Bullet Jump

[CoElkHunter]

IIRC (and it's been months since I read them), the point of th8s PRB / COL series was that:
Precision accuracy can often be found with the bullet backed well off the lands. And when this is the case, that precision can be maintained longer - possibly over the life of the barrel - without ever adjusting the load.

A pretty big deal to serious PRS competitors that burn through barrels and ammo much faster than the average bear.

I guess when the bullet gets a "running start" to the barrel lands so to speak, it makes sense the bullet velocity would stay "up" and thus accuracy improves in theory?

 

[crs]

CEH,
How about a 3/10 inch bullet jump?
I always thought that when something "jumped" it was in free flight and totally unsupported.
I have seen this debated and often misunderstood on other forums.
One of the frequent statements was that any jump ruined accuracy.
The proper definition is essential to any meaningful discussion of this term.

The most common such statement came up when discussing the use of factory .45-70 ammo in a .45-90 chamber. Old wives tales of poor accuracy, powder and lead deposits in the chamber, copper and lead scrapings due to bullet non alignment with the barrel, and other such gossip (usually by 45-70 shooters with NO .45-90 experience).

When I bought my Miroku/Winchester 1886 .45-90, I measured the .458 300 grain bullets used and the .3 inch distance from the front of the case to the entry into the .45-90 barrel. When fired, the bullet would always be well into the barrel BEFORE it left the case! Therefore it would never be unsupported or in free flight until it left the muzzle. Even more so for Longer, heavier bullets.

In firing thousands of rounds, none of the threatened dire accuracy killing events happened. In fact, the 26 inch barrel seems to be very accurate with both .45-70 and .45-90 ammo. Both cartridges shoot minute of angle at hunting ranges.

The same is true of my .45-90 Double Rifle. I plan to blood it later this month on a cull hunt on a Texas exotic ranch.
Rant over.

 

[shootist~]

I guess when the bullet gets a "running start" to the barrel lands so to speak, it makes sense the bullet velocity would stay "up" and thus accuracy improves in theory?

Short answer is I don't have a clue.

Best SWAG is old Roy W figured out how to slow the peak pressure curve to allow more power and V - then told the barrel makers to solve the accuracy part.

 

[fourfive8]

"Usually" some jump tends to minimize chances of pressure spike "surprises" as you approach maximum loads or load combinations that tend to go "spiky" as that max is approached. Consistent shot to shot Inertia of the bullet itself during the deformation into the lands may minimize surprises- the "get a run at the lands" approach... sounds kind of silly but has been proven in the lab by the way. A bullet with zero or negative jump is starting from dead stop with likely variable resistance to starting the deformation process and is relying on building progressive powder burn pressure that can respond in a likewise variable, progressive manner. Most gilt edge bench rest shooters usually find that for getting the last smidge of accuracy out of a load, that bullet jump number can be at near zero up to maybe .010". The trick is getting that constancy in the mechanical "geometry" in the load to begin with, especially where extremely small variations in case to chamber sizing and bullet ogive shapes can affect that jump number significantly. Many chase those last smidges of ever elusive accuracy gain down that rabbit hole for entire lifetimes.

I figured long ago, reliable hunting ammo was more important to me than chasing that elusive last .1" MOA accuracy so I tend to always load with a good margin for error in bullet seating depth starting with at least .020-030" bullet jump. Sometimes much more than that without compromising any accuracy at all. Sometimes, especially with heavy recoiling DG ammo, I simply seat the bullet to crimp in the crimp groove without even considering bullet jump... but always verify to make sure there is plenty of jump with that load combination. One very tiny reloading booboo with a hunting round that may cause balky chambering or sticky extraction due to a high pressure spike is not worth it to me at all. Plus, there is some anecdotal evidence that monlithics, like the Barnes TSX and similar, actually prefer significant bullet jump, maybe in the .050"+ range. If nothing else, preventing potential spiky pressure surprises especially with a really "hard" monolithics makes intuitive sense, even if accuracy differences aren't always realized.

 

[rookhawk]

Relating to @fourfive8 ’s comments about monolithic bullets, here is what Barnes says about bullet jump for their copper bullets:

When loading a Barnes TSX, Tipped TSX or LRX bullet, your rifle may prefer a bullet jump of anywhere between .050” up to .250” or more. This distance off the lands (rifling), aka “jump” may be limited to the rifles throat length, magazine length and bullet length.

When selecting the cartridge overall length (COAL) we recommend starting with a minimum “jump” of .050” off of the lands. You can test different seating depths and find a “sweet spot” that your particular firearm prefers. We suggest working in at least .025” increments as follows seating the bullet deeper to allow a further jump. Your test plan could look something like this:

1st group- .050” jump
2nd group- .075” jump
3rd group- .100” jump
4th group- .125” jump
5th group- .150“ jump
6th group- * see below

This length can be determined by using a “Stoney Point Gauge” or other methods. You do not have to seat the bullet at, or on one of the cannelure rings. Remember there are many factors that may control or limit the seating depth for your application. You may find that you need to start at around 0.150” off the lands and are not able to get any closer due to limiting factors including proper neck tension and magazine length.

*In rifles that have long throats you may be limited on how close you are able to get the bullet to the lands. In these instances, it is not uncommon to find the best accuracy with a jump of .200” or more.

This jump may possibly stay the same regardless of powder or charge weight within a given rifle. If preferred accuracy is not obtained, we certainly recommend trying another powder, for the powder type and charge greatly affects the overall accuracy of each individual firearm.

FYI- An accurate load requires a bullet with the proper consistent case neck tension which leads to more constant pressures and velocities.

 

[Hogpatrol]

Must be something to it.