bump and into the lands?

[Steve]

Ok, educate me here, maybe I am actually doing this and call it something else.
When reloading, lets say you have only one gun in a specific caliber. Ok you fire your gun and keep your brass, now this brass should fit your chamber. You have neck sizing dies with different bushings for your desired neck tension.
Say you want 4 thousands neck tension or 2 thousands, doesn't matter for my question coming but stick with me here, so you put your bushing in that will give you that amount of tension on the bullet.
Ok, I hear some people say after so many firings with that select brass they bump the shoulder back x thousands, wouldn't the very next shot put you right back to your chambers dimentions?

Now into the lands question.
If you are setting your bullet to be longer than the freebore of your chamber by x amount...pick one...2, 5, 10 thousands or more...wouldn't your bullet given with light neck tension just actually be seated further into the neck of the case when chambered and only be tightly against the lands at the actual freebore length?

 

[britz]

as for the first question: It is my understanding that you can neck size for a few loadings but the case does stretch a little with each firing. This will eventually cause a sticky bolt while chambering the round. When you bump the shoulder this starts the process over again giving you the opportunity to neck size for several loadings again.

As for the second question... that's above my pay grade..???

 

[BoydAllen]

The need to FL size is determined by pressure level and number of firings. In Benchrest it is common to use a closely fitted FL die every firing. Beware of so called bump dies. Cases get tight and need sizing right in front of the head as well as at the shoulder, and a bump die only works on the shoulder and neck, not the back of the case. When used with factory chambers regular FL dies size excessively causing distortion of the case and significant reduction of fit. A custom FL die does neither. What caliber are we discussing?

 

[Cheechako]

Steve

What britz and Boyd said on the first question.

On the second, you are right. It is possible to seat "into the lands" only so much. A light neck tension will result in the bullet being pushed back into the case virtually to the point that the ogive is only touching the lands. More neck tension means that you may be able to push the bullet into the lands but the amount will be very small, measured in ten thousandths. Shooters have done tests which confirm this - contrary to what some may tell you. Even a bolt action cannot force the bullet "into" the lands by any significant amount.

JMHO

Ray

 

[Steve]

Ok, Thanks guys.

 

[Steve]

I may look into getting the chamber reamer used by my smith to make a special full length die or should I say just get him to make me one.

 

[BoydAllen]

The chamber reamer is too large to make a FL die. You would need a special reamer for that. Call Hornady about a custom die...much cheaper and well done.

 

[Fred Bohl]

Steve & Ray

The practice of seating the bullet to "jam,bullet seated out further than throat clearance)" into the lands has a potentially dangerous consequence. At some combinations of "neck tension,interference of neck ID and bullet OD)" and jam, the bullet will get stuck in the barrel and if the cartridge has to be removed without firing,such as for a "cease fire and remove bolts" command), the bullet will remain in the barrel. The case will extract spilling powder in the action and trigger and the relay will be delayed to clear the barrel and action. This combination of activities is potentially dangerous and may cause damage to the extractor, barrel and trigger even if done safely but in the rush of a delay.

The probability of such an occurrence is higher for long nose bullets,VLD and secant ogive types) than for tangent ogive bullets but is possible for both types. It is also not very predictable since the forces actually retaining the bullet in the neck are much more complex than the commonly discussed "neck tension,interference of neck ID and bullet OD)". These also for example include the hardness of the neck, deposits and distribution of powder residue and/or lube on the neck ID, deposits and distribution of powder residue and/or lube in the barrel throat, and the presence of contaminants on the bullet.

Passed experiments have shown that the force required to extract a bullet from a case is less than that required to seat it,20 to 40% less after 24 hours). Slightly increasing the seating depth 0.010 then extracting it 5 seconds later, the extraction force was found to be only 10 to 20% of the re-seating force.

