Jump, jamb, FPS and ES

Status
Not open for further replies.
JCSC

JCSC

Member
Joined
Jul 24, 2019
Messages
597
Location
Columbia SC
Will a jumped bullet have different speed and chrono numbers vs a bullet that is .010 in to the lands? I understand everything has an effect, but I am curious if there are any general rules.

ex: a jamb bullet will be faster, due to more complete powder burn? FYI This is just a fictitious example.
 
Oh man, here's another subject with some twists and turns.

Yes, a jammed bullet will have a higher pressure than a bullet seated back off the lands, and it's more to do with overcoming inertia with an accompanying pressure increase. A jumped bullet gets a bit of a head start (see leede, throat, freebore) with pressure buildup before hitting the lands and groove engraving starts. Just how far back off the lands does pressure becomes "normal"? Read the attached: Internal Ballistics - Hornady Manufacturing, Inc

A jammed bullet is not faster due to causing "more complete" powder burn behind it. One of the advantages of a long rifle barrel is that we can burn slower powder longer behind the bullet including right out of the barrel. Powder should be matched to barrel length, i.e. shorter barrels, faster powders and vice versa. There are all kinds of sites that offer advice on powder and barrel length including in this forum.
 
I never jam anything, in fact I always start 20 thou off the lands and work back further. This was further reinforced by testing on my part, starting at 20 thou off, then increasing my jump 3 thou off as far back as I want to test. As bullet ogives keep changing and getting better there is more and more evidence that a 30 or even 40 thou jump is actually best. Berger hybrids are a great example of this.

If you want a very in depth and accurate video of this, I suggest looking up Eric Cortina's video on YouTube.
 
The cause for my question was a pair of loads that I had and their varying FPS. Both had identical powder charge and bullet. The variation was one was jammed in hornady brass with LRP. The other was .015 jump in lapua brass and SRP.

The jammed round was faster.

I didn’t figure the brass would have much affect, so this left the primer and jump as the culprits in my mind.
 
I did a test years ago when I got my crony on seating depth. Pressure increases when the distance from the lands is 0.020" or less, velocity increases too. Once you back off 0.020" the velocity did not jump near as much. It did increase some due to pressure increase with less volume in the case. But it was not enough to put you in a high pressure condition. I did not notice any SD or ES change.

I normally start at 0.030" off the lands when I start load workup, or max mag length. After I have found my most accurate load, I then start playing with the OAL to see if it improves the group. After that depending on use I do a primer test with the same load.
 
The pressure curve on a rifle can be U shaped. Shove the bullet way back in the case can raise pressures, even with a lot of jump. Some jump can allow pressure to get past the bullet and give it a running start before it hits the lands, both can lower pressures vs having the bullet pushed into the lands, despite having more volume in the case.
 
jmorris said:
The pressure curve on a rifle can be U shaped. Shove the bullet way back in the case can raise pressures, even with a lot of jump. Some jump can allow pressure to get past the bullet and give it a running start before it hits the lands, both can lower pressures vs having the bullet pushed into the lands, despite having more volume in the case.
Click to expand...

Yes, I should have mentioned that seating too deep can really foul up your day.
 
If I wanted the exact answer to those types of questions I would buy and read the applied ballistics books. I'm sure he covers just about everything. It's on my reading list just havent got there yet.
 
JCSC said:
The cause for my question was a pair of loads that I had and their varying FPS. Both had identical powder charge and bullet. The variation was one was jammed in hornady brass with LRP. The other was .015 jump in lapua brass and SRP.
Click to expand...
So, they really had nothing in common then.

JCSC said:
I didn’t figure the brass would have much affect,
Click to expand...
Did you measure to know?
I find brass to be quite influential.
Less volume, more pressure.
More pressure, usually more speed.

Some rifles I jam, most I don’t. I know why I do it, with the ones I do.
The factors that are included in my decision are,
Purpose for the rifle and load.
Accuracy.
Optimal barrel time from ignition.
Accuracy.
Conditions of ammunition transport.
Cleanliness of action and throat.
Accuracy.
Condition of throat.

Notice muzzle velocity wasn’t on there.;)
 
DukeConnor said:
Nobody knows. It is just theories
Click to expand...

All of that can be and has been measured using test barrels. There are more variables than meet the eye though. Along with the OP’s variable, burn rates, friction of the bullet and bore, length of barrel, all can certainly have an effect on muzzle velocity, that can be proven and repeated beyond theory.
 
Demi human
You are correct they had several differences. Stupid question....

I’m learning here with the chrono and didn’t jam for velocity. It was hopefully for reduction of group size with a match bullet.

thanks JC
 
JCSC said:
Demi human
You are correct they had several differences. Stupid question....

I’m learning here with the chrono and didn’t jam for velocity. It was hopefully for reduction of group size with a match bullet.

thanks JC
Click to expand...
Jam is used on certain bullets namely vlds. You may be interested on looking at secant vs tangent ojive shapes and it may help understanding why the choice.
 
No scientific charts graphs or any of that stuff..just a hunch.. But..maybe fps increase or variances as the bullet gets jammed into the riflings or compressed as the bullet is jumped is something that just gets lost in the white noise of too much information when all we are really doing is changing the barrel timing slightly through pressure changes resulting in group shape and size variances.

Whatcha think ??
 
I've seen a chart where pressure vs distance to lands was ran. It was at 0.020" where it started changing direction by shorter jump and less jump ( higher pressure due to smaller volume). It's been decade since I've see that graph. I pretty sure it was in the front of one of my hand loading books But I could not tell you which one, and some have been lost over the years too.
 
JCSC said:
Stupid question....
Click to expand...
Not at all. :thumbup:
There was just more to it then that, with that test.
If one is testing for velocity, or accuracy or whatever, everything should be as exactly the same as can be, save for the single test variable.

