Reloading

[Douglas Kiser]

Can someone explain to me, the jam and jump in reloading

 

[Creedmoor shooter]

Can someone explain to me, the jam and jump in reloading

Its in reference to where the bullet is seated whether the bullet is off your lands or into your lands.

 

[unclefish]

Basically your projectile on the lands of your of your chamber.
Very important on accuracy of your weapon.
Usually you want your projectile (bullet) away from lands so it has no chance of getting stuck in there if you are hunting and will be removing the cartridge. Also if you plan on using a all copper bullet they seem for me to work about .100 away from lands. Hope this helps

 

[Alaska2006]

Jump is how far the bullet is way from the lands. think of it as giving the bullet a running start before engaging the rifling lands.
as a rule you will see less pressure when seated off the lands. That said you need to be careful in seating the bullet too deep in the case. The deeper in the case the smaller the combustion chamber so to speak and this too can increase pressure. Another issue with seating too deep, if you go too far you run the risk of the case neck not holding the bullet correctly and the bullet can push inside the case when trying to chamber a round or in some instances simply recoil can push the bullet the case if all things are not correct.
Jump/jam is critical to great accuracy and every rifle is different. Start out by finding here your lands by using a modified case and COAL gauge.

I'm sure I haven't explained it in the best way but hope this offers and idea of jump and jam

 

[AZShooter]

An alternative no cost way to find the lands is to seat the bullet long, polish up the jacket with fine steel wool and insert the dummy round into the chamber. Obviously if the bullet is out too far you may not be able to close the bolt. If that is the situation seat a bit deeper, repolish the jacket and repeat. Eventually you will see evenly spaced rectangular marks on the jacket. You can continue seating deeper in small increments till the marks are barely visible and use that as a rough reference as to distance to the lands.

 

[Douglas Kiser]

Thanks Alaska, AZ shooter, and Uncle

 

[DocDoc]

Rather than polishing with steel wool, just color the tip with a black marker, insert, remove and check for contact marks from the lands. A comparator makes it even easier to find the contact length and measure the CBTO, cartridge base to ogive.