Humidity, Temp, Air Density, specific question

Vern

Vern

Morethan1waytoskinacat
Question.
If we eliminate the powder, primer and humidity inside the case when loaded, what affect does the humidity, temp., air density or any other terms, in any form other than rain have on the size of a group?
Remember this is just concerning the bullet ONLY as it begins its path down the barrel and on to the target AFTER the combustion , burning or whatever term would be used for the actions inside the case?
 
I keep trying to understand this subject better. Progress for me is slow.

The past couple of weekends I was shooting my 22 Waldog (Twist is 1-14) with N-133 and Berger 60 gr moly coated flat base Match bullets.

At an A.D. (Altitude Density) of 1,000' @ 75 deg or so the bullets were stable and shot quite well.

At an A.D. below 750' @ about 65 deg the bullets were not stable and the group started to open up to over 3/4" @ 100 yards.

The lower the A.D. and the temperature the bigger the group. At A.D. below 250 feet the group was over 2" and I could see the bullet holes were not round.

At our range here in SE FL it is common for the A.D. to start at about 800+' and rise to over 2,000' by noon or so. Over the years I have noted that a change in the A. D. of 400-500 feet will usually put my 6 PPC barrels out of tune. That is the groups will go from about 0.200 to 0.400+ If the A. D. moves up another 500+ ft the barrel will sometimes come back into tune.

With the PPC I usually adjust the "tune" by changing the powder slightly.

I have seen similar tune change with my 25 BR when the A.D. and temperature change. I use the barrel tuner to correct tune on this barrel.

With my 30 BR which is fitted with a barrel tuner the A.D. does not seem to be so much of an issue. It just keeps shooting in tune. I have never adjusted the tuner since the barrel was first tuned.
 
Octopus, By your statement then I would take it that you are reloading at the range. If so your groups are/can be affected by the temp and humidity within the powder in the case.
I suppose what I am looking to find out would only be able to be determined by preloading under controlled environment.
I know in long range these weather variables can affect the bullet impact in elevation or horizontal, but in shooting a 100-200 yd group do these variables have any actual affect only on the bullet itself in relevance to the group. Even if the poi changes from 1 group to the next is not as important as the actual group size itself. It has always been talked about that humidity, temp and other variables affect the powder, load and ultimately the tune.
I noticed that there has been a recent potential world record shot using preloads. I started experimenting with this last year and have found the results to be quite positive.
Now I am looking to see what affect the weather variables can actually have on group size having removed them from the powder and combustion.
I have another match next weekend and we will see how it goes. I will try the same load I used at the final match last year and see what happens. Hopefully I was able to recreate the same loading environment.
 
Question:
If you were to preload a quantity of each of several slightly different powder charges under identical conditions (same time and location), would doing at test by shooting them in different conditions tell you if the optimal load changed with conditions?
 
In my post above all the loadings were done in advance in my shop 12 hours or so earlier with good Temp and humidity control. The 30 BR loads were loaded in TN back in early 2012.

When I am chasing the "tune" with the 6 PPC and N-133 powder I usually load at the range.

In regards to adjusting the load to match the A.D. and Tune the barrel what little I know came from Beggs in a post a few years ago on this site. These are my notes from this post: (You may be able to find the info on this site somewhere in the Forum history.)

