Stupid reload question: Bullet seating depth , jump and OAL

[199 398 500 A]

I'm looking to get into reloading and have one question. Does "jump" just mean how much shorter (due to deeper bullet seating) your round is compared to prescribed maximum OAL?

For example, a 77gr SMK for a .223 is listed as OAL 2.260". If you want 0.005" of jump, you seat the bullet 0.005" deeper into the case and now have an overall length of 2.255".

If this is incorrect, can someone better explain it to me?

 

[_Raining]

It is in reference to where the bullet touches the lands in your specific rifle. There are different tools and methods to get that touch value.

 

[199 398 500 A]

ah okay. that makes more sense now. Thanks

 

[seanh]

And unless you have a custom barrel, if you want to shoot from magazine you'll be quite a bit far out from engaging the rifling

 

[Simonsza1]

Hornady overall length gauge, there modified case and a comparator will be the easiest way to figuring out your measurement to the lands and then you can set from there. There are lots of videos using this tool watching some will probably give you a good visual of the answer your looking for

 

[paulwardd]

This might be an overkill explanation, but if anything hopefully it can help clear things up and get you started. As stated above, jump is how far away from the lands of the rifling you are. Generally the less jump you have the more accurate your load will be, as there is less "wobble" and unguided travel for the bullet to go from where it sits in the case to when it hits the actual lands (at the 'ogive' - the point in the curve of the bullet that becomes the actual diameter of the barrel). Most often, you need long, heavy, high-bc bullets to be able to seat out far enough, since a bullet of less weight will be too short to reach that point. Remember to always keep at least the diameter of the bullet seated into the neck, or close at least. Generally a good starting point is around ~20/1000 off the lands, +- 10. Increments of 5 are usually good. It's dangerous to seat the bullets right at the lands as that can lead to pressure problems (causes dangerous pressure spikes). The way most people do load development that way is to figure out charge weight that will get you the most accurate, and once you have found the best load then start playing around with seating depth. Again, that's just one way and I'm sure other people have their ways of doing it, that's just mine and what I've seen the most. Hope this helps!

 

[Jordanwickham]

Bullet design also has alot to do with amount of jump that would be desired. VLD (secant) style bullets are very jump sensitive. Tangents and hybrids are less sensitive and can handle more jump.

 

[Dthomas3523]

“Less jump is generally more accurate” is an inaccurate statement.

You can jump .080-.100 or more and be fine.

 

[seanh]

As mentioned, it depends on the individual bullet. I remember being extremely surprised at this when I tried some Berger 185, forget another one that I tried. But it seems it didn't matter where I see the bullet, accuracy was very good. The smks, between 15 and I think about 40/1000 they shot one way, deeper seating I had to work up a different load.

 

[Case7140]

I actually hit a target somewhat consistently at 1000 on Sunday. I appreciate all the time people put into this forum, I have a LONG way to go.

 

[AKMarty]

My Tikka in 6.5cm shoots 1/2 MOA w factory ammo. It has been doing that from a starting jump of .030” new until today with 1800 rounds thru the tube and it’s jumping .107”.

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