Bullet Jump(ing)

[stagpanther]

Found this very interesting article with some perhaps surprising results. A good read on a debated topic.

Maybe Roy Weatherby knew about the "long jump" long ago.

[Unclenick]

That and another were cited in a recent post. It is interesting. I would like to see some of the military researchers look into the reasons for it, but it certainly seems that for every claim you must be in the lands or 0.020" off, there is other published experience to the contrary. I am left thinking you just have to try the possibilities and see what happens in your gun.

[nhyrum]

That is an interesting read. While I don't currently have time to read it I looked over most of it. An helpful tip I believe I heard Brian litz talk about, once you find the jump window, you want to load on the long side of it (closest to the lands) so that as your throat erodes you still keep accuracy.

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[Unclenick]

That or some folks predict the throat wear and seat progressively longer to chase it. Got to shoot the rounds in the right order, though.

[stagpanther]

A factor not mentioned is variation in psi--which I suspect is also correlated (maybe that's what varies according to bullet design?). What seems to be implicit is that a better strategy would be to choose the longest jump which hits the widest sweet spot while at the same time minimizes throat erosion. I didn't know erosion was really that fast with close to the lands COL's.

[nhyrum]

Originally Posted by Unclenick

I am left thinking you just have to try the possibilities and see what happens in your gun.

Yeah. Good "windows" that work are nice. But also, all the barrels/chambers they were using, were they match barrels with throats cut? Or factory actions? Would that matter?

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[nhyrum]

Originally Posted by Unclenick

That or some folks predict the throat wear and seat progressively longer to chase it. Got to shoot the rounds in the right order, though.

I'm not doing too try to chase wear in my 300 rum I started loading at 20 off about 200 rounds ago... I bet I'm close to 70 now... I'm only semi serious about that

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[stagpanther]

were they match barrels with throats cut? Or factory actions? Would that matter?

The results towards the end showed that Mark tested the jump across a variety of guns and cartridges--the consistency was there and more specific to the bullet type.

[nhyrum]

Originally Posted by stagpanther

The results towards the end showed that Mark tested the jump across a variety of guns and cartridges--the consistency was there and more specific to the bullet type.

I did see that part. I didn't look too much at the particular guns, but if there throats were cut to fit a particular bullet, you can't really their them in the same batch as factory cut/saami chambers/throats, because that changes two variables. Being prs blog, I'm assuming they were all throated barrels. I will agree that their results show more jump to me more consistently accurate. I've also seen data that shoes 3 jam to 0 jump is better

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[ghbucky]

Found this very interesting article with some perhaps surprising results. A good read on a debated topic.

Thanks for posting that, Stag. Lots of very good info in there that pertains to stuff I'm trying to figure out right now.

[stagpanther]

You're welcome--but thanks should be directed to the Precision Rifle Blog.

[stagpanther]

Of course the cynical part of me figures it could be a conspiracy to get you to burn you barrel out faster trying a multitude of loads.

[Geezerbiker]

I mostly shoot light loads from my .300 Blackout and the bullet jump is really long. I mostly shoot it at 30 to 50 yards but it's phenomenally accurate. I think of it as a modern .32-20 and I mostly use it like I would a .22 rim fire...

Before this I was dead serious about .002 to .005" jump to the lands but with this rifle it's more like 3/8" to the lands.

Tony

[hounddawg]

I have ran everything from hard jams to .075 jumps with good results on various barrels, bullets, and powders. The rifle I am working with right now likes hard jams with compressed loads. I would rather have some jump but the barrel/bullet says no way.

[Unclenick]

Originally Posted by stagpanther

I didn't know erosion was really that fast with close to the lands COL's.

Erosion is mostly a function of the quantity of powder burned in each round and the peak burning temperature reached during the burn. What changes with throat jamming are two things: one is that it takes less powder to reach either a given peak pressure or a given barrel time, and that should reduce throat erosion. But it achieves those objectives at lower muzzle velocity. That may seem counterintuitive, but the higher start pressure from the jammed bullet causes the round to reach peak pressure before the bullet has travelled as far down the tube as would be the case with some jump and a larger charge that produces the same barrel time. The earlier in the peak in the bore, the more running start the bullet has down the rest of the barrel, so it takes less pressure in the rest of the bore to get it to the muzzle in a given barrel time. But that also means lower muzzle velocity for the given barrel time as the running start got the bullet a little ahead of schedule so it didn't need quite as much velocity to make the same barrel time Folks don't like losing muzzle velocity if they are loading for long range, so, instead they often end up looking for a faster barrel time sweet spot that actually runs at higher peak pressure, thereby throwing away the erosion advantage.

The second thing is that the longer your jump, the less difference, percentage-wise, a change of a thousandth of an inch makes to the total jump. I think this is probably why deeper seating produces wider sweet spots.

There are two strategies for hanging onto a narrow sweet spot found close to the lands or jammed into the lands. One is to keep track of base-to-ogive measurements with a reference gauge, whether it be one you make yourself or you use the Hornady type tool. Changes in that number over the first couple of hundred rounds will provide an estimate of how much you lose per round. Then, assuming you use a seating die with a micrometer adjustment, you can lay out a chart based on that erosion rate that rounds it to the nearest thousandth. You then always fill your ammo boxes in the same order, say, from upper left to lower right, like reading, and set your cartridges in the box in that order as you back the seater off the occassional thousandth to compensate.

The other method is the one Mid Tompkins and his clan of international champions uses, and that is soft-seating the bullet out too far and allowing chambering the round to finish seating the bullet. This used case necks sized loose to allow you to move a bullet in them by finger pressure alone. This has the limitation that you have to remember to tip your barrel up or turn the gun so the ejection port is at the bottom or some combination of the two to clear a live round from the chamber as the bullet will likely have stuck in the throat and you don't want to dump the powder into your action when you eject the case.

[ghbucky]

Of course the cynical part of me figures it could be a conspiracy to get you to burn you barrel out faster trying a multitude of loads.

It's trigger time!