A few weeks ago, I tested a few 3 round groups for hunting for the 308 using H4895 and 150 SST Interlocks. Worked up from 44 grains to 45.6 and did not use a chrono at that time, and checked for pressure signs and there was none. The best charge was 45.2 grains which shot a ragged hole @100 . The other day I made 15 rounds of 45.2 grains H4895 to test and sight in for deer season and took the chrono this time, and unfortunately it did not shoot like it used to, and it was very humid, some groups were .75- .9 MOA instead of the .2 MOA I done previously a few weeks ago. After firing several rounds thru the chrono, and ES spread was 6 and velocity average was a whopping 2980 fps. That is close to 30-06 speeds. Hodgdon data shows 45.5 grains as max with a 150 Ballistic Tip @ 2800 ish. Before I have fired the rounds today, I have shot about 80 rounds of 155 Palmas with RL-15 ( 3 trips to the range ) and shoots an inch or less @ 300 yards and gave it a good cleaning before I used the SST's.

Would humidity have any effect on patterns compared to a sunny day?

I am working on another set of 3 round groups but with a CCI 200 primer ( saving the Fed210M ) this time. Hopefully I see some change from the last patterns.

This most likely caused your open groups. Foul the rifle back in, get the barrel seasoned back up, and then re-shoot. But .2 moa groups are very good, and not may rifles will repeat that day in and day out.

Humidity, probably not, but I have seen the sun and sun angle change up the POI a little. I love shooting on a cloudy day for this reason.

In my case it ain't the rifle that is not repeatable. LOL Daniel

Agree with JG on fouling the barrel.

I know some days I just shoot better than others, I know i need to shoot more... When working up loads or testing loads for accuracy I will generally take a rifle and ammo that are very good on accuracy as a test for myself. Often I am the loose nut in the system, rifle, ammo, scope, rest, shooter.

If I knew something is shooting good , since this load shot several .3 MOA easy @100 , not just once, but the best was this, then I know its repeatable but not every time.

Kmon and Chad, possibly the foul could be the issue, but strange that before I shot those tight groups, only two rounds was needed. I possibly could have done something wrong, maybe used the RCBS full size die with the expander instead of the Redding full sizer with the bushing without the expander. I use the .307 mandrel after. Time will tell when I go back to test again. I don't like to be pulling on the necks with that sizing button on the upstroke.
Thanks

A change in your brass sizing can easily make a difference. The name of the game for accuracy is consistency. If you are changing things up, you're changing the consistency of the ammo which will effect the accuracy of it.

That is true, I am going to repeat the same 3 round charges again.

There it is....maybe just shot average for a day. i don't think that projectile is capable of .2MOA accuracy either way.