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.