+5 Degrees F = About 500 Ft A.D. = - 0.3 Grains Powder Barrel nodes every 120 fps. 0.1 grains = 10 fps 2 baby clicks= 30 fps Max out of tune is 1 click # =0.6 gr
A 6 mm PPC H.V ? vibrates at a frequency that the nodes occur about every 120 fps as you work your way up the ladder from 27 grains to 30.2 or so.
The powders we use produce about 10 fps per tenth grain. So let's say our pet load is 29.2 grains or about 53 clicks on our Culver type measure (Harrell).
Each click/number on a Culver is .6 grains. 2 baby clicks and you will change the muzzle velocity 30fps. We know the nodes appear at 120 fps intervals.
This means that when we go to the line ANYWHERE ANYTIME we cannot possibly be more than one click/number (.6 grains) out of tune.
We can go either up or down and get the rifle in tune. When we get there, make a note of the powder charge and temp and that is our benchmark.
On a Culver type measure, each full number represents .6 grains of the powders we normally use in the 6PPC, i.e., N133, H322, Benchmark, etc.
Make your adjustments in .3 grain increments which is half a number, which in turn equates to 30 fps change in muzzle velocity.
A five degree increase in temperature will result in an increase of about five hundred feet in density altitude (DA), which is the point where I make
my first adjustment to stay in tune. I usually just use ambient temperature to make my adjustments because I know that is the reason DA is
going up and RH is going down. When you get your rifle in tune, make a note of the powder charge and temperature. That becomes
your benchmark. If temp goes up five degrees, reduce the powder charge .3 grains. Of course we are talking about a 6ppc in a 22 inch barrel
using N133, H322, 8208 etc. I worked this out over a several year period and it works. The first time I applied the formula was in Phoenix,
at the 2003 NBRSA Nationals. I won the HV Grand.
A sporter barrel vibrates at such a frequency that the nodes occur about every 120 fps as you work your way up the ladder from 27 grains
to 30.2 or so. The powders we use produce about 10 fps per tenth grain. So let's say our pet load is 29.2 grains or about 53 clicks on our
Culver type measure. We know from experience that each click/number on a Culver is .6 grains. Move it two baby clicks and you will change
the muzzle velocity 30fps. We know the nodes appear at 120 fps intervals. This means that when we go to the line ANYWHERE ANYTIME
we cannot possibly be more than one click/number (.6 grains) out of tune. We can go either up or down and get the rifle in tune.
When we get there, make a note of the powder charge and temp and that is our benchmark.
 
Boyd I preload for tuning.
I load for d round groups using 1 powder charge and then 4-5 different depth seatings.
Last year in July I could see my barrel was pretty much shot out (about 1700 rnds) with only 1 match left for August I had just about decided not to shoot it. Then I decided I didnt want to miss the match but I was just too tired to take everything to the range unpack and repack, so I made the decision to pre-load and not take anything extra.
I set the house to the same temp and humidity as when I had done my tuning loads at the beginning of the year.
The gun shot surprisingly well. First group out was a .16? followed by a couple of 2s and a couple of low 3s I think. The increase was I think due to higher winds. In the 100 I used a 29.5 load and I had set up a 30.3 load for the 200.
The 30.3 shot some amazing 4 shot groups. Some of them were in the 2s however they all had 1 flyer each time that opened them up to 4's. So this time I am sticking with 29.5 across the board to see what happens.
As I remember I came in 4th or 5th overall in the grand.
Well things have been a little tight and I have not yet had the barrel replaced. I was contemplating not shooting this year but I looked at last years Aug targets for the preload and decided I would try it again. If for no other reason I like the matches and I like to shoot. So Ill see this weekend how things work out.
 
On your four and ones....I believe that Tony Boyer has said that he tunes for a half a bullet of vertical at 100, and a full bullet at 200 to avoid that sort of thing. How have you worked with seating depth as your barrel has worn?
 
As I remember I backed it out 2 -4 thou dont remember for sure.
I am considering having the barrel set back, just not sure yet. Will see what this match brings first.
I didnt notice any problem with the 29.5 thats why I went with it for both yardages. The 30.3 has always seemed to have 1 hight but I thought it might be better for the longer ranger and higher winds we have in the afternoon.
I may have been right but I guess Ill know after I try it.
 
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I've pondered that question a lot and one answer that seemed to fit is that the air density factors as related to load and barrel tuning are somehow correlated to the concept that a bullet and burning powder gasses are a piston in an open-ended cylinder. It must push the contents of that cylinder out into the general atmosphere which provides a certain amount of resistance based on its "air density". Other explanations seem to have alternate factors that don't fit in the equation (like why the retained heat in the barrel from 4 rounds doesn't warm up the air inside the barrel and make the 5th round out-of-tune).

Rod
 
There was an article in Precision Shooting that was very well done. In it the author measured weight changes, and differences in velocities of the same exact samples of 133 out of the bottle, humidified to the max., and thoroughly dried. The difference in velocities was striking. It seems to me that unless you have powder at a constant state of humidification (note how I sidestepped the use of moisture content ;-) that there will be some variability in the velocity produced, and if tune is velocity specific.... But as always, results rule.

Once, a long time back, at the end of a day that had become hot, I decided to tune up and preload for the first match the next morning, when it would be perhaps 30 degrees cooler. That afternoon, the groups were good, and the next morning conditions were very light, but the loads did not shoot as before. I realize that this is not the same as loading in an air conditioned space, but the loads were the same, and the conditions that they were shot in were not, neither were the the results. You might say that this has caused me to be somewhat less receptive to the idea of preloading.
 
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