In my 220 Swift, the Nosler 64 gr BSB shot really well in warm weather, even though ballistics info suggested that it wouldn't or shouldn't. I assumed that it was marginally stabilized, and I feared that cold weather would push things the wrong way. This morning was rather cold and my fears were realized. No key holing of bullets, but groups opened up to well over 2 inches at 100 yards. Dang.

If I ever have that rifle rebarreled, I'm going to get a faster twist.

Which powder are you using?

IMR 4064, at max load (per Lyman 49th). I knew it would be either marginally stabilized or just shy of it. I wouldn't have mentioned it, but I had earlier said that it did stabilize, and I wanted to at least post the change.

Do you suppose that one of the more temperature tolerant powders might work a little better? Do you know what the velocity was in the summer and what it was this morning?

Wonder if h4350 or varget would get you there. They do seem a bit less temperature sensitive

My expectations on stabilization were from that JBM website. What I've got is a bullet that's a small bit too long for happy stabilization in a 1 in 14 twist. At summertime temps, per the app calculation, there was a higher chance that it would stabilize than if the temp was lower. Even at the higher ambient temp, the calculation showed that the bullet should not stabilize - but in actual practice, it did stabilize. I was just hoping that as the temp fell as we got into wintertime, it would continue to stabilize. Like I said, it wasn't keyholing but the groups opened up a bunch and the bullet wasn't going where I was trying to send it.

So what I'm saying is that I doubt that the powder sensitivity to temperature is the problem. Now, I could be wrong. That's certainly possible. Maybe if I could push the bullet a bit faster, just maybe it would again stabilize and shoot small groups. I don't have my loading books in front of me, but I'm thinking that 4064 gives me about all the bullet speed I can get (I will need to recheck that).

I do not know what the velocity was in the summer vs where it was today. One of these days I really need to buy a chrono.

This is new territory to me, in that I have never had a bullet/powder/rifle combo that was like this. I think the ultimate answer is a faster twist barrel, if I want to shoot that bullet. It will shoot the 63 gr Sierra SMP, so I could use that, but I really wanted to try out that 64 grainer on big pigs. I can always shoot it in the 223, but I really wanted to use the Swift. The Rolling Stones said it best...You can't always get what you want.

It will change your life! Okay, maybe that's a stretch; but it will make a major difference in understanding what is going on with your loads.

I was doing load development on a 6x45 and was going NUTS!!! I was on the bottom end of the load data, but getting overpressure signs. The data showed that it should have running around 2450 fps, which I was not happy with. I was hoping for around 2700 fps. I kept backing off of the load until I quit getting pressure signs then a little more for a safety factor. It was shooting good groups, but I was not happy with the velocities (I thought.) I finally got a chronograph, and the truth was revealed. I was getting a 2705 fps with a very small ES. The rifle's tight/short chamber and the LC brass was building pressure faster than the test data.

That opened my eyes to the fact that test data is not set in stone. It is just their experiences with their equipment. Chronos are also very valuable for helping with true ballistics. Having an accurate (not approximate) velocity will give a more accurate flight path when using JBM Ballistics and others.

I reckon I'll order one before too long. They aren't expensive.

No. Not compared to what we are willing to spend on other gun-related items. Everyone has their preference, but I have nothing but good to say about my C.E. ProChrono Digital. Around $100...