Another and potentially more serious consequence of seating to "jam" is the high probability of causing excessive pressure when firing. Others have reported typical overpressure of 5,000 to 10,000 psi for "jammed" versus "jumped" seating of the same load. I witnessed a trial that was conducted on a 6x47L using a HV barrel equipped with the RSI - Pressure Trace. A load developed slightly compressed at "touch" seating depth showed 58,000 psi peak with no pressure signs. The same load seated to "jam" 0.015 showed 79,000 psi peak with primer fall out and start of case head separation.

 

[Forum Boss]

Fred raises some important points. Need to clarify some terminology though,and that's not Fred's fault). Short-range BR shooters often refer to "jam" as that seating length to which, having seated the bullet very long, the bullet is pushed back by the rifling. In other words, the maximum length the bullet can go into the rifling.

I don't like that terminology and I've also found that this length is not particularly repeatable,because of the many factors Fred enumerates). This "jam length", let's call it "max jam" can vary based on neck tension, jacket hardness, many other things. I think it is imprecise for that reason. It is also hard to really measure, because most of the people loading to "max jam" can't extract a round without leaving the bullet stuck in the rifling.

For that reason I think it is better to refer to the amount of "jam" as the distance beyond first bullet to rifling contact.

Using this concept of "jam", you'll understand that when reloaders refer to "jamming" they'll be talking about seating to a point,any point) past initial bullet jacket to rifling contact,past "just kissing"). Then you must add the "amount of jam", which could be .005, .010" etc.

Fred also talks about some worst case scenarios, and it's important to be mindful of safety considerations. "Jamming" the bullets,using the second definition) will have the effect of raising start pressure. However, the effect is not always as dramatic as Fred explains from the one example. If reloaders adjust their loads carefully, a BR shooter can often attain improved accuracy by seating bullets so that the bullets are engraved by the rifling. With many modern bullets, .010" into the lands is actually a good place to start,if you load down accordingly).

But again I want to stress that here, when we are talking about .010" "jam" that means ten thousandths past initial bullet-to-rifling contact. This is not .010" past "jam" as some short-range benchresters use the term. I really wish that use of "jam" was replaced by "absolute max" or something like that, to eliminate any confusion.

 

[Cheechako]

I don't think I would characterize experimenting with jam as flirting with a "potentially dangerous consequence". It's been my experience that any load that is reasonable and safe is little affected by the degree of jam, whether it is .050" in or .050" out.

But, the range is not the place to experiment. Do it at home with empty cases. For any particular neck tension it is easy to determine the maximum jam possible without sticking the bullets. That leaves only one direction to go when at the range.

JMHO

Ray

 

[billmo]

Ray, I think you are correct in your asumptions but only if testing has been done with the empty cases as you have mentioned. Another method of testing that I have used is to load 15 cases, 3 - 5 shot groups ) with everything equal as far as I can tell including the same amount of powder in all 15 cases except for the first 5 five cases would be seated .010 out of or from kissing lands, the second 5 kissing lands, and the third 5 .010 into and past kissing lands. You must chrono all of these loads starting with the 5 cases .010 out or from the lands first and watch for MV increases as you progress to the next 5 shot groups. In my testing with this method I have not seen more than about 50 fps increase. Neck tension was @ .002 for all loads as well. Accuracy has always been better either at .010 out or .010 in the lands rather than just kissing lands. My testing was done with a custom 6 X 47 Shehane 28" barrel 1 - 8 twist with 41.0 gr. of VV-N560 underneath a 105 gr. VLD Berger bullet with 450 CCI MRP. ********* Warning ********** Any initial loadings should be worked up to with this load. Bill

 

[Fred Bohl]

Paul,esteemed Moderator/Editor in Chief)

I found the following in an email from me to you on 1/5/08 regarding the terminology problem you raised in post #9 of this thread:

