Originally Posted by Bart B
Regarding accuracy with semiautos..... Few have had their bolt faces squared up with the barrel's chamber axis. This is the reason military teams shooting top scores with new ammo (arsenal and commercial match stuff) could never get reloaded cases from them to shoot accurate in their M1 and M14 platforms. Same thing for AR platforms. Those fired cases had their heads press way out of square and resizing them did not square them back up. So, unless your AR's bolt face gets squared up properly, new cases will probably produce best accuracy.
This got me thinking, so I did a test.
I placed a Sinclair case holder for 223 Remington into my lathe. I then inserted a series of cases into the case holder, so the case head faced away from the chuck. Then, using a test indicator, measured axial runout on the headstamp part of the case head. This is difficult due to the headstamp itself causing the indicator to jump around, but got some useful data I think.
In addition, I took of couple of the case and carefully faced the head, noting how much I had to take off before the bottom of the case was dead flat. This assumes of course that the Sinclair case holder is straight, and after some measurements, am confident it is. The test results.
- Home built AR-15.
- Krieger barrel chambered in 223 Remington minimum SAAMI spec.
- Young Manufacturing Match chrome bolt carrier group.
- Sun Devil upper.
- "WCC 09" w/NATO stamp from box of Blackhills 223 Remington FMJ 55 grain ammunition (more on this later).
New unfired Blackhills with WCCbrass: .0025"
1x Fired Blackhills with WCC brass: .0025"
1x FiredBlackhills with WCC brass FL resized: .0016
Frankly, this in inconclusive since the dial indicator needle jumped around so much on the headstamp, I could never be sure of very high accuracy.
So, used the lathe to face off a couple of fired cases, until the head had been completely touched by the cutting tool showing shiny brass. I did this on two fired cases and both required a tad under .002" to square them up.
I did not do this on unfired cases, cause I didn't want to ruin a case by reducing its rim thickness, however small.
I may do more testing, but from where I sit, I am not seeing much difference between new cases, fired cases, and possibly even resized cases (even if they show a bit flatter). I need more confidence in the accuracy of the measurements.
There was one other thing I might test, but this is getting to be a lot of work!
As the case sits in the chamber, it is held tight against the right side wall. This is because the ejector plunger is pushing it that way, pivoting around the extractor claw. I wonder when the gun goes off, and the case is plastered against the walls, does the case straighten itself out, or does the left side blow further than the right? If it did, then the case centerline is not square with the bolt face. One way to find out is to mark where the extractor grabs it and then measure to see if measured case head runout is in any way consistent with where the extractor grabbed the case. A wild theory I know, but just wondering.
On the Blackhills ammo I said I would talk about. This ammo showed pressure signs. Yup, factory ammo. I wonder why 223 Remington ammo from Blackhills is using a 5.56 NATO case. The primers were slightly flattened and the brass had signs of ejector plunger smearing as well as the impression of the extractor seam on the bolt head. I am not shooting that stuff anymore.