Tiger By The Tail

K

Kim Merrill

Guest
I fell like I have a tiger by the tail and don't know what to do with it. I got his bright idea to load test tubes with precisely measured loads starting at 27g progressing up to 30g in .1g increments. My goal was to find out the starting point for tuning this gun. So here I go, five test tubes of each weight. The morning is perfect for shooting. Consistent light wind puffs with dead calm lulls. I started out with the 27g producing a .279 group. When I reached 27.5g the group was down to .132. The average FPS was 3191.2 and the temperature was 79F. Then the groups started getting bigger. I kept on going, cleaning the gun after each fifteen rounds. Then the groups started to tighten up again. By the time I loaded the 29.5g my group size was down to .172. This average FPS for this group was 3396.2 with the temperature up 6 degrees to 85F. As I continued on with hotter loads the groups started spreading.

It appears to me that I have found an harmonic high and low point. The trouble is :confused: that I don't know how to best use the information.
 
While one group might give an indication of the capability of a certain load, it's the aggregate that matters. To definitely prove the one is better than the other would likely require more rounds fired than you'd care to fire.

The best I can offer is to go to the range with enough weighed charges at 27.5 and 29.5 gr so that you can fire 5x5 shot groups, and sighters/foulers. Fire your groups in 7 minutes in attempt to duplicate match conditions too. Firing a five shot group over fifteen (or more) minutes to pick the best conditions for the group isn't really a fair test IMHO.

A question though, are you planning on weighing every charge before a match and packing the weighed charges in vials/tubes to be dispensed at the match, or are you going to throw your charges at the range? Sometimes a load weighed exactly will shoot more or less well than a similar charge thrown from a measure. Just a thought.
 
it's hard to ride around with a tiger in your car or change film with a kid on your back.
you've got a tiger by the tail, it's plain to see.
lots more testing is in order. what was the humidity? what was the D.A.? what will the load do if you took it out tomorrow and the humidity had changed 40% and you weighed it today? what if you changed the seating depth .002 on the biggest group's weight- would it tighten it up? powder weight is only one small piece of the puzzle and dropping it from a harrels measure at the range between relays may not be all that bad- it may surprise you. the load I shoot zeros with in arkansas may only get me 5th place in a 4 gun in alabama the next day- I've seen it happen:eek:
 
Next Step

Larry, thanks for the advice. I plan to throw and check the weight at the range as I load. I will work around the two loads and will look for repeatability.

Dusty, the humidity was 76% at the time of the 27.5g load and 62% during the 29.5g load. What is D.A.? Do you mean S.D.? The S.D. for the 3191.2 string was 18.5. This high value makes me question the casings setup. The 3396.2 string was much better 6.5. I am looking for a starting place to tune. How do you pick a starting place? I guess that you could just ask Billy!
 
what I was getting at is that it will be different tomorrow. DA is density altitude and some think it is important, moreso than humidity. where to start would be to find a seating depth for your reamer and bullet combo. find out if it likes to jump or get jammed. then fine tune it with the powder. I feel the right seating depth will make your load more consistent with powder variances more than powder weight or volume and consistent FPS alone. also need to know how repeatable your powder is going back to a certain atmospheric condition. mine is good but the newer stuff makes me wish I would've bought more back in 99.
 
Dusty,
Your dad set up my gun with a Match reamer and I am using Barts BT's. At this time my seating depth is what I call "0", no jump just touching enough to make a tiny little marks on the bullet. I will experiment with it also. BTW congrats on your shooting at Mickey's.
 
Welcome to benchrest

Kim,

What you have experienced is what everyone who has done this for awhile comes to know... getting a gun to shoot its best groups is NOT a static event. Everything effects the group size: temp, humidity, powder charge, powder lot, bullet lot, primer lot, brass wear, barrel wear, ... , the list goes on and on and on and on. And they all add up! Some things affect your group a lot, and some things affect your group a little. The trick is to learn which things have which effects at which times.

It is generally accepted that a particular PPC will shoot "small" groups in 2 or 3 load windows. We generally call these the "light", "medium", and "hot" loads and for most rifles, they are .6 to .7 grains of powder apart. Start at 27.3... if suddenly your gun tightens up at 27.7, it will likely shoot well at 28.3 and 28.9 too. In between those loads though it seems to blow up and shoot big groups. Now, this assumes the groups are tested at identical (static) environmental (temp/humidity/whatever/) conditions. Let the temp drift up 5 degrees, and the new "perfect" load might be 27.8, 28.4, or 29.0 grains -- or it might be 27.6, 28.2, and 28.8. Load testing is about knowing a starting point, and then knowing the increments based on whichever outside factors you choose to correlate to.

The holy grail of benchrest is a single barrel/bullet/charge/powder/seating depth/primer combination that shoots 0's every time. It exists for only fleeting moments, the rest of the time you have to work very hard to stay in the game. When you know this, you will start shooting top-quarter. Instead of searching for a single perfect load, start looking for predictable behavior changes, and predictable responses. Watch your conditions and think about what they will be in 15-30 minutes when you walk to the line again. If the temp is going up, and your rifle wants correspondingly less powder, then knowing how much less powder for each 5 or 10 or 15 degree temp shift is the knowledge that will move you into the winner's circle.

Two things will move you to the top (or bottom) quarter of the pack fast: gun handling, and load tuning. Load tuning is actually the easy one. The other takes practice, practice, practice.

Rod
 
that's what I was getting at Kim. there's no way to say a particular load is gonna shoot the same tomorrow. best thing to do is learn what a particular change is going to do to ahead of time. find a load that shoots decent and see if you can shoot that decent group 5 times thruout the day. let the gun go out of tune and go work to bring it back in. make sure your "table manners" are not contributing to group size too. I like to tune my gun one cool morning then come back that afternoon when the wind is blowing and try to get it back in altho I don't usually get a chance to practice since I don't have a good bench to shoot from. practice and learning your barrel/powder/bullet combo and what it's gonna do to you is about 75% of the game I'd say- and every one of em are different.
 
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