Re: Neckturned Chamber?
Sure.
We'll use a 6mm BR as an example. Out of the box the neck of the cartridge measures .271" in outside diameter. Just store that in the hard drive for a moment.
The production process for brass hasn't really changed all that much over the years. You start with a little slug of brass and then basically squish the snot out of it with several tons of force and "poop" it into the shape of a shotgun case. From there the rims are either machined or again "squished" into shape. Then it's drawn through a series of dies to produce the final cartridge. The better companies have invested heavily in ensuring this process is repeatable over long production runs.
They also anneal (soften) a portion of the case (usually the neck area) afterwards because brass likes to work harden. (take a piece of wire and bend it back and forth over and over. Eventually it snaps at the root of the bend. This is work hardening)
Now, with millions and millions of little cartridge cases rolling off the assembly line on an 8 hour shift its a testament to modern manufacturing that they even go into the firearm's chamber with any kind of reliability. The fact that they hold these tolerances over and over again is quite impressive.
Be that as it may there is room for improvement in some instances.
Ok, back to the BR. Take a micrometer (not dial calipers) capable of measuring the wall thickness of thin wall tubing and make note of the wall thickness around the periphery of your cases. It's an almost certainty that its going to vary some.
In the interest of small groups and selling gadgets tools exist to remove material and make the walls a uniform thickness.
It does work and in some instances it does make an improvement. However one should be aware of all sides of this practice before diving into it head first. First is application. If a guy is truly an "operator" with any kind of law enforcement agency he's kicked the door wide effin open for litigation/liability if he's ever called upon to fire his weapon. The shark lawyers will eat him alive claiming he's a psycho gun nut hell bent on murdering people (why else would he craft such lethal ammunition?) or that he's an incompetent loon for tampering with ammo that then failed to fire (for some reason) when he needed to react to a crisis situation.
Next and far less serious but just as annoying. Lets say your going to Africa or some other "once in a lifetime trip". You spend thousands on a gun and hundreds of hours preparing your cases for the trip. Some jackoff loses/steals your ammo and now your stuck in the boonies with a gun that uses one of a kind bullets that no one else for 10000 miles has. - The guide service will get a chuckle out of you for sure. . .KA CHING Sucker. . .(seen this more than once)
Last. You do all this work only to discover that your cases now look like a mountain range when you stack them side by side because the neck tension wasn't high enough to resist the bumpy roads to the range/match/campsite. Your seating depths are all over the place. Many a competitive shooter driving across the US on their way to Camp Perry has learned this lesson the hard way for the exact same reason.
Never mind what they are going to do when rattling around in the magazine all day.
During my trips to Perry there's been many a late night I've seen some poor bastage up all night seating bullets under the romantic glow that only 40watts of sodium insect attracting sexiness can provide in the huts after thawing out his VISA card at the Sinclair store on commercial row just so he can shoot the presidents 100 or whatever the following morning.
Last (and then I'll shut up) A case MUST have room to grow. So many guys build a tight neck gun and set the necks at .0000001" smaller than the case. Only to find that they now have excessive pressure, a gun that runs poorly, give horrible accuracy, and is a general pain in the butt to shoot. Never mind the potential trip to the ER to have gun pats removed from face parts cause it blew up due to excessive pressure.
You also have to appreciate what happens to pressure curves when the case can't yield to release the bullet. They spike (violently) and this causes huge variances in velocity. Your super dooper rifle is now likely to string from one end of the 8 ring to the other.
A case is a balloon and it has to have room to expand when the fires get lit. You need a MINIMUM of .0015" (.002" is better IMO) between the outside of the case and the inside of the chamber. This is all the way around so it'll be a .003" to .004" difference in diameter. On our .271" ND 6mm BR the bullet measures .2428" in most cases. The case is .271". This gives a case wall of just over .014" thousandths. You never want to get below .01" so your only going to take .002" worth of material off the case at any given point.
Now ask yourself if a bullet traveling at 2+ times the speed of sound over hundreds of yards really cares about something smaller than the width of the hair on your head?
I have to say in most cases no. Especially if your using boat tail bullets. . .(thats another story)
Bottom line. Neck turning should (in my opinion) begin, stay, and end with bench rest guns. They are purpose built specialty (novelty) rifles that operate within very narrow parameters. For any kind of working gun its just not worth it IMO.
Hope this answered your question.