JCSC said:
I’m learning here with the chrono and didn’t jam for velocity. It was hopefully for reduction of group size with a match bullet.
Click to expand...
I haven’t used my chronograph very much, not as much as I thought I would.:oops:

Something I notice a while back when doing a load work up...
All: Lapua brass. (REDACTED)grains CFE223 CCI450s Hornady 75 BTHP
Variable: seating depth CBTO length.
5: 1.929”=+.016 interference
5: 1.926”=+.010
5: 1.919”=+.003
5: 1.916”= 0
5: 1.910”= -.006
5: 1.906”=-.016
5: 1.896”=-.026
5: 1.886”=-.036 jump
I don’t know why I chose the numbers I did instead of fives...:confused:
I shot these shortest to longest. The groups got slightly tighter the closer to the lands they were.
Those that were right on, .003 in and .010 in were visually the same shape and size, just printing slightly higher in relation to point of aim.
Whether this was from my barrel releasing them slightly later in the harmonic wave, or they had slightly less drop from increased speed, I am unsure. I need a LabRadar!;)
The one with the most interference was slightly larger, perhaps from a pulled shot, but the firing pin hole primer extrusion was looking rather proud, so judged “too much”.
So it does increase pressure slightly. But I think there is a limit to it. The bullet will go down the bore eventually, and the amount of powder is the ultimate source of pressure, whether that makes a little more speed, or sticks a bolt or pops a primer out is more dependent on the powder charge.


Whew! That was a lot. Which brings me to the important question...

How did it shoot?:)
 
Blue68f100 said:
I've seen a chart where pressure vs distance to lands was ran. It was at 0.020" where it started changing direction by shorter jump and less jump ( higher pressure due to smaller volume). It's been decade since I've see that graph. I pretty sure it was in the front of one of my hand loading books But I could not tell you which one, and some have been lost over the years too.
Click to expand...
I proved it again not long ago by changing two things and not working back up again. Always, always, drop back down if you are in the lands and change something, especially if it's the bullet. ;) (Fire forming load by the way) :)
 
Blue68f100 said:
I've seen a chart where pressure vs distance to lands was ran. It was at 0.020" where it started changing direction by shorter jump and less jump ( higher pressure due to smaller volume). It's been decade since I've see that graph. I pretty sure it was in the front of one of my hand loading books But I could not tell you which one, and some have been lost over the years too.
Click to expand...


3C549A4D-CB3C-4BBD-9116-AD47C49C4A6C.jpeg
 
When a bullet is jammed into the lands, it's like riding a bicycle. When you were a kid, did you ever roll your bike's front wheel right up to a curb, and then from a standing start try and jump the curb ? It's nearly impossible. All that's needed to get over the curb is a small "head start". Once the bike is rolling, hopping a curb is easy. It's the same with a bullet and the rifling.
 
Demi-human said:
Not at all. :thumbup:
There was just more to it then that, with that test.
If one is testing for velocity, or accuracy or whatever, everything should be as exactly the same as can be, save for the single test variable.


I haven’t used my chronograph very much, not as much as I thought I would.:oops:

Something I notice a while back when doing a load work up...
All: Lapua brass. (REDACTED)grains CFE223 CCI450s Hornady 75 BTHP
Variable: seating depth CBTO length.
5: 1.929”=+.016 interference
5: 1.926”=+.010
5: 1.919”=+.003
5: 1.916”= 0
5: 1.910”= -.006
5: 1.906”=-.016
5: 1.896”=-.026
5: 1.886”=-.036 jump
I don’t know why I chose the numbers I did instead of fives...:confused:
I shot these shortest to longest. The groups got slightly tighter the closer to the lands they were.
Those that were right on, .003 in and .010 in were visually the same shape and size, just printing slightly higher in relation to point of aim.
Whether this was from my barrel releasing them slightly later in the harmonic wave, or they had slightly less drop from increased speed, I am unsure. I need a LabRadar!;)
The one with the most interference was slightly larger, perhaps from a pulled shot, but the firing pin hole primer extrusion was looking rather proud, so judged “too much”.
So it does increase pressure slightly. But I think there is a limit to it. The bullet will go down the bore eventually, and the amount of powder is the ultimate source of pressure, whether that makes a little more speed, or sticks a bolt or pops a primer out is more dependent on the powder charge.


Whew! That was a lot. Which brings me to the important question...

How did it shoot?:)
Click to expand...
All the jammed bullets shot decent, but not anything spectacular.
The lapua loads showed promise. These where the first firings and sub MOA on all 5-5 shot loads up to 41.5 gr of H4350. I was unhappy with ES on them though, so the search continues.

B230C127-A89F-418A-9BEE-D76D96CF76A8.png 9DA6B569-5BB7-45E0-AB46-49B66F31C01F.png C586A55C-BB5A-42C2-BB24-A1D1C1905CFC.png
 
Status
Not open for further replies.

Similar threads

MDof2
  • Locked
Hornday 6.5mm 140 ELD M Bullet Jump
Replies
2
Views
2K
MDof2
MDof2
Andrew Leigh
  • Locked
Is bullet jump a mechanical or pressure phenomenum?
2 3
Replies
50
Views
3K
Andrew Leigh
Andrew Leigh
B
  • Locked
Pressure
Replies
1
Views
761
Ol` Joe
O
Suedenflames68
  • Locked
9mm workups with chrono
Replies
19
Views
975
Average Joe
A
LiveLife
  • Locked
Plated bullet differences - Mythbusting at 25/50 yards
2 3 4
Replies
85
Views
17K
Walkalong
Walkalong
Back
Top