Twist Rate in AR

[MCMXI]

Now, do you think the cases being SAAMI -.004 is affecting my accuracy given my chamber dimensions?

K3, I would say yes given my personal experience with my AR and other rifles. I've found that the best accuracy I've ever achieved in any of my rifles is when I use a case three, four or five times only neck-sizing as I go. As I mentioned, it may take a number of firings to conform to the chamber. With my bolt action rifles I don't bump the shoulder back unless it's absolutely necessary i.e. the round is getting hard to chamber. With my AR, I simply neck size every case and then bump the shoulder back 0.002 inches (from SAAMI + 0.001 inches which is the headspace dimension of my AR chamber) so that the rounds will feed easily and still give me great accuracy. I'm guessing that the new cases you're using are sized SAAMI - 0.004 inches so that they'll function in most rifles.

I've got one AR that takes new brass and pushes it to SAAMI +.006 or .007.

That's still within spec though. If I remember correctly, according to my Redding literature, SAAMI + 0.010 inches is considered to be within spec.

 

[K3]

That's still within spec though. If I remember correctly, according to my Redding literature, SAAMI + 0.010 inches is considered to be within spec.

I think that's right. That particular AR that gives my brass such a workout is quite a shooter, and it's not a varmint rifle. I cloverleafed 5 shots when it was scoped and I tried some of my varmint loads in it. I took the scope off and put a detachable handle with NM type sights on it. Wanted a 'rifle' rifle, if you know what I mean.

Just to be clear, you run your brass through the FL die, but only enough to bump the shoulder back where you want it. Right?

 

[MCMXI]

Just to be clear, you run your brass through the FL die, but only enough to bump the shoulder back where you want it. Right?

After cleaning the fired cases I do the following:
1. Neck size (and decap primer) using a Redding competition neck sizing die.
2. Measure headspace of each case using a Redding instant indicator comparator with a dial indicator.
3. If needed, bump the shoulder back (typically around 0.002 inches) using a Redding body die that doesn't touch the neck of the case.

So this sounds similar to what you're doing.

 

[Wildfire]

These guys are right.

Hey there;
These guys are right .... New brass will most often be short on Head space. No big deal. Rare that 1st firing with new brass is the most accurate. Most of the time fire formed cases are. That has already been said.

The Mic. Have and use these same MICs. I have found that some do not line up right . I now use the same MICs but ignore the numbers on them. I use my dial calipers on the ends of the mics. That gives me a more consistant reading. I have a set for the .300 win Mag. the numbers are so far off that they are worthless.
Some of my mics are dead on. But some are not that is why I stsrted useing the dial cailpers. Once I got used to that it went faster. I always keep (1) fire formed case for each gun. That is how I know where I am at.
I too set mine at .002 to .003" .....
I do not care for their Free bore tool. It usually sticks in the throat and does not always give the same measurements. They break pretty easy too.
One of my rifles is a REM. .308 PSS. It has a very long lead. .250" That one is very hard to set the bullet depth with the RCBS free bore tool.