bullet shape vs jump or jam

[ghermitage]

Has anyone found a correlation between the following aspect of bullet shape and whether they do best jumped or jammed, or is it the usual 'you just never know' situation?

The particular aspect of bullet shape I am wondering about is how abruptly the bullet engages the rifling. If the front of the bullet is more like a cone shape,e.g. Hornady Interbond), then it may be that the bullet doesn't ease onto the rifling like a design that is blunter at the front but quite gradual where the rifling is engaged.

What I am wondering is, depending on how the start of your rifling is cut, might it be better to jam bullets that do not engage the rifling gradually?

 

[spclark]

It seems that bullets by Sierra,their SMK line mostly) shoot best either just touching or jumping somewhat,how much is the question) to the lands in the various calibers we shoot. Bullets having a more gradual taper - VLD styles, Lapua's - seem to mostly get seated long enough to be well onto the lands when the bolt closes.

A lot may depend on just what the chamber profile you're shooting is; some leade angles are shallower and/or longer than others, making the point-of-first-contact harder to determine with any accuracy whilst easing the bullet's movement onto the lands once it begins it's journey.

 

[AckleymanII]

standard rule of thumb, the more pointed a bullet is, the more important it is to have the bullet touching or jammed up to .030
into the lands.

 

[Cheechako]

Go to Dan Lilja's web site. I think he has a tutorial on this very subject. And a very good one, at that.

Ray

 

[jeffreybehr]

Go to Dan Lilja's web site. I think he has a tutorial on this very subject. And a very good one, at that.
Ray

Ray, I did and didn't find it; care to post a link?

 

[Cheechako]

Jeffery,

Only kidding. I know it's Jeffry. I mean Jeffrey.

http://www.riflebarrels.com/articles/bullets_ballastics/throat_angles.htm

Ray

 

[jeffreybehr]

Ray, I saw that one; it has nothing about bullet shapes v. jump or jam, but it has LOTS of info on how to calculate groove-throat angles.

 

[ghermitage]

The question was particularly focussed on shape to the extent that it determines how the rifling is engaged,e.g. throat angle) and Ray's answer attends to that focus. However, it is all good, and pursuing any relationship between shape and jump or jam be it through fitting to throat angle or not is welcome.