neck clearance to chamber

[TackDriver284]

When I got the bolt back from being bushed for my new 6BR, I did some measuring before going further and found an issue on high pressure !! I got a fired case that I fired many weeks ago and tried to slide in a 6 mm bullet and it would not slide in freely and it was pretty snug and had to push the bullet in. I know I have a .271 chamber neck and I went ahead and resized two fired cases using the .268 bushing and seated two dummy rounds and measured the neck diameter. It came out to .270” So .001 “ clearance is not good, so i neck turned 49 pieces of unfired brass. I turned .0005” off and remeasured again and it measures .269” with a loaded round now. Should I go .003” instead of .002”? It should lessen the pressure with .002” but is .003” better for a better consistent bullet release? Would .268" on a loaded round be more ideal to fit in a .271" neck chamber?

 

[thatguySHM]

I don't think the pressure difference will be enough to say one way or another to be totally honest. I think I understand the question, but to clarify, what are you turning your necks to for thickness? What is the application for the rifle? I turn a lot of my necks, but not for tight necked chambers, more so for even and consistent tension and roundness. For hunting rifles I like a minimum 0.002" clearance, which would be 0.004' under chamber neck since you need it all the way around. For bench rifles 0.003" will provide a nice release and will give you a bit of wiggle room (0.0015") if some carbon gets in the neck area.

As far as neck tension, I've had better luck with higher velocity cartridges with a minimum of 0.0025", which I generally aim for 0.003" under loaded size bushing and will turn the other 0.0005" to custom fit. My 22-250 AI likes a 0.247" bushing with 0.013 necks in a 0.255' chamber. I tried a few different bushings until I settled on that set up.

Trial and error/success are your best path to find out what will shoot best, I wouldn't turn in big bunches until you have a rough idea of what works best. Just my two cents.

 

[TackDriver284]

For hunting rifles I like a minimum 0.002" clearance, which would be 0.004' under chamber neck since you need it all the way around. For bench rifles 0.003" will provide a nice release and will give you a bit of wiggle room (0.0015") if some carbon gets in the neck area.

Actually there was .001" clearance under chamber neck ( .0005" on one side ), its reason I had so much pressure where the primer flowed into the firing pin hole even at mild and minimum powder charges, bolt bush was one fix and the clearance was another issue that I had found. I had neck turned it the other day to measure from .001" to .002" under chamber neck, thought I could do it in increments and give it a run at the shooting range and see before going another thousandth neck turn to make it .0025 or .003". I had to order a full size die because I needed the neck sized all the way to the shoulder before neck turning, the bushing die I had can only size about 80% of the neck, and the other 20% of the neck by the shoulder junction was .002" larger than the top 80% of the neck which measured .267" after sizing with the bushing. I would not want to remove that material at the base of the necks because it would make it thinner with possible neck separating after so many firings. Ran a K&M neck turning mandrel prior to neck turning. Thanks for the reply, appreciate it.