Jump or jam, how does it affect the results?

[SteveOak]

Lots of good information. Especially interesting was the response of barefooter56 who finds the optimum OAL first and then tunes with powder charge. That method would seem to have a lot of merit as whatever volume is defined buy OAL is not changed after the work has been done finding a charge that shoots well.

This seems to argue in favor of there being two effects in play. One being the effect on powder burn, the other being mechanical.

Unfortunately this thread has kind of veered off from the question. The original question was, why does jumping or jamming or changing improve grouping? What is the mechanism?

 

[spclark]

What is the mechanism?

System’s harmonics, why tuners have become in vogue for center-fire after taking over in rimfire.

It’s all about vibrations.

 

[SteveOak]

System’s harmonics, why tuners have become in vogue for center-fire after taking over in rimfire.

It’s all about vibrations.

So you are saying to ignore jump or jam and get a tuner?

 

[spclark]

So you are saying to ignore jump or jam and get a tuner?

No, not exactly.

What I’m suggesting is that barrel tuners have become legitimized as one means to improving consistency in accuracy when other, more traditional means have proven to be either not as dependable or not as successful as expected.

They’re certainly useful on a day-to-day basis when ammunition being used cannot be ‘managed’ (i.e., rimfire) with regard to load profiles. I don’t shoot benchrest but am aware tuners are being employed with greater frequency now, where shooters have traditionally relied upon more arcane methods for tuning loads to conditions.

Tuners obviously have found a place in the toolkits of some shooters desiring improvements in reliable, consistent accuracy. Whether you choose to join them is entirely up to you! If you feel that move would perhaps further your goals, let us know what you learn!

 

[jamesdmock]

All interested in this topic should read today's Daily Bulletin (Feb. 8th).

 

[DanSavage]

I remember not long ago all you heard was don't jam a bullet in the rifling or you will get a pressure spike and blow yourself up!

In my experience with 3 different rifles moving the bullet in the case more gave more velocity or pressure,, moving the bullet out of the case it slowed down a little UNTIL the bullet was at the land's then the pressure went back up,, about to where pressure was with bullet seated into the case fairly deep.

 

[DanSavage]

There is also the theory that a bullet being jumped May not be as accurate,, the powder explodes, bullet hits rifling causing minute disturbance to the rifle before the bullet can exit the bore.

A jammed bullet is already seated at the land's and disturbs the rifle less as the bullet is getting out of the barrel.

But like said the rifle will let you know what it prefers. Most of mine like .005 off or a little bit jammed.

 

[Fred Bohl]

Tangent and hybrid ogive bullets produce <0.300 groups at 0.010 to 0.020 jump relative to touch point for 1800 rounds. Secant ogive bullets produce the <0.300 groups at soft seat jam for 1200 rounds.

Per an email that implied doubt about the above being suspect even numbers......

The testing method was to shoot 25 rounds as 5 relays of 5 shots for aggregate each trip and alternate trips shot at 100 and 200 yards for each configuration. Therefore:

Tangent and hybrid ogive bullets produce <0.300 MOA groups at 0.010 to 0.020 jump relative to touch point for 1800 rounds (aggregates 1-72 were good aggregate 73 failed).

Secant ogive bullets produce the <0.300 MOA groups at soft seat jam for 1200 rounds (aggregates 1-48 were good aggregate 49 failed).

I hope that clarifies the results.

 

[retired]

i can understand that, if you are shooting sierra bullets...
but just not true in competition benchrest/long range precision shooting.

I will tell you this, it will get me flamed I am sure, but years ago the Sierra tech told me that most "standard" designed bullets will shoot best at 20 to 25 thou jump.

With a exceptions being these bullet designs with the pointy noses, I have found that still works.

 

[dkhunt14]

The simple answer is shooting a lot with both barrels. Over the course of three seasons (here in Wisconsin at 68 years old when this trial started a season is April thru October) the two barrels were shot at a private range at both 100 and 200 yards. The criterion I use for barrel life is aggregate group size that would have been competitive back when I was still competing.

Tangent and hybrid ogive bullets produce <0.300 groups at 0.010 to 0.020 jump relative to touch point for 1800 rounds. Secant ogive bullets produce the <0.300 groups at soft seat jam for 1200 rounds.

The two barrels used were Krieger 4 groove SS 26 inch HV using my reamers.

