Slow motion of bullets leaving muzzle

[kenbro]

I’ve thought of this before.
Logic says that the front of the bullet must engage in the lands long before the base of the bullet exits the case.
If that’s correct, then there’s a ‘gas seal’ made by the bullet leaving all the pressure from the powder burn behind it, with only normal atmospheric pressure in front of the bullet. But of course, that atmospheric pressure is being pressurised by the bullet coming down the barrel. At least it must be something like waiting on a Tube stn platform - you can feel the wind from the approaching train long before it appears.
I’m sure there’s people on here with much more knowledge than I who can confirm or deny my theory.

I would think that some bullets will have left the case neck before the full diameter of the bullet engages the lands, thus allowing some gas to pass the bullet in the barrel.
K

 

[Smellydog]

Rarely does a jacketed bullet fill the grooves.
In the image below you can see how the jacket has not filled the grooves.

With lead bullets it's not necessarily the speed of the bullet that can lead a barrel but the increase in hot pressure leaking past the bullet and melting the lead surface and depositing lead on the barrel.
Hence why bullet lube rings were added and paper patching methods and metallic gas checks.

 

[Lateral]

Hmmm, is it just me, or is a 30-06 shooting a 150gn bullet at 3740fps somewhat optimistic ??? Typo ? lighter bullet ?

Just checked, and a typo, should be 2740fps for 2500ftlbs.

 

[Rutland lad]

I would think that some bullets will have left the case neck before the full diameter of the bullet engages the lands, thus allowing some gas to pass the bullet in the barrel.
K

I understand your point, but I’m not sure that’s the case.

Follow my logic for a second - if we’ve loaded 50 thou off the lands, then (in my minds eye) the ogive of the bullet will fully engage with the bore of the barrel once it moves forward 50 thou. If that’s the case, then unless the seating of the bullet in the neck is less than 50 thou deep, the hole is sealed.
And hunting bullets are certainly not seated in the case neck anything like that - they’re much much deeper.

Please note I’m not an expert by any means. I’m just trying to ‘visualise’ in my head what I cannot see going on in the breech of the rifle.

 

[kenbro]

I understand your point, but I’m not sure that’s the case.

Follow my logic for a second - if we’ve loaded 50 thou off the lands, then (in my minds eye) the ogive of the bullet will fully engage with the bore of the barrel once it moves forward 50 thou. If that’s the case, then unless the seating of the bullet in the neck is less than 50 thou deep, the hole is sealed.
And hunting bullets are certainly not seated in the case neck anything like that - they’re much much deeper.

Please note I’m not an expert by any means. I’m just trying to ‘visualise’ in my head what I cannot see going on in the breech of the rifle.

Some rifles have a lot of freebore to help reduce pressure, 1/10” is not uncommon.
Then you get a bit extra free ore from the spring point of the ogive to the start.
All adds up.
Ken.

 

[Rutland lad]

Some rifles have a lot of freebore to help reduce pressure, 1/10” is not uncommon.
Then you get a bit extra free ore from the spring point of the ogive to the start.
All adds up.
Ken.

Yep - I can see that

 

[kenbro]

Yep - I can see that

Me thinks we need someone who understands internal ballistics.
Ken.

 

[borbal]

Me thinks we need someone who understands internal ballistics.
Ken.

Check this out. Page 48

Absolute chamber pressure in center-fire rifles

deepblue.lib.umich.edu