theoretical question on pressure (compressing the load vs seating on/near lands)

[whysman]

Newbie question here....

I realize there are many variables involved, but generally speaking what effects pressure more, compressing the load or close proximity to the lands?

Thanks all!

 

[BoydAllen]

I am not aware that moderate compression of powder by the bullet as it is seated has any effect on pressure. As far as proximity to the rifling goes, it is commonly reported that there is a difference of 5-6,000 CUP between seating off the rifling, and touching or into. I can say my experience has been that as I load longer than touching, pressure goes up more as I get farther into the rifling. Bullets take more pressure to engrave when they are touching or into the rifling, as opposed to having a run at it.

 

[Nomad47]

Right on, Boyd. QuickLoad says to add 7200 PSI to the shot start pressure when seating at or into the lands.

 

[Shynloco]

Excellent question and great to know the answer. I personally don't compress any of my loads, however do seat bullets both off and into the lands depending on the specific rifle and it's like and dislikes that I've learned while testing. And I can say with relative certainty, the closer I got to and into the lands, signs of pressure became more apparent as I neared a maximum load for each particular rifle while also seeking the seating depth that provided the most accuracy. Learned something new tonight for sure.

 

[BoydAllen]

Kind of a funny story:
An older friend of mine tells me that he has never gotten good accuracy loading into the rifling. See if you can figure out why. He works up a good powder charge off of the rifling, jumping a little, and then starts loading longer at a time until he is touching and then into the rifling. Every time he does this, accuracy gets worse as he goes into the rifling. I hasten to add that he generally finds a load that works, but all have a little jump.

 

[gotcha]

tsk tsk! Boyd, You said you'd keep that a SECRET!

 

[whysman]

Thanks for the great insight guys. I sincerely appreciate your thoughts on this.

Here is a follow-up question to chew on....

If I am, for example, .030 off the lands and not compressed, would you say that if I compress moderately and go to .050 off the lands, then my pressures should be reduced? I would think so given the comments, but I am sure that there is a point of diminished returns. Again, I realize this is just hypothetical and depends on the specific load, etc... (in this case, I am tinkering with a .308 with a long OAL).

As a follow-up question (and I am sure this is covered elsewhere, so accept my apologies) but what is generally considered a "long jump" vs just off the lands? I would think a few thousandths off would be "just off" but does anything more considered a "jump"? I just want to see if I have the terminology down.

Thanks again!

 

[bheadboy]

in my experience Boyd is dead on. to add to his comment of pressure building as you touch, i find that baciking off on powder level keeping the pressure within reasonalble range, not sharply elevated i find the most accurcy. if you keep the power level near max (non touch) and move the bullet out i find the pressure becomes excessive and accurcy falls off

your second question -some powders seem not to be bothered by compression, others seem to burn differently. i do not think backing the bullet off additionally will change the way they ignite, but would alter the way the pressure curve develops as the bullet free floats longer.. this may be false evaluation howerver, i have experience slower powders highly compressed require magnum primers to burn consistantly.

Bob

 

[BoydAllen]

Let me put in a plug for something that relates to all of this, working up loads at the range.
Sometimes I think that many of you are trying to figure this out at home, put together some loads, and go to the range to test. In addition, there seems to be huge resistance to doing anything different. As a result, unless they get lucky, shooters spend an inordinate amount of time, gasoline, reloading components, and barrel steel in an attempt to find out what shoots best in a particular rifle, and if truth were known, probably never get to its ultimate potential. Tune depends on many variables, some of which we only experience the effects of without fully understanding in a way that allows accurate prediction of outcome. That is why short range benchrest shooters load between matches, and some of the very best in the world go to the line, for a match with more than one load, and choose what they will use after looking at how things come together on the sighter target. If you don't use wind flags, and have not loaded at the range, let me suggest a new adventure. Try it, with your most accurate rifle, and use a SYSTEMATIC method for working up loads, that does not involve wasting a lot of rounds shooting loads that look bad after the second one is fired. I think that if you pay attention to what you experience with this approach, your time to find a load, and understanding of the variables affecting tune will improve greatly. I know that many kinds of competition are not structured so that you can adjust loads, or even clean between matches, but for load development, there is a clear advantage to loading at the range. End of sermon

 

[Nomad47]

whysman, when speaking of compressed loads it is generally understood that powder is added and seating depth remains the same. You mentioned moving the bullet deeper in the case to create a compressed load. Moving the bullet deeper changes the case capacity and will increase pressure. But that pressure will be somewhat negated by the longer jump.

 

[Steve Blair]

Boyd, how would you know where barrel time nodes, optimum seating depth and low ES are without shooting the load?

 

[BoydAllen]

I shoot them, I just don't waste any shots if two look ugly. Looking good at two is no guarantee of great accuracy, but if it starts out ugly....
For short range benchrest, ES is not nearly as important as for long range, where it is very important. Generally, I find that the position of the bullet holes, relative to each other, tells me what I need to know. With a new rifle, once I have one good load, with a given weight of bullet, I may chronograph it, so that I can have a reference velocity to load to if I want to try a different powder. This assumes that for a particular combination that accuracy nodes are related to velocity. None of the short range benchrest competitors that I know of consider barrel time persae, and there is not much done with chronographs either. I use a chronograph and pay attention to ES when helping friends work up loads that we know may be used at long range, and our results have been good. We have done this very efficiently by taking the whole kit and caboodle to the range. I usually run the loading, and they only have to shoot. If the rifle is a good one, we usually go home with "the load".