Secondary Pressure Indications

[ET]

Have been busy trying to make sense of secondary spikes seen in pressure traces by reviewing info RSI provides. Trying to break down the info for an easier review by extrapolating what I feel are key points has not been easy because of all the variables in play.

Secondary spikes exist more often than what we believe from what I read from RSI Tech Article.
A small secondary spike when repeatable in size should not be a concern if good results are obtained.

Searching through RSI Tech Articles a sentence found is this article makes the following statement.

“In every instance when secondary pressures are detected they can be eliminated by using a faster powder, heavier bullet or bullet with more bore contact area.”

In the last part of the statement about bullet weight and bore contact area I seem to relate this also meaning bore resistance to movement during pressure build up. So if a slower powder is being employed bore resistance needs to be increased to prevent the bullet from racing ahead of the burn to prevent or reduce a secondary spike. So then the rule used of tight is right is a good rule in this case.
Another factor would be amount bullet obturates and its contribution to bore resistance.

Now if we add Chris’s theory of bore expansion and contraction where the amount is governed by amount of peak pressure developed would affect bore resistance during the bullets travel down the bore. How much I don’t know?
So in the end a balance of the mentioned variables needs to be obtained to reduce or eliminate secondary pressure indications is what I’m getting.

If someone else sees something I missed or has another view of the interactions occurring please feel free to jump in and express your view. I’m striving to learn like others.

Here I might as well toss in a question for sabotless shooting.
Would a finer knurl over a coarser knurl increase bullet contact area?



Ed

[Savage Shooter]

Mar 3, 2011 1:17:22 GMT -5 ET said:

Have been busy trying to make sense of secondary spikes seen in pressure traces by reviewing info RSI provides. Trying to break down the info for an easier review by extrapolating what I feel are key points has not been easy because of all the variables in play.

Secondary spikes exist more often than what we believe from what I read from RSI Tech Article.
A small secondary spike when repeatable in size should not be a concern if good results are obtained.

Searching through RSI Tech Articles a sentence found is this article makes the following statement.

“In every instance when secondary pressures are detected they can be eliminated by using a faster powder, heavier bullet or bullet with more bore contact area.”

In the last part of the statement about bullet weight and bore contact area I seem to relate this also meaning bore resistance to movement during pressure build up. So if a slower powder is being employed bore resistance needs to be increased to prevent the bullet from racing ahead of the burn to prevent or reduce a secondary spike. So then the rule used of tight is right is a good rule in this case.
Another factor would be amount bullet obturates and its contribution to bore resistance.

Now if we add Chris’s theory of bore expansion and contraction where the amount is governed by amount of peak pressure developed would affect bore resistance during the bullets travel down the bore. How much I don’t know?
So in the end a balance of the mentioned variables needs to be obtained to reduce or eliminate secondary pressure indications is what I’m getting.

If someone else sees something I missed or has another view of the interactions occurring please feel free to jump in and express your view. I’m striving to learn like others.

Here I might as well toss in a question for sabotless shooting.
Would a finer knurl over a coarser knurl increase bullet contact area?



Ed

One of theories you have touched on above holds a certain amount of credence that I am just beginning to understand. That is the amount of expansion the bore suffers when under peak pressure and as you said it is a "balancing act".

i.e. As the bore expands (stretches during application of pressure) the bore is growing in diameter, so what started as a tight fit may not stay a tight fit during barrel expansion if the bullet is not obturating at an equal rate to maintain "fit". It would make sense that it is going to be harder to maintain that tight "fit" with tougher or thicker jacketed bullets.

IMO, if the pressure at some point during burn is just the right amount to stretch the barrel enough to (even if momentarily) "lose" grip on the bullet then the bullet is going to "outrun" or jump ahead of the burn rate.

Catch 22 is that as you increase pressure with intent of "kicking in the pants" to start, increase or maintain bullet obturation to increase gas seal you have at the same time increased bore expansion which increased the need for more obturation to maintain barrel "fit" with bullet.

