Tac powder odd behavior

[wvbuckbuster]

Last year I worked up a load in my Sako Forrester 308 Win using Tac powder, Nosler 125gr AB, Win case and CCI 200 primers. Results were as follows:
Average for four shots
MV= 2957.2 ES= 20.8 SD= 7.7
Couple days ago, I shot this same load with the same components with these results"
MV= 3029.2
ES= 85.4
SD= 32.7
Was using my Garmin to check with. Group size went from .5825 to about 1 in. Temperature was within 10 degrees or so of each shooting session. I know the lower the ES, SD the tighter the groups normally but I have seen good groups with high ES, SD before. What has me puzzled is why they are higher now than last year using the same components? The load groups well enough that I plan on using it to try out on a whitetail. Just wondering if any of you have seen Tac powder act like this. I have used it in 223 and 35 Whelen without any major changes in readings. Your thoughts?

 

[salmonchaser]

I have seen that kind of behavior out of my 338 Jarrett; My 338 WM; my 06 and my 300 HH. In my case with the aforementioned I was tickling the tail of the dragon pressure wise, so to speak. I wan’t using Tac, probably RL 26 but I don’t specifically recall. No idea where you would have been pressure wise with Tac however. Actually with the 338 Jarrett and the 300 HH I saw erratic behavior by seating bullets into the lands also.
If it’s the same lot of powder and only 10 degrees warmer it sure seems like something happened to boost pressure.

 

[Chuck R]

How many shots per chronograph reading?

It looks like there was a "bad" round in your 2nd string that caused an ES of 85.4.

 

[wvbuckbuster]

Four

 

[Todd NE WY]

With only 4 shots recorded you really do not have enough data to be valid. I would shoot a 10 shot string, letting the barrel cool appropriately, and see what the data shows. 10 is the minimum I will use for any significant analysis, more is better but I think I have a decent look at the big picture at 10.

 

[Chuck R]

With only 4 shots recorded you really do not have enough data to be valid. I would shoot a 10 shot string, letting the barrel cool appropriately, and see what the data shows. 10 is the minimum I will use for any significant analysis, more is better but I think I have a decent look at the big picture at 10.

That was the exact point I was making, not enough of a sample size to be accurate and 1 "bad" or off round in a 4 shot string can skew the results drastically. 10 is the minimum, better is multiple 10 shot strings over varying temps to get my data for my ballistic app.

I use TAC in; .223, 358Win and 350RM. It's not as temp stable as say H4350 or H4831SC, but it's not anywhere near as bad as causing an ES of 85.4 unless there's an issue with a round or two. I'm guessing that if the OP looks at the individual rounds, he'll find the culprit.

 

[RL338]

ES of 85 in a 308 Win is possibly an 2gr variable. I’d be taking a close look at my scale.

 

[wvbuckbuster]

Looking over some of the comments let me add some more info. I first worked up this load last year without any extreme spreads. I shot this load again within the last couple weeks using the same components, two groups total of eight shots. I have looked thru my records of having used this powder in the 223 and 35 Whelen having shot it back in May without any extreme spreads. Monday, I did a ladder test in my grandsons 308 using this same lot of Tac but with different bullet, case and primer and the closest velocity spread was about 25-30 feet shooting nine different loads in this test. The scales are the same ones I used to develop the loads listed under the post put together 270. Personally, I haven't had powder to go bad, and this lot is two years old and showed no issues three months ago, still looks and smells ok. So, having said all that, the jury is still out. At worst I'll dump this powder as there are others, I have available. This is just one of the weirdest things I have encountered in reloading. Good reason to use a coronagraph.

 

[Chuck R]

Looking over some of the comments let me add some more info. I first worked up this load last year without any extreme spreads. I shot this load again within the last couple weeks using the same components, two groups total of eight shots. I have looked thru my records of having used this powder in the 223 and 35 Whelen having shot it back in May without any extreme spreads. Monday, I did a ladder test in my grandsons 308 using this same lot of Tac but with different bullet, case and primer and the closest velocity spread was about 25-30 feet shooting nine different loads in this test. The scales are the same ones I used to develop the loads listed under the post put together 270. Personally, I haven't had powder to go bad, and this lot is two years old and showed no issues three months ago, still looks and smells ok. So, having said all that, the jury is still out. At worst I'll dump this powder as there are others, I have available. This is just one of the weirdest things I have encountered in reloading. Good reason to use a coronagraph.

I'm thinking more along the lines of a neck tension issue, bullet, or bad primer.

It could even be a chronograph issue, which is why I go through the shot string and look at the individual shots. When I run a vibration trigger on my Labradar for 22LRs and suppressors, every once in a while I'll pick up a bolt closing which will give me a false round. Non-issue as I delete them as they happen.

I've got powder that's 5 years old that still shoots fine because it's stored properly I.E. "cool dry place", so I doubt that a lb of powder that shot fine, went bad so quickly.

 

[Todd NE WY]

@wvbuckbuster I would not throw out powder over an episode like this. Sample sets of 4 in the statistics world are meaningless. in order to really evaluate this you need a minimum of 10 shots to see what both the group size and velocity are doing. I would highly recommend listening to the Hornady podcasts on sample size. They do an excellent job of explaining why 3-5 shot group and velocity samples are not really valid. It explained to my why in the past I have shot a 3 shot group that was gnats a$$ tiny in one session and could never replicate it. I think this may be a little of what is going on with you, unfortunately that original 4 shot string just happed to be 4 of the best for that load and gun and the results show that. Tiny group, small ES and ST.
Todd

 

[RiverRider]

I'm thinking more along the lines of a neck tension issue...

Keeping your case necks annealed will eliminate a variable that is often overlooked. A "small change" in the temper or ductility (or whatever the proper terms may be) of a case neck can have much larger impacts on the burn/pressure curve than we might expect.

Larger sample sizes are more meaningful as well. It could be that what you have observed is truly representative of what is really happening with your loads. Or NOT. You just giotta have larger sample sizes in order to be able to deduce anything at all with any real confidence.