Bullet jump

[thomasem]

I have been reloading for a while now and happy with results to date. My question being - I am looking at trying a few different weight bullets; so does the required jump to lands vary with bullet weight or does it remain a constant once you have found the sweet spot.

Thanks

Tom

[Adamant]

Tom,

I've been wrestling with this one for a while when loading for my .270 - my experience so far is that for most bullets, the 'sweet spot' is the same. However, I have found that one or two bullets behave very differently in my rifles, particularly Nosler Ballistic Tips - these were all over the place at my usual 0.020" jump to the lands and I've found they like to be only about 0.005" off (I think it might be to do with bullet shape?). Thankfully, .270 brass is long necked, otherwise the Nosler bullets would be seated too shallow for comfort.

My .223 seems to prefer a jump of 0.015 for everything

This is one of those jobs where an OAL Guage and a bullet comparator are worth their weight in gold!

Adam.

[menial 1]

I'd say you've pretty much answered your own question Tom, most changes in weight will have a different length. Stops it geting boring though.
Pete.

[dicehorn]

Yes, A heavier bullet for example, if seated the same distance as a lighter one, being heavier it will have to fit further into the case and that will in turn reduce the air space above the powder causing the bullet to perform differently.

[richness]

each one is different IME
I mean, they may be within a few thou....but you end up redoing/testing the whole load anyway, to be sure

[thomasem]

Thanks chaps.

Best get another tub of powder in then!

Tom

[flims]

due to the different ogive of bullets, different bullets tend to have/require different seating depth. instead of OAL, use a bullet comparator which will measure from the Ogive to the headstamp of the brass rather than from the headstamp to the tip of the bullet. This will make sure that different bullets are all seated the same amount off the lands example 0.005" off.
http://www.ktaccuracy.com/images/bullet%20comp.jpg

I would also use the bullet comparator to check different ogive to base-of-bullet length. if one bullet is very close to the other, than you might try seating them to the same seating depth. If however you find that the ogive to bullet base length is very different, you will have to work out seating depth again.
http://www.accuracyone.com/images/bulletcomparator.jpg
you can read more about using this over here
http://www.6mmbr.com/medlerTUBBBSC.html

[CollieCross]

instead of OAL, use a bullet comparator which will measure from the Ogive to the headstamp of the brass rather than from the headstamp to the tip of the bullet. This will make sure that different bullets are all seated the same amount off the lands example 0.005" off.
http://www.ktaccuracy.com/images/bullet%20comp.jpg

Not so.

All that a comparator can do is is measure the distance from where it happens to sit on the "ogive" of that specific design of bullet to the base of the case.

There is no standard for the profile inside the comparator. It is improbable that the comparator will touch the bullet in the same place as the bullet will touch the lands. In fact it is impossible, unless you had a custom made comparator built using your exact same chamber reamer. Even then as the throat recessed with use it would become inaccurate

It is improbable that a significantly different shaped bullet will give any useful comparative measurement.

All that a comparator can do is give you a comparison measurement between identical profile bullets. Change the bullet make/profile/etc. and you are back to square one. I.e. you will have to once again set up a dummy cartridge/bullet to kiss the lands, then measure it with your comparator, then back the bullet away.

A comparator is just that, it can only give a comparative measurement between otherwise identical cartridges (i.e identical bullet shapes). It cannot give an absolute measurement that is at all useful when changing bullet types.

You can spend lots of money on fancy calibre specific comparators, or just buy the simple Sinclair hex nut design e.g. http://www.reloadingsolutions.com/15.html

[baz]

Dont make the mistake of thinking that all bullets like to be close to the rifling in every gun, I shoot different guns all the time and it used to drive me nuts when I first started testing ammo, factory rifles can shoot best with lots of "jump" with some bullets

A Sako 75 I loaded for would only shoot well if the bullet was set with ogive at 60thou off lands, and it only liked "stubby" bullets such as Speer Spitzers, anything else would give groups of around 2+ inches at 100yds

Custom rifles are different as closer tolerances whereas factory rifles need to accept factory ammo, just about any factory rifle in good condition can be persuaded to shoot well, quite a few are easy to load for but some can be a real bitch!

[richness]

well said baz. Even vlds don't like to be crammed in my gun.
Some loads like to be shorter than saami (i'm thinking vmax in 204ruger)
Generally, a little over saami specs is the way to go but essentially there is no formula or shortcut- every gun and every load needs fully tested