Riddle me this. I shot identical ammo through two different rifles. One one, the spread was 3, yes 3 fps. This was a 20" Tikka CTR in .308. Shot the same ammo from the same lot, (box actually) and the spread was 70 out of a 24" Remington 5R Milspec. What could be the cause of the spread in the 24" 5R, over the Tikka?

The 3 fps ES rounds were a fluke, and the 70 ES is about normal for factory ammo, assuming it's factory ammo. Sample size would also probably too small for the CTR. Also, if the 5R had been cleaned and not fouled back in, you can have a high ES. You also may need more rounds than just a few on the CTR to know for sure. Most factory ammo will easily have a 50+ fps ES. My bulk ammo is the same way. My match ammo with the powder charge loaded to .02 grains easily turns in sub 20 ES.

Factory ammo or handloads?

Factory, Hornady SST Superformance, 165 gr. I've shot this for years. I back calculated and was thinking the CTR would be right where it was based on barrel length and what I was seeing in elevation at known distances. The published velocity was spot on.

Then, there is the variable of the chrono.....

The Superformance ammo is loaded with the Superformance powder, which is a double based ball/spherical powder. It is a slightly slower burning powder for the rounds/cartridges it was designed for. They cram a lot of this slower burning powder in the case to get these speeds, and that's it. You read that most Superformance ammo does not shoot well. This is why. It is a Ricky Bobby powder- I wanna go fast! Put this ammo in the heat, and speeds will pick up. Shoot it in winter time, and speeds will go down, a lot. The factory ammo will have a large ES to begin with, and will have a large variation in hot to cold temps. I don't use this powder.

How many rounds to calculate the ES on the CTR?

One other thing I've noticed is that cheap chronos can tend to throw an outlier result in with a list of good numbers. Might be something to think about if the 70 fps was closer to 20 or 30 fps with one or two rounds far from the average.

I have found it to be very accurate, consistent and with the bullet (SST) it's a hammer.

Shoot 10 from each rifle and that should give good base line.

For factory ammo, an ES of 70 fps is not at all uncommon. If you are like the average shooter that does not shoot past 300 yards, it won't matter, as long as it's accurate. Assuming your chrono results are accurate, and the factory ammo is consistent enough to shoot within 3 fps,(which I've never seen with factory ammo, and very few handloads), it underscores the fact that "every rifle is a rule unto itself".

Want a low ES all the time?

Find the most velocity producing, temperature stable, appropriate powder for the cartridge, bullet weight, and barrel length. And tune a rifle for it, via a hand load.

I'll settle with this factory stuff.

5 is good, 10 is better.

I've seen retarded low ad from prime and the new federal 6.5 and 6mm gmm but most were on custom tubes with special reamers. Otherwise I agree with your statement.

A five round sample is not enough to give you good reliable stats. I agree with Chad...the 3 fps ES is a fluke.

I think patriot07 has something up his sleeve.

I forget what the sample size needs to be to get a statistically meaningful SD, but I remember it's a lot more than five. I know we're talking ES, but we're also talking about meaningful stats. Five shots don't cut it.

I was asking the OP how many he had used to calculate the 3 fps ES. I can't imagine it was 5-10 rounds...or it was a complete fluke or some type of chrono error.

Assuming no chrony issue, likely just an outcome of a small sample size. If you had a deck of cards and you randomly grab 3 cards there is a small a chance you get all 2s. Not likely but possible

Here's my take away. I've shot this ammo for about five years. Close to 350, 400 or so rounds. When I first started shooting it, I shot it at ranges of 100, 200, 300 and 400 yards. I measured drop, put that against the charts and determined that the velocity was very close to what Hornady published. I've killed everything I've shot at, longest was a whitetail at 402, many shots at 250 and 300, and the best for me was a coyote at 323 (small target, quick shot). In my opinion, this ammo is fantastic, and has been tested and proven by myself for my use under the situations I hunt in. It kills, quick. On the range, no complaints about the groups always under an inch, mostly <. I'll take the velocity with the chrono that day, along with my own field tests as assurance that Hornady is very accurate with the numbers they put on the box. Thanks for the opinions and information.