- - - - -
One of the things that seems to promote the ugly exchanges IMHO is the semantics issues or at least a lack of common terminology,didn't Winston Churchill once say that "England and America are two great nations separated by a common language."). The most confusing and oft repeated issues revolve around reloading terminology. Maybe you could start up a reloading glossary. A good starting point might be to solicit input from the die makers such as: Forster, Harrell, L. E. Wilson, Lee, Neil Jones and Redding while you are at the Shot Show. I would also suspect that you could get valuable input from the shooters and gunsmiths that are regular contributors to the site and/or forums. I think it would be in the interest of all precision shooters, reloaders and the suppliers thereto to have a common glossary of terms to provide a more clearly defined set of basic definitions to facilitate clear and concise mutual communication.
- - - - -

At both "point-blank 100 & 200 yard" group Benchrest and Hunter Benchrest matches I've seen and experienced a small but troubling number of bullets stuck in barrels when an unexpected "cease fire and remove bolts" command has occurred. In several cases the recovery looked more like a "Chinese Fire Drill" than a safe and orderly anticipated procedure.

More disturbing and the reason I responded as I did is the steadily increasing number of rookie reloaders I see at our club range "jamming" bullets in new barrels, factory barrels, and re-chambered factory barrels and having no clue how or equipment to remove them. I do assist them and we keep the necessary gear at the range. When discussing the problem post event with them, the common thread is that they read a reloading article or forum post on how the "jam and upper load window" was the place to start for accuracy and/or small groups. Of course none remembered any cautionary notes on how to get there safely or what the consequences may be.

Ray

Yours is the voice of reason and experience and I would tend to agree with you and your method. However, the young and inexperienced beginning reloaders seem to be in too big a hurry for instant success and are big believers in the philosophy "If all else fails read and follow the instructions".

 

[Cheechako]

Fred . . .the young and inexperienced beginning reloaders seem to be in too big a hurry for instant success. .[/B] .

Fred

So true. The Internet is a wonderful thing but it is producing an entire generation of those seeking instant gratification in all things. Not only that but much of the information posted is downright scary. I have quit more than a couple of "shooting" sites and forums because I have a hard time holding my tongue.

Ray

 

[Fred Bohl]

Ray

First thank you for not abandoning this forum. You may be correct that the Internet is the core of the problem, but some of the shooting magazines are also making dubious claims and do not include sufficient detail or cautionary information particularly in reloading articles,warnings don't sell ads).

Bill

That is the useful kind of detail particularly with that great warning reminder about working up to the load that is missing from many posts and articles. Thank you for that example.

 

[billmo]

Thank you Fred, for the great comment. I agree with you totally that SAFETY should be first. It is our responsibility to the new and inexperienced handloader to include a warning anytime that we post any load data. Bill

 

[Obtunded]

This was an unexpectedly interesting thread! I am as guilty as the next person for using the land-to-jacket contact as the basis for anything beyond being referred to as "jam", but I also have a good understanding of what is happening.

I have experimented with measurment of COAL on 6BR cases using 2 thou neck tension and tried to establish the point at which the bullets,Berger 105 VLD) ceased to extract or extracted with eveidence of being driven back in the case.

I did so on the heels of a fellow shooter who was showing astonishing results by jam seating his bullets with very light neck tensions.

Using Krieger 4-groove .236 bore barrels, I never had a bullet stick in the barrel, but 40 thou beyond the point of first contact, I was only driving bullets into the cases. I concluded that 2 thou neck tension did not allow enough resistance to the bullet to allow it to become wedged into the barrel.

I had not contemplated the effect of time on the elastic recoil of the case necks and this is a very interesting point!

Thanks you guys for some very interesting readng!

 

[Forum Boss]

FYI, I've found with Scenar 105s in a 3-groove Lilja barrel, somewhere between .025-.030" past "first contact". Beyond that the bullet would push back or I had trouble closing the bolt.

I experimented with .020-.025 in the lands. 100-yard accuracy was good but the ES and SD spiked. With bullets seated about .012 into lands I was able to maintain 5-shot ES in low teens with low SD.

 

[Steve]

Thanks guys.