? Why will Jump have higher pressure than Jam ?

[necchi]

??? I read this in a different post;

In my rigs, I'll show pressure with a given load if the bullet is "jumped" to the lands. That same load with the bullet jammed "into" the lands won't show any pressure and I'am able to boost the charge without and problems.

I believe the poster, I'm not trying to counterdict at all, just trying to wrap my head around the "why for".
Given the same charge and neck tention, obvious no crimp, Why does this occur?
My brain says "bore obstrution" pressure should be higher.
Can someone of ya explain the phenomina? ???

 

[NorCalMikie]

From what I read a while back, it has something to do with the way the pressure spike builds as the powder is burned. By jamming the bullets into the lands, the pressure builds a little different than if the bullet has to jump to the lands.
Not really sure how or why but it works for me in my rifles so I make it a point to jam everything. (not including my M1A) The larger calibers seem to like a jump to the lands??
My shooting buddy has a HS Prcision in .308 that has a .150 to .200 jump and is a bug hole maker. Go figure.
What works for me may not work for you so each person needs to decide what works best in their rigs. It'a all a learning game and if we're not learning something new, we're in a rut. We're never too old to learm something new. JMHO, Mike.

 

[jonbearman]

I would field this question to someone like Germa salazar on this forum.Tony boyer or any of the real super knowledgeble people here.

 

[Rust]

There has been some experimentation with seating depth vs pressure done with a strain guage pressure system which operated at a much higher frequency (meaning smaller time divisions between measurments) and what was found seems a little coutnerintuitive. With most jacketed bullets, pressure did not spike when seated into the lands. The reasoning was that pressure built up until the bullet started moving and the bullets kept moving. With bullets not seated into the lands, the bullet would hit the lands and stop momentarily, thw powder burn would be further along at that point in time, and pressures would rise higher before the bullet would start to move again. This with the same powder charge.

There would be many contributing factors: thickness of the jacket, hardness of the jacket, hardness of the lead core, angle of the leade of the rifling, angle of the bullet where it would meet the lead, surface finish at the engagement point between the bullet and the leade of the rifling, profile of the rifling (land and groove depth and width), pressure curve of the powder (pressure rise vs time), hotness of the primer etc.

Things like this are why factories are careful what they load and sell, and why reloading manuals tend to be conservative, they have to develop loads that will be safe for pretty much an conceivable application or misapplication. Which is why a well developed handload for a specific single rifle is tough to beat.

And why handloading should be done with a certain amount of caution. Think about it, what other sport involves setting off multiple explosions a few inches from your face. A given powder is safe (or as safe as it gets) within a cerain set of parameters given cartridge volume, caliber, bullet weight etc. Going too far over or under that safe operating envelope and life could get interesting real fast.

At 1,000 yards I'd rather be 1/2 moa tighter than 100 fps faster.

 

[minnesota]

Look at this web sight, I don't believe that pressures would go up when bullet has to jump, or barnes would recommend jamming into rifling.

http://www.hornady.com/ballistics-resource/internal

 

[Shynloco]

Rust,
I like your explanation! And I have to agree somewhat with what Necchi had to say. But there has always been ANOTHER unresolved ongoing issue and that has to do with powder and powder burn rates. I suspect that powders not only have different burn rates, but that those burn rates cause a difference in pressure. SO logically (and thats just my logic) pressures can be controlled by the powders we use. I'd kinda wonder what you thoughts are about issue Rust? I'd just like to know if perhaps it is the powder that that fellow is using that is creating the pressure during the jump. Thanks!

 

[Erik Cortina]

QuickLOAD always predicts higher pressures when bullet is seated to touch the lands. Start pressure is higher therefore Chamber Pressure is also higher.

The QuickLOAD instructions for setting Start Pressure read "WARNING: With bullet seated to touch rifling, add 7200 psi. to Start Pressure!"

So, as far as I know, and also by QL instructions, pressures are higher when bullet is seated in the lands than when jumping into the lands.

 

[HPMike800]

One day I chronod 3 cartridges 6BR(VLD) 260R(VLD) & 308W(SMK) at +0.010", -0.030", -0.060", -0.090". I only varied seating depth. Taking -0.030" as a baseline the jambed rounds were 15-25fps faster, the -0.060" were 1-2fps slower and the -0.090" were 2-4fps slower. All three cartridges showed the same pattern. I thought jambing would make more difference.

Just my experience.

 

[Forum Boss]

Erik is correct that QuickLOAD posits a +7200 psi when loading into lands.

