OzFclass

Dave, thanks for providing that link, it was an interesting read and confirms work I did at a similar time with a Border Pressure Tester lent to me by James C and a Pressure Trace which I have.

In a previous post, a while back now, I presented some temperature, pressure, velocity graphs based on the Palma round and the Radway Green round. These graphs reveal the instability in the first few shots which I put down to barrel temperature too.

I also commented on some of these earlier posts that the faster I shot from the machine rest at 1000 yards at Rosedale in another test, as monitored by Peter P the tighter the groups were in several barrels. This was like as fast as I could load and fire. The powder did not have time to heat significantly in the chamber and the barrel heat still stabilized in the heavy barrels. This particular test was conducted in 100 F degree heat. It is the very reason why I have advocated four warm up shots prior to conducting load development. I tend to chamber a round just before I make the decision to fire and don’t give it time to cook. It is also a reason for having a slightly positive compensation tune rather than a nodal tune to make a coach’s job easier during that period of instability.

If shooters wish to investigate this, rather than using an infra red heat sensing gun, Neconos supplies temperature indicators which you can see change colour and value and they are stuck on your barrel. Choose the Benchrest mid range as you will not see the barrel go much higher than 130 F on hot days with normal firing procedure. By that I mean taking a short time to read the conditions.

However, in load testing I use a delay or 45 seconds between shots which reveal to me the what the barrel will do as it cools in the worse case scenarios. This is just as important as finding what the gear is capable of because, how often do we have stop during a shoot? It’s important to have rhythm in shooting to maintain velocity if you are chasing centres.

That being said, ambient powder temperature is still a factor and by testing the ammunition at various temperatures on a target it will become apparent where stability areas lie with different powders. I found there was sufficient soak time for the powder to reach temperature relatively speaking with methods I previously mentioned, albeit a bit agricultural. It doesn’t matter so much because nodes or OCW tunes are reasonably velocity tolerant areas. It is better still if you can sneak a positive compensation tune in close to the nodes or OCW tunes. I had some very good Pressure Trace data that did not group in an untuned barrel. So the key to testing the powder effects is to start with a barrel tuned to that powder.

You may be tipped off the node if your test procedures have taken place in the cool of a morning. Conversely, if your nodes have been established in the heat of the afternoon it will take a big temperature change on the day of competition, in the afternoon to tip you out; but morning shoots of competition might not be as tight with the nodal tune. Each setup is different. David.

A while ago I did some testing on the temperature sensitivity of AR2225 and think doing a similar test with AR2209 would be worth while for anyone that is concerned.
The problem with testing powder for velocity variations due to temperature change is that you have to shoot the stuff in order to get a result.
Getting the exact temperature of a powder just before ignition would require instrumentation that is not available to the average target shooter. So my testing was more application specific to cold bore, long range shooting under real life field conditions.
For target shooting where long warm strings are being shot, the shooter could come up with a different test procedure that would be more applicable to that situation.
Dave has given some great information on how barrel/powder temperature effects accuracy and nodes so I can’t add much to that.

Anyway have a read of my testing as it may stimulate some interest in doing your own testing with your rifle. Some of my methods have been left a bit obscure on purpose due to the inherent danger of this type of testing. So take care. http://www.austargets.com/powdertemp.htm

Very interesting Norm - has given me some ideas for doing something similar.

AlanF wrote:Very interesting Norm - has given me some ideas for doing something similar.

It would be interesting to see what you came up with Alan, are you going to try VV powders?

Norm,
Very interesting test you did for temp variation.
I wonder if the odd slow shot at sub zero could be due to the moisture inside the case being effected as temp dropped?
Freezing around granules?
Maybe if the rounds were loaded at zero humidity(might have to move to Alice springs) there would have been less effect??
I'd guess brass would also absorb moisture from the atmosphere?
More questions than answers??

Norm, I can identify with the pocket full of crackers joke, but seriously what you were doing has great merit. Thanks for providing that insight and you prompt further comment. Similarly, a few years back while shooting with the Wimmera Team in F class in the Victorian Championships, the morning was cold at Bendigo, so I put the rifle and ammunition in the motel bed with the electric blanket on a couple of hours before the shoot and kept them warm in the vehicle by insulating them in blankets. Boy did I get some ribbing. But that’s me. The team won with great coaching from Peter Shultz and the rifle performed well enough to top score for the championship considering it was first cab off the rank with the cold start. In the warm afternoon, at longer range, the rifle sat beautifully on the node. At 900 I used another rifle which was tuned specifically to positively compensate. Not good results the next year, when the rooster became a feather duster when a new Leopold scope shat itself.

You rightly say you have to shoot the rifle to see what it is doing in a test situation. So the devil is in the detail and the answer is blowing in the wind. Tim you make a sound point about moisture.

You cannot escape a discussion on powder without looking at its little brother the primer. Testing certainly takes away preconceptions. The link DaveMc provided to Denton Bramwell, provided more insight with regard to magnum primers, and is counter intuitive in the test example, where pressure was reduced by the magnum primer. This also links to thoughts about firm seating pressure, jamming projectiles and air gap - more variables but essentially they affect the efficiency of the powder burn too.