I believe the difference is in the barrels. I have shot many different Dasher barrels, all 4 groove Kriegers. I always shoot in the lands and some barrels starting going away at 350 roumds. Some started at 500 rounds and some lasted 1000. I am talking BR Accuracy. I truly believe it is barrels because they all shot the same lot of powder, same primers and same bullets. Also the same load. Matt

 

[bobcat93]

I do the dowel rod method with ever bullet im gonna use..
ill open my bolt , slide wood dowel down barrel and make sure rod is on bolt face , close and lock dolt. Mark dowel
Remove bolt, drop bullet in chamber, (gently) hold bullet against lands with another dowel from rear of rifle
Now i push the dowel back in barrel untill it contacts bullet and mark the dowel.
This is the most accurate way to measure C.O.A.L
In my opinion
I will start with a distance off the lands and climb back up to my C.O.A.L.
And see what my target tells me to do.

Get this book if your about rifle accuracy
it my Bible and full of amazing information

High Power Rifle Accuracy: Before You Shoot https://a.co/d/dPoBqZ9

 

[William Vogel]

Case capacity is increased by seating the bullet farther out....BUT, this length is limited to an overall length that will feed through a magazine.....thus Weatherby avoids the jam. Maybe some on here has one of the Oehlers that are capable of measuring pressure and can determine this theory by testing. James

It’s only limited IF you feed through a magazine. Then the limitations are freebore length and bullet to case seating depth. I shoot 223 that’s 2.645 coal.

 

[dusterdave173]

I will tell you this, it will get me flamed I am sure, but years ago the Sierra tech told me that most "standard" designed bullets will shoot best at 20 to 25 thou jump.

With a exceptions being these bullet designs with the pointy noses, I have found that still works.

Brad made my barrel so I asked him this question about 80.5 bergers
He says 020 out 23.5 Varget
First five rounds made a tiny little group
I am hard headed so I worked it from .015 in to .050 out and guess what? Brad was dead on the money at 020 out on this chamber / bullet combo
I don't need Quickload now I will just call Brad--so...just another Perk if you buy from Urban Rifleman Barrels

I don't know the reasoning but depth tests are crucial to finding that perfect load--I depth test first then powder Loading has become easier since I started doing it this way Some combos are forgiving some are so sensitive it is amazing really

 

[SGT. Slaughter]

As far as the seating depth goes, exit timing of the bullet in the barrel in it's harmonic wave will dictate in tune or out of tune. Small groups or not so small groups you decide. Seating depth whether you jump or jam is very important.

 

[urbanrifleman]

Brad made my barrel so I asked him this question about 80.5 bergers
He says 020 out 23.5 Varget
First five rounds made a tiny little group
I am hard headed so I worked it from .015 in to .050 out and guess what? Brad was dead on the money at 020 out on this chamber / bu

llet combo
I don't need Quickload now I will just call Brad--so...just another Perk if you buy from Urban Rifleman Barrels

I don't know the reasoning but depth tests are crucial to finding that perfect load--I depth test first then powder Loading has become easier since I started doing it this way Some combos are forgiving some are so sensitive it is amazing really

"Nobody knows..."

It's true.

 

[David Milisock]

Here is the essence of my question. Varying the OAL of a cartridge, either pushing the bullet into the rifling (jam) or holding it short (so that the bullet jumps to the rifling) is commonly used to fine tune accuracy. Can anyone tell me what is being affected by the change and how does that effect translate into a change in accuracy?

I can think of two general categories of effects;

Powder combustion
Bullet alignment

Some possibilities I have pondered;

Causing the rate of powder burn to increase or decrease
More complete ignition earlier in the combustion cycle
Higher pressure before bullet moves
Higher pressure before bullet moves significantly
Bullet is more centered in the bore
Bullet is free to self center

It is interesting too that the testing to determine optimum OAL is commonly performed after a primer/power/charge weight is determined. That would seem to indicate bullet alignment rather than powder charge as the mechanism.

Secondarily there is the question of why is it not universal? Sometimes, seemingly with everything else being the same, cartridge, powder, bullet, primer, charge etc. jamming or jamming harder improves accuracy while other times jumping is the answer. If I remember correctly, which happens less and less often, certain bullets are more likely to respond positively to jamming and some are more sensitive than others to jumping or jamming.

Jumping will cause a more sloped leading edge to your pressure curve. Jamming will cause a more steep leading edge to the pressure curve.

How the bullet will react requires individual testing.

 

[SteveOak]

Thank you @David Milisock

 

[urbanrifleman]

It is most likely the shape of the bullet nose that requires jam or jump at various dimensions.

 

[Fire Dog]

System’s harmonics, why tuners have become in vogue for center-fire after taking over in rimfire.

It’s all about vibrations.

Good, good, good…. Good Vibrations…

 

[Tomiboy]

Bullet Jump Research: Executive Summary & Load Development Tips

This is the last post in a series of articles focused on bullet jump research that has been conduct...

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