However, I have discussed this issue with Quickloads creator, Hartmut Broemel and explained to him that, in my experience, one gets more accurate velocities by raising the default 3626 to 5500 psi when jamming typical long-range bullets.

Harmut concurred that my results "make good sense" and he believes the +7200 is probably to high, but it was chosen to "err on side of safety".

Reduced Case Capacity.
In my experience, the reason you see pressure increase when jumping is when you have fairly small cases and start impinging on the case capacity. Quickload actually does predict this increase (due to reduced case volume) pretty well, if you go from, say .010 away from lands, to .040 away from lands.

 

[tenring]

In his book, Tony Boyer says he always jams 6ppc loads,

Harold Vaughn writes in "Rifle Accureacy Facts", page 29: "The peak chamber pressure will drop about 1000 psi for every 30 mils (thousands) of additional distance (free run) between the bullet and contact with the lands in the throat. In other words, if you seat the bullet so that it has about 60 mils of free run before contacting the lands, the peak chamber pressure will be reduced by about 2000 psi. This means that the chamber pressure is not very sensitive to seating depth."."

 

[Erik Cortina]

In his book, Tony Boyer says he always jams 6ppc loads,

Actually, Tony Boyer loads with bullet in the lands, but does not jam.

 

[AlNyhus]

Shooters that actually work with stuff have long known that 'jamming' doesn't have the boogey-man effects on pressure that these old wives tales...and computer programs... would have them believe.

It's amusing that shooters will input data into a computer program and cite the results as 'fact' ....when many of the programs defaults (which contribute to the final 'fact' spewed out) are, in fact, ....just guesses.

Good Shootin'. -Al

 

[CanusLatransSnpr]

Necchi, I have a little different take on this issue that your having and I haven't seen anything mentioned of it here yet! What exactly are the pressure issues that you're seeing? Is it cratered primers, flattened primers, etc? You haven't made mention of the rifle your shooting: make, model, etc. What I'm seeing here, and this is just my opinion so please don't get upset, is that your rifle may have a little excessive headspace. Please listen to my rationale!! If when you jam a bullet into the lands, you don't see any pressure issues ie. flattened primers, cratering, etc. but, when you jump the bullet you see flattened primers, cratering. That would be indicative of excessive headspace to me. Think about it. When you jam a bullet into the lands, the cartridge head is held securely against the bolt face whereas when a bullet is jumped it is not. So when the cartridge head is not held securely against the bolt face, the primer "may" protrude from the cartridge and leave a cratered or flattened primers after firing. You smelling what I'm stepping in here ? Here is another idea. Take some of your once fired brass for that rifle and just neck size it and shoot it with the the bullet seated to jump and tell us if you still see any of the pressure issues that you were seeing before. I'm willing to bet that you will not because the brass is fireformed to your chamber and therefore no excessive headspace! Hope this helped!

Mike

 

[NorCalMikie]

Mike:I believe that necchi was refering to the post that I had made. All the brass is the same and sized alike. 6BR Lapua. Shoulder bumped .001. Bushing necked sized.
Happens to be on a Savage action with the head space set on the snug side to keep the brass from growing and more than needed.
Powder charge was aprox. 30 grains of AA 2230 with a 60 grain Berger.
With the bullets jammed .010 into the lands, everything is fine. That same charge with the same brass and the bullet jumped .010 would show pressure signs. Stiff bolt lift and flattened primers.
Stuff the bullet back into the lands .010 and the pressure signs go away and I could boost the powder charge even higher without any pressure signs showing. Accuracy is deadly.
Seems that it doesn't work that way for everybody but it works that way for me. Go figure. ???
That's the reason I jam all my small caliber loads.
Hope that explains things, Mike.

 

[necchi]

Thanks NorCalMikie,,
,,For adding the specific info.
So far this is an interesting thread with a few different ideas. As you stated this dosen't happen ALL the time

What's made sence to me so far was the powder reaction issues that Rust pointed out and the bullet characteristics.

 

[NorCalMikie]

Some times it'd hard to figure out "why" something works so you just go with the results. It happens "all the time" in my rigs but not with other folks or, they arn't saying.

 

[CanusLatransSnpr]

My mistake guys ! I was just trying to be helpful. In one of my Savage actions I was having a similar problem but the cause was two fold. One, there is excessive headspace on that chamber and two, the bolt face isn't completely flat. It is dished around the firing pin hole. I was getting flattened primers with factory Fed ammo. Just got me thinking about that when you said you were experiencing high pressure "symptoms" when jumping the bullet. Thanks for clearing that up though!

Mike