What is 'Jump'

[parrotheadkjc]

I am fairly new to the 'precision' reloading and shooting,I have reloaded for my hunting rifles for over 25 years) and while researching loads for a 6.5 x 47 Lapua, the author was listing jump, i.e. .015' Jump

What does he mean by that? Is that seating the bullet into the lands?

Thanks for the Help,
Parrotheadkjc
Medford, OR

 

[Preacher]

Nope, not seating at the lands, it's jumping that much distance to the lands.....

 

[parrotheadkjc]

Preacher:

I am not following you- what do you mean it is jumping that much distance to the lands?

Thanks
Parrotheadkjc

 

[Preacher]

Yes it is jumping that much distance to the lands.....
It will move forwards .015 inches before it hits the start of the rifling...
Fifteen thousandth is not a lot to jump, Barne's bullets recommends jumping their
solids .050...

 

[parrotheadkjc]

I think I understand, You seat the bullet to a set OAL of the cartridge so that the bullet, when the round is fired,'jumps' or moves 0.015' before contacting the lands. Is that correct?

Thanks Again for helping this newbie!!
Parrotheadkjc

 

[Posit]

Not to interupt... but you've got the idea. Also referred to as 'from the lands', or 'off the lands'. To be precise, you have to measure from the ogive of the bullet to the base of the case, because OAL may vary slightly among bullets in the same box due to minor fluctuations at the tip. Note this is a pretty precise measurement.

The next point is you need to measure each type of bullet,manufacturer/weight) you intend to load, since the shapes will be different.

The converse is 'jam' - loading the bullet long enough that it actually contacts and presses slightly into the lands.

Thus you have 'jump','at the lands' and 'jam'.

Aren't you glad you asked?

 

[parrotheadkjc]

THANK YOU TO BOTH OF YOU!!

I am glad I asked, now I know, and I am not guessing.

Thanks Again
Parrotheadkjc

 

[Preacher]

Posit gave you good solid take it to the bank information, be sure that you caught the part about measuring to the ogive of the bullet your shooting not to the tip of it !!!!!

 

[Forum Boss]

I would only add that you will see some folks,particularly short-range BR shooters) use the term 'jam' to mean a specific OAL at which the bullet can go no further into the rifling without starting to push back into the case.

Some guys like to seat the bullets very long, close the bolt, then extract the case, measure it and call that 'Jam'. Then they'll seat their bullets .00X' 'off jam' or 'less than jam.'

I personally don't like this use of 'Jam' because our tests have shown that it is not particularly repeatable, and it can vary with neck tension. Also some guys deliberately load longer than this length and let the bullet seat itself... so what do we call that?

I prefer to designate the point of first bullet to land contact as the zero point and calculate 'in the lands',longer OAL) or 'off the lands',shorter OAL) from there. For example, if you use a tool to measure the point where the bullet just touches the rifling, and then go 15-thou longer, then you'll be .015' in the lands.

Just be aware that when some guys use the term 'Jam' they are talking about 'maximum effective load length' which could be 30-35 thousands beyond initial bullet to lands contact.

 

[gambleone]

Like Preacher said, by measuring to the ogive of the bullet, brings up oal & coal. With oal overall length, is measured from tip of bullet which can vary & is inaccurate, if you want to be precise. coal comparator overall length, which measures from the ogive of bullet, will give you the precise measurement.
Hope this helps.

 

[Mikem]

Great info, and I will only add, that 'Stony Point' has made test tools for this purpus for years, but now I think they were bough out by someone else, but are still available.
The other thing is, record all of the measurements for every bullet type and weight you plan to use, and retake the measurements every 250 rounds or so, as throat erosion will slowly move the lands out, and your numbers will slowly increase.
M.

 

[gambleone]

Stoney Point was taken over by Hornady who sell the oal gages. Sell for about $30.00, depending on where you buy them.

 

[NORZ]

Is there any 'rule of thumb' or such, that will give you an estimate on what bullets prefer in respect to COL? Lets say for 308Win?

 

[Mikem]

I know of no rule of thumb, except most factory ammo will have quiet a bit of jump, but then most on this site dont shoot much factory stuff, and most move the bullets out at least closer. Some rifles and loads like to jump, my 6BR with 80g Berger's likes a .010 jam.

You asked about the 308. I have a factory stock H.S.Precision HTR 308 that will shoot factory Federal gold Metal Match, at better than .240 moa with a jump of .090. And better with my hand loads.

If I were starting out with a new rifle, I would start with about a .010 jump, and move in to a .010 jam, with a reduced powder charge, and checking for signs of excessive pressure.
It can take time, numerous loads,OAL, bullet brands, weights Etc before you find what your rifle like best.
M.