Load development and accuracy problem questions

[SS427]

In "The Book of Rifle Accuracy" Tony explains a load development process for the 6PPC where you find the "jam" point for a particular bullet and neck tension, and then progressively seat the bullet farther and farther away from the jam point.

My question is, is this a good development process for other calibers as well such as the 6mmBR?

The reason I'm asking is my 13.5 twist 6mmBR is not shooting worth a darn any more. Groups used to always be in the .2s and 3s with an occasional group in the 1s. Now I'm getting groups in the 3s, 4s and some even in the 5s. I only have about 1,200 rounds down the barrel. Could the barrel be worn out this soon?

Another detail worth noting is the spread is showing up as vertical stringing. There is little to no horizonal dispersion. I'm shooting a load of 30 grains of N133, 70 grains SMKs, loaded about .003 into the lands. This has been my standard load from the very beginning that has shot very well. Could this load now be out of tune? (Already checked scope screws....everything else is exactly the same as before.)

If the load is out of tune, I'm thinking of starting over w/ N135 using Tony's load development process.

 

[crewchef]

Have you re-checked throat measurement? If throat is wearing you're freebore would get longer so instead of jammed .003 maybe you are jumping them now? Worth checking.

 

[SS427]

I just checked the lands prior to my last 50 rounds. I had to move the bullet out .003 more as a result. Didn't fix the problem.

I don't think it is a scope issue. Everything is tight and nothing has changed....have not bumped the scope or did anything to break it. Some groups are good, so again I don't think it is a scope issue. (Sightron SIII 8-32x that has performed perfectly.)

I brushed out the first 6 inches of barrel with JB 25 rounds ago. I could feel it being a little bit tight and wasn't tight when I was done. I've read about the carbon ring right in front of the case but have not worked on that yet. I just checked my cases and they are getting close to the limit where I have to trim them. I'm going to trim them to 1.550 and see if that helps. If it does I will go after the carbon ring.

But that leads me to another question....if it was a carbon ring causing the problem, inaccuracy would not be limited to vertical, correct? It would cause horizontal as well is my thinking.

 

[Con]

What I know about the 6mmBR can be summarised in 'not much' ... but an acquaintence that shot one for considerable time has found it harsher than he expected on barrel crowns. He was re-crowning every 500 shots to maintain his top level accuracy.
Cheers...
Con

 

[bheadboy]

vertical isopften a result of velocity changes, thus, consider the following, as you indicate no horizonal, the afore mentioned solutions would usually address dispersion - not vertical

powder lot deviation of burn rate/pressure
primer lot ignition diff
bullet lot bearing surface, diameter, ojive differences (all slight but can cause change in inpact)
Temperture effect on velocity from powder
additional jam as .003 is very slight
check and clean your dies as the size die may have a lub buildup on the shoulder giveing a greater setback

Bob

 

[dieselten]

Also check your fired cases for donuts. Take a flat base bullet and make sure it is an easy slip fit into a fired case - all the way - same tension all the way. This might be easier with a longer bullet but you get the idea.

 

[lpreddick]

agree with above. the carbon ring is a real problem and is not easy to get out...boyer describes how he gets his out and it works. i am a complete convert to seating depth(distance from lands to bearing surface on the bullet) to find a cartridge/bullet/powder/charge weight combination that produces the smallest group. this is chris long's shock wave theory of rifle accuracy. the idea is fairly simple..get the bullet to the muzzel when there is the least amount of "disturbance, vibration ,harmonics, whip, twisting" and other discription of what the barrel is doing during the time the bullet is in it. the easiest way to achieve this is to start the bullet at different points in it's travel down the barrel...seating depth of "jump". 0.005 thous difference will make a difference in group size and interesting enough in group location! load 3-5 rounds seated at 5 thous increments and you will usually find the jump your bullet,etc. "likes".

 

[Shynloco]

Otter,
Just now saw your post so I'm a bit behind. I had the same thing occur in my Savage Benchrest 6mmbr Rifle. I checked everything you did as well as what others have suggested. My groups had declined in accuracy and it was driving me nuts! I too was using VVN133 and using Bart's 68 gr bullets being pushed by 29 grs and the bullet was set just touching the lands. Bullet count was at 1200. So I tried an experiment by increasing the gr of powder. First went to 29.5, then to 30.0 and end up settling on 30.2 gr and my accuracy was back as it was a year ago. I hadn't changed a darn thing other than the amount of powder and I cannot understand why. Frankly, I don't give a darn and am very happy I'm back to being able to group in the .1's and .2's which is all I care about. I'll leave the technical explanation to the guru's on this blog.

 

[Nomad47]

My guess would be barrel friction has decreased.

 

[SS427]

Thanks for that input Shynloco. I'm going to try going up in powder weight then. Problem is I'm already at 30 grains. Using the same ratio that you used, that would put me up over 31 grains, and I think that might be pushing things. Will have to check my records and see if I have tried using that much powder.

 

[CPorter]

I've found that powders slow with age. It might be that the double base powders I use lose one component slowly due to evaporation.

Vertical might also be due to the brass necks needing anealing. My ES always goes up after a few firings. If you have several firings the necks will be very inconsistent from work hardening.

 

[lpreddick]

Shyloco: i have noted the same esp with vhitavhouri powders...i usually have to increase a load in the winter to get tight groups and decrease it in the summer. there might be an explanation in what we're seeing in reference to the shock wave theory of accuracy. increasing powder charge increases pressure and speed of the bullet thereby DECREASING time the bullet is in the barrel allowing it to reach the muzzel ahead or behind the shock wave. bullet exit at the millisec of muzzel quietness. we are obviously limited in the range of charge adjustment but seating depth adjustments are almost limitless. powder charge adjustments to find an "accuracy node" is the concept behind the audette ladder test. so many variables and we have some control of so few.

 

[SS427]

I wanted to thank everyone for the advice and report back the results. Accuracy has returned.

I made a couple changes but pretty sure it came down to a couple things. The two things I think that helped was I gave the area in front of the throat a good work over with JB. I could feel the carbon build up when I really focused on it. I also trimmed all the cases back to spec, as they were all close to .010 over trim to length.

I made a couple other changes that I feel made a difference, but not as much as the above changes. I worked on seating depths and I'm fairly confident putting the bullet into the lands about .006 more resulted in an improvement. Seating deeper than that started to degrade accuracy, but not much. The other change was increasing powder weight 1/2 a grain. I shot some control groups at the old weight of 30 grains of N133, and they were good, but increasing to 30.5 the groups tightened up just a little more. I have to make note that the temps were in the 30s....so as the temps go up in the spring, moving back to the lighter load might work better.

Again, thanks everyone for the advice.