Now I think Tony (Ecomeat), made the important observation when he found primers with similar extreme spread and SD’s produced different velocities within a small range. Twenty foot a second is enough to tip you off some nodes and is a sound way of dealing with fluctuating ambient temperature that is typical of autumn with cold starts and hot afternoons where we live anyway. Whether this primer or that primer is better amongst all the good ones we can try can come down to a subtle burning rate difference even though we know that in general terms a primer can contribute about 5000 psi to pressure. Tony well knows the importance of tertiary vibrations in a barrel that these subtle ignitions and speeds can affect. So to all those pedants that weigh primers to ensure they have the same fuel load, you might have something, but a change of primer within a certain range might be a simpler way of doing things. Maybe there is a subtle difference between a pedantic condition and an OCD condition too.

I like to advocate doing primer tests prior to load development and tuning having posted samples from project Penumbra before. This is important to do with a known pet load before commencing with a new barrel as each set up is different in the harmonics at the micro level, and Tony has given shooters a reason because there is more to it than looking in general terms at the primer compatibility with powder. It is even more important if you are considering a compensation tune. While we are down in the mud we still crave super centres and like to eat bullets for breakfast, dinner and tea. David.

Aaron got me thinking about the single and double base powder. For those that are unaware, a single base powder is basically nitro cellulose apart from stabilizers and solvents you smell coming off as acetone, alcohol or ether. Whereas a double base powder has nitroglycerine added to the mix. It doesn’t take much of a guess to think which has more energy and which may be dirtier with extra components.

But I think the issue here is that smokeless powder is designed to burn in a container. The more the powder is contained the faster the burning rate. That’s why a compressed load has more energy released quickly compared to say a pile of powder in a tray that is lit with a match that just generates a flame. This is not dynamite that goes bang!

Now if you think about this, altering the seating depth assuming the throat has not been lengthened has a couple of big effects on group. Firstly it changes the air gap and so the powder density in your cartridge which in turn changes the burning rate, its efficiency of burn and the harmonics due to pressure changes. A large air gap slows the burn rate. So, a minor few thou of seating depth adjustment can have a big effect on burn rate if the case is relatively full of powder and less effect if the case has a big air gap. Secondly, if you make big changes in seating depth, assuming the barrel length enables a complete powder burn, the greater effect lies with timing the bullet exit with the upward or downward swing of the muzzle with the lower peak and forward movement of the pressure curve. So we have a few variables to optimise.

If you lengthen the throat you are in effect increasing the size of the pressure vessel and so you reduce pressure and effect exit timing a fair bit. The Pressure Trace system can optimise this, or you do it by trial and error in load development. If you are game enough you can use a throat reamer. I posted a group plot taking the throat out in .010” graduations for compensation purposes a while back.

So Tony’s observation that a primer change, rather than a powder volume change for less velocity, if it is still compatible with the powder, does have less variables associated with it. Similarly, the use of a tuner has less burn rate variables to deal with once a tune is established rather than playing with charge volumes to meet environmental changes. I note with interest on Benchrest Central a recent comment by Jackie S that in low humidity he packs the case with powder aside from external ballistic velocity he must be considering.

Different powders are essentially designed to give a burning rate that is safe and best suited to the weight of the projectile. So I think it is important to use recommended powders for the job, but generally speaking a case gives more a consistent burning rate if it is full of powder with no air gap and the adjustments to accommodate this should be in the length of the barrel throat if your projectile does not sit on the powder at the junction of the shoulder and the neck.

I conclude by saying, that after a lot of experimentation, the jammed bullet, with no air gap will give a consistent burn rate. It also has the tendency to raise the muzzle for compensation purposes even in heavy barrel pulses; and it leaves less chance for in bore yaw which can be small to affect the bullet more so in the downward path to the target at long range when velocity is falling irrespective of a compensation or nodal tune. The jammed bullet will give a complete burn. Similarly, the neck tension can affect burn rate. The jammed bullet largely mitigates the need and you can get away with low neck tensions with a jammed bullet if you wish. With regard to wall thickness of the neck, I think there is less chance of bullet run out with a thin wall in any seating depth and less need to anneal. There is no doubt you can reach a fine tune with any method, but you have to ask yourself is it going to be predictable in a variety of environments and distances and that’s why consistency is so important.

Good post Dave.
I think there are some excellent tips amongst that advice.
Given a good barrel and chamber, It sounds like a good formula for accuracy is.

Thin neck wall, light neck tension, seat bullet into lands and fill the case to the bullets base with a suitable burn rate powder.
At a safe pressure.

As an indication of the fact that every single individual chambering and load will have a different reaction to Primer changes, consider the following :
In fireforming new Nosler Custom cases (made for them by Norma) for my first 280AI barrel, I played with primers and did a lot of shooting with a MagnetoSpeed attached to get the data.
On these brand new cases, Federal Magnum LR Primers gave me over 50 fps faster velocity over 11 shot strings than the Federal 210 GM primers i normally use.
CCI BR2 primers were "in the middle", being 20 fps faster than the Federal 210 GM's, and over 30 fps slower than the Federal Magnum primers.
In hindsight, i should have continued with the Magnum Primers, to see if i could get away with quite a bit less powder in well prepped, second-firing cases.......but i didnt think of it at the time. That barrel is having a spell for a while now while i shoot 284 Win for a bit.
Tony