One of my biggest fears is the VERY REAL possibility of an unobturated bullet getting "jumped" ahead of pressure due to barrel expansion which will create instant pressure loss causing barrel to instantly contract because pressure was lost and instantly again "grip / grab" bullet so pressure almost instantly jumps back up........now if this hypothetical unobturated bullet gets hit with this 2nd pressure at, lets say, 50+K as the 2nd spike occurs and now this pressure suddenly DOES obturate bullet due to the 2nd spike..........WELL lets say your barrel just got a lot SHORTER.

IMO the short of it is when shooting sabotless increasing pressure can create the need to increase pressure...........See the circle!

When the .45 hit the scene I thought the smaller bore would help a lot with the physical ability to easily make pressure......now this is correct but only to a point. Point being if you are using equal bullet weights to what we used in the .50's for adjusting pressures.

But as most have dropped bullet weight to 200gr, now the bullet weight is not equal but I still expected the 45 with saboted 200gr to be "similar" to the .50 with saboted 250gr and it is NOT. This shows by the tracing these guys have provided. Seeing the ability to shoot 50+ grains of N110, 4759, 5744 etc. at pressures less than what the .50 created at equal weights proves this IMO.

Again ET your observations are accurate and we need to remember what RB said:

"Secondary pressures often occur with the items you mention but I'd add there might be other factors in a muzzleloader such as very low bullet inertial resistance with light weight bullets".

I see a great need to determine if 2nd spikes being observed "are" pressure spikes or anomalies of other origins, i.e. gun handling, barrel harmonics, sabot disruption or other theoretical causes etc.

Bottom line regardless of origin some of the 2nd spikes we are seeing are VERY REAL dangers. Determining which is the challenge.

[edge]

I almost always shoot toward the low weight end of the bullets used. I find that with the exception of the fastest powders that we use I need duplex to avoid very large muzzle flash and generally inaccurate loads.

With rigid sabots and also sabotless using what I would consider "correct" powders I have not seen much in the way of large second pressure spikes.

edge.

[Richard]

I am going to offer my $0.02 on this pressure trace thing:
I have played with the PT and have been making observation on the TGinPA's results and listening to the comments by others. I think at this time most of us who have done a lot of shooting and playing with various high velocity/pressure loads, are pretty well convinced of their safety. In my case, I have achieved up to about 3200 fps with 200 gr. bullets using sabots and my gun is still in one piece My bulged barrel was MY fault (double loading) not the result of a high pressure load. I don't really see 95% or even more, of the shooters on this forum running their loads much above 3,000 fps. I say this because they have to be aware that a blown sabot on a once in a lifetime buck is not worth the extra 100 fps or so velocity That being said, we are looking at most shooters working in the 2700 to 2900 fps range.
All this chit-chat about extra pressure spikes and the like don't recall seem to be doing much in the way of improving what is already, great accuracy? If I watch my chronograph and develop loads (yes, by trial and error?) that give me low ES's and good accuracy, I could care less about those miscellaneous spikes
Yes, it has created lively discussion between a few members, but the bottom line is accuracy..................since we have already established a safe range of loads with numerous powders and bullets. Switching powders and mixing duplex's and triplex's around have failed to unsafely raise pressures, in my experience. Yes I know, my PT's have been suspect of low readings.............even so, the mixed loads and even my highest pressure loads with an additional 10,000 psi added, are not dangerous? Anyway, thats just my thoughts
Richard

[ET]

Richard

I welcome all comments to any threads I start here, as this is open forum for discussion.
Nor will I challenge or debate anything you’ve said here because this thread is about learning with the use of the PT unit and the significance of what it can reveal and can be learned from it.

You’ve done a lot of good work and research that has allowed you to reach your own conclusions. Some of us need to look a little deeper to what lies underneath before reaching a conclusion.



Ed