6BRinNZ said:
Do the groups indicate that the FPS has moved out of the accuracy node? I am wondering if fowling is pushing up FPS and moving me out of the node as the final groups are approx twice the size of the first 3 groups... How would you guys interpret the data?
Data? The proof is on the target. You tune with bullet/brass/powder/primer combinations, not FPS. You fine tune with neck tension and seating depth.
The first three groups look great for two hundred yards. How much did the conditions change for the fourth and fifth groups?
Were you shooting over wind flags and using the same condition for all your targets? [Five-shot groups or much better than three, and ten targets at each load are a much better comparison than one each, which tells you very little to nothing for comparison.]
In a no wind situation like a tunnel, using one bullet hole as a base line, if you are one bullet hole higher [one on top of the other, the edges are just touching] you have one bullet hole of vertical and are .3 of a grain out of tune, if you have two bullet holes above the base you have two bullet holes holes of vertical and are completely out of tune by .6 of a grain.
But again, how much of a difference between the first three targets and the last two are a function of missing changes in conditions?
And here's another item to consider, how much of a change in temperature took place from target one to target five? Did you know you can go from in tune to completely out of tune with a 20 degree change in Temperature?
Here's a discussion to consider regardless of caliber. Gene Beggs responds to the question:
Q: 6ppc using v 133 powder. Is it just humidity or is humidity & temp that affect tune? When the Humidity goes down I go up on the powder to keep my gun in tune? Is it that the powder kernals weigh less with lower humidity so I have to go up on the charge? Is it that the conditions change the harmonics of the barrel. Is it that the bullet travels through the air differently with the changes in conditions?
A: Changes in air density makes our rifles to go out of tune. If the bullets are exiting when the barrel is stopped at either the top or bottom of the swing, the rifle is in tune. If the atmosphere becomes less dense due to an increase in temperature, the bullet encounters less resistance as it travels down the bore and exits early before the barrel comes to a complete stop, in which case, we can either reduce the load or adjust the tuner to compensate. I think it's much simpler and easier to use a tuner. You NEVER have to change the load and can make your adjustments at the line.
Atmospheric density is the result of pressure altitude, temperature and to a far lesser degree, moisture content. Contrary to what many believe, dry air is heavier. Steam rises and it's 100 percent saturated.
So .. altitude, temp and humidity in that order. What do we mean by altitude? Range elevation above sea level. The Midland Shooters Association range is 2800 feet above sea level; that never changes so we can forget about it. Yes, slight variations in barametric pressure can change the pressure altitude but for all practical purposes, we can forget about it.
****Temperature? This is the big one! It's actually the only thing you must concern yourself with. The temperature range from being perfectly in tune and being completely out is 20 degrees F.****
Yep, if your rifle is perfectly in tune during the first match of the day when temperature is 70 it will be completely out of tune when the temp reaches 90 if you do nothing to compensate.
To keep the rifle in tune throughout the day, reduce the velocity 30 fps for each five degree increase in temp. With most powders, N133 being the classic example, this equates to one half click per five degrees. For example, let's say your rifle was in tune for the first match; temp was 70. Your Culver type measure was set on 54 clicks. When temp reaches 75 the proper click value is 53.5, for 80, 53, 75, 52.5 and for 90F, 52 clicks. Would you have ever believed it would take two full numbers to stay in tune with a twenty degree spread?
"Now wait a minute Beggs; are you telling me relative humidity has nothing to do with it?" Yep, that's right; very little, so little that for all practical purposes we can ignore it.
Relative humidity (RH) is expressed in percent of saturation at a given temperature. If RH is reported as 30%, temp 85 degrees, this means the air contains thirty percent of the moisture it is capable of holding at that temperature. If temp goes up, RH goes down and vice versa even though the amount of moisture in the air is exactly the same.
Some shooters say they adjust the powder charge by relative humidity; RH goes down, they go down on the load and vice versa insisting it is humidity that affects the rifle's tune when in fact it is the change in temperature that is actually responsible.
BTW, you asked if changes in conditions affects barrel harmonics?" NO
If the rifle is in tune at 100 yards, it will also be in tune at 200, 300, etc. With tuning, all we are doing is timing bullet exit to coincide with one or the other stops of the muzzle. It has nothing to do with yardage. Some say you have to retune the rifle for a headwind, tailwind, left crosswind vs., right etc. That makes no sense to me. External ballistics apply only to the bullet's flight path after it leaves the muzzle.
If you had to change the tune at 200 yards from that which shot well at 100 yards, my question is, "Was the temp the same; how much time elapsed between the last group at 100 yards and the first at 200?" A ten degree increase in temperature will make a noticeable difference in vertical.
What many shooters do not understand is the fact that tuning is condition specific. The rifle will be in tune only as long as there is no change in atmospheric conditions. You cannot set the tuner to a specific position today and expect the rifle to remain in tune tomorrow unless there is no change whatsoever in atmospheric conditions.
Before the first match of the day, always zero the tuner by turning it all the way in and backing it off one revolution, placing the reference mark in the 12:00 o'clock position. This will always be your starting point regardless of location or temp.
My feeling is that it doesn't matter whether or not you load in an air conditioned trailer. The thing that matters is the outside temperature and its affect on air density at the firing line.
The test group on the sighter will tell you whether or not the rifle is in tune. Make all tuner adjustments in reference to outside air temperature.
If tuning with the powder charge, the same thing applies. If the rifle shows a full amount of vertical on the sighter the charge can either be increased or decreased .6 grains to bring the rifle in tune.
