350, RL-15/16; it didn't matter, it just shot well. <shrug> It may be correlation doesn't equal causation, but it was an odd thing to see, and since, I have always run loads in that velocity node, without any accuracy issues.

 

[gixer1]

Just my personal experience, so take it with a grain of salt, but I ran a load for 6.5CM at 2650fps, due to the last powder shortage. I had to use AA-4350, which runs a lot slower than H-4350.

At any rate, that load was so finicky, I finally gave up. Scott Satterlee had a theory, which has mostly held true, that a given case volume and bullet weight has a sweet spot, velocity wise. And that the 6.5CM was at 2850fps. Didn't matter what powder or bullet, everything just seemed to shoot reasonably well at that velocity node with a 140gr bullet.

That seemed to mirror what I was seeing when running that 2650fps load. It would shoot fine in calm winds, at distance. But as wind increased, I started to notice some erratic accuracies. I still can't explain why, but after moving to a powder that kept velocities up in the 2850'ish range, the flyers went away. H-4350, RL-15/16; it didn't matter, it just shot well. <shrug> It may be correlation doesn't equal causation, but it was an odd thing to see, and since, I have always run loads in that velocity node, without any accuracy issues.

I might well start stepping it up a bit to see how it does…

 

[MarinePMI]

I might well start stepping it up a bit to see how it does…

I should also caveat my post above with "If you're running a Tikka 6.5CM with a factory barrel, drop 100fps off that sweet spot." Tikkas (for whatever reason) tend to have "slow" barrels in the 6.5mm bore size. Usually 100-150fps slower. No one really seems to know why, but it's pretty much a known thing.

 

[Kiwiguy]

I might well start stepping it up a bit to see how it does…

When you originally worked up your load what chronograph did you use?

You mentioned that you have bought a Garmin chronograph and I am wondering if the original velocities were gained using a different chronograph and maybe there is a discrepancy between the two chronographs?

 

[Border]

If you haven’t seen it, take a look at the Erik Cortina video. He explains it well, and knows what he is talking about.

 

[gixer1]

When you originally worked up your load what chronograph did you use?

You mentioned that you have bought a Garmin chronograph and I am wondering if the original velocities were gained using a different chronograph and maybe there is a discrepancy between the two chronographs?

All done on the Garmin, so should all be pretty reliable,

 

[cjm1066]

My old 308 needed a 150g bullet 2mm out of the case to touch the lands.

At that stage groups were opening up. But until that stage velocity and accuracy were fine.

 

[gixer1]

I mean we’ve all seen it right??

To me - he did actually measure it when he did the jam measurement - that’s what he uses for his datum….ok, it’s not the lands but he’s doing the same as using a OAL gauge where you push the bullet in until it stops - this is pretty much jam (and the reason a cleaning rod needs to be used to push the bullet out sometimes.