ogive

[Link]

MMMMmmmm how am I going to ask this so it makes some sort of sense?

I reload for my 6br. When using my Sinclair bullet comparator

there is .034 difference between the overall length of 2 rounds [Bart original vs Bruno 00s.

Does this mean there is near that much difference in jump?

thanks

 

[benzy2]

Yes that would mean there is that much difference in jump. Both bullets will contact the rifling at the ogive of the bullet. As such when you measure from the ogive you are measuring how long the round is from the head to the point on the bullet the rifling will touch when fired. Measuring from the ogive lets you ignore the differences in the tips of bullets and focusing on how they meet the rifling. What you want to do is find the length from the head of a case to the point the throat in the rifling makes contact with the bullet. You can either use a cut up dummy round,many pictures on this site about how) or a Hornady OAL gauge which will let you know how far out you can seat a bullet,all based on the ogive point and not the total overall length) before it touches the rifling. Once you know this you can make your bullet lengths set to be so far away from the throat or so far into the throat based on what your gun seems to like.

The problem with measuring from the tip of the bullet to the head of the bullet is that the tips often vary within a given batch of bullets by a bit while the ogive point tends to stay consistent. Second is that two bullets of even the same weight can have greatly different shapes and so if the ogive sits farther one way on the bullets even if you load to the same length based on measuring from the tips the part that matters, that is the part that touches the rifling, is actually in a very different position. This makes using any load data for bullets that seem close though not identical a very dangerous event and can give a big change in group size even though a similar weight bullet was used.

 

[Link]

Great explanation Benzy.

Maybe one bullet - powder combination would like a little different jump or jam than the other but .034, does that sound like a lot?

thanks Link

 

[mikecr]

It's alot.
You have to start over completely w/regard to seating depths with different bullets. And you can't just adjust seating by that .034 and assume things are right. You have to recheck actual jam, and determine the most accurate depth -for different bullets.