Brass trim length v. velocity/pressure

[kelbro]

I (inadvertently) let some brass grow .002 between trimmings and noticed my velocity kept rising with a known load over several firings. Accuracy was also affected. Trimmed it back .001. Velocities dropped a little. Trimmed it back another .001 and saw my original velocities and groups.

Has anyone else seen this behavior? If this is a rule, then a generous chamber and less trimming might get you enough extra fps to stay above transonic in some situations.

 

[spclark]

Reamer designers typically make the transition from chamber neck to free bore an abrupt but not "squared-off" or step, it's more like maybe 45 degrees. Too, it's recommended that one ought to trim cases to stay clear of the beginning of this transition by something like 0.010" minimum. Some will trim back even more.

0.002" isn't much at all but if you're super-diligent trimming brass to an exact length over time you may end up with an artificially-shortened chamber neck due to a build-up of carbon (and other materials left behind from burned powder) that could be the cause of what you're describing. Such a 'carbon ring' is gonna be both hard and abrupt.

I know I wouldn't want to rely on such an unknown over an extended string of fire in competition.

 

[mikecr]

I vote carbon ring.
You can get this regardless of chosen trim length and stepping into it is not a benefit.
You need to find & fix it.

 

[kelbro]

Hawkeye borescope here. No carbon ring.

Just checked and the QucikLoad simulation software also shows higher pressure and velocity with longer brass.

 

[Nomad47]

Kelley, I assume you adjusted "case length" in QL to arrive at your findings. Did you also adjust case capacity? I guess you could add area of a circle (pi*r squared) times added length to your original case capacity.

 

[mikecr]

I don't know of any way QL would show higher MV with longer necks

 

[kelbro]

Yes, I just adjusted the case length. I did not change the H2O volume.

Cartridge length did not change so the net volume (under the bullet) did not change. Not sure if net volume is how QL calculates. Many of the numbers there are derived from gross volume.

This has me scratching my head. Doggone it, now I have to run more experiments

 

[mikecr]

Ok I see what you're saying and seeing with QL.
By changing case length without changing COAL, you're changing usable case capacity. In this case reducing case capacity by the bullet seated 2thou deeper in the case neck.

This is such a tiny change and result predicted though. Hard to picture it as measurable with a chrono.
Have you measured chamber end clearance from your trim length?

 

[kelbro]

My chamber plug says that I have another .004-.005 to go.

 

[broncman]

How much head space do you have? .002" headspace then add .002-.003 extra brass length and you just ate your clearance up when brass is pushed forward by ignition???

I would trim shorter....

 

[Shynloco]

I (inadvertently) let some brass grow .002 between trimmings and noticed my velocity kept rising with a known load over several firings. Accuracy was also affected. Trimmed it back .001. Velocities dropped a little. Trimmed it back another .001 and saw my original velocities and groups.

Kelbro,
If I might ask (since I lack the technical expertise some of the others have), but wouldn't it make sense that when you establish a certain case length and find the sweet spot for seating your bullet, that any increase in case length would also raise your pressure and velocity? Many experienced shooters say those increases don't necessarily equate to better accuracy and often lead to a drop off in accuracy. Now perhaps I'm looking at this issue much too simply, but I'm not all that surprised by your finding with a "slightly" longer case length. Just curious.

Alex

 

[broncman]

If one does not change COAL AND bumps to same case dimensions when resizing... How does that increase pressure!!

If the case is sized and bumped to same headspace, then the brass migrates up into a longer neck. This would theoretically mean MORE internal volume as the brass has gotten thinner and migrated into longer necks...

An increase in case length should not raise pressure unless it is on a carbon ring or getting into the edge of the chamber... Causing a tighter bullet grip.

???

 

[broncman]

If one does not change COAL AND bumps to same case dimensions when resizing... How does that increase pressure!!

If the case is sized and bumped to same headspace, then the brass migrates up into a longer neck. This would theoretically mean MORE internal volume as the brass has gotten thinner and migrated into longer necks...

An increase in case length should not raise pressure unless it is on a carbon ring or getting into the edge of the chamber... Causing a tighter bullet grip.

???


Dougnut forming??

 

[brian427cobra]

Do you have the chronograph readings or are you assuming higher velocity due to points of impact.

 

[kelbro]

Magnetospeed chrono.

No doughnut.

I can only assume an increase in start pressure due to a little surface area hence more neck tension. May also show up with a crimp. I may try that too on my AR.

 

[Wildboar]

I'm with the idea of increased bearing surface in the neck (more brass gripping the bullet) causing higher pressures/ velocity. Sounds logical.

 

[mikecr]

What was the velocity change amounting to Kelbro?

 

[kelbro]

16-20 fps in a 260 Rem.

 

[mikecr]

The cause can't be 2thou of tension then

 

[broncman]

How long did the cartridge stay in the hot chamber before firing? I have cooked a few waiting for conditions and had velocity change....

I no leave bolt open and unchambered till readu to shoot...