I am new to precision reloading, I have been reloading for my factory guns for a few years, and now have a couple customs with smaller than factory necks in the chambers, one of my guns has a .297 neck how undersized from that diameter should a loaded case be to insure safe bullet release when its fired??

Hey c c

It depends upon what caliber the .297" neck is on.

.277 and you have a serious problem, .264" and you are fine and may not have to turn.

.003" is the usual recommended clearance around the neck. So the math would be .297"-.003"= .294"-.264" (I assume you have a 6.5)=.030"/2 (each side of neck)= .015" neck thickness on brass. That is still pretty thick and it will depend upon what type of brass and how thick that batch is as to whether you even have to turn. Normal accepted limits for turning are no thinner than .010".

I have a .292" neck and I have to turn that brass down to .0125" each side.

so would .003 be the minimum??? yeah the gun is a 6.5 loaded necked are about .293-.294 range

Serious BR shooters often aim for maybe 2 thousanths of clearance.

"Safe" can mean different things. I believe any standard caliber sporter rifle needs at least a minimum SAAMI spec. neck so it won't KABOOM if someone chambers a factory round later. No tiny bit of increased accuracy potential is worth anyones life or vision. IMHO.

so is it safe to have .002 clearance as long as I keep a close eye on it?

.003" is a good place, especially for a hunting gun. It is also good for me since I can inside ream with a Forster reamer which is .0025" to .003" larger than caliber. After fire forming the .003" outside neck clearance transfers to the inside neck diameter and then I can ream.

I recently assisted a friend who wanted a 6.5 rem mag like mine but wanted to be able to shoot factory ammo. I bought a box of factory shells and measured each one and we decided on a .297" neck. Only trouble is that the neck came out to .301". Don't be surprised if your chamber neck is not exactly on what you wanted. Just measure the outside after fire forming.

Some bench rest shooters use .0005 clearance.
But you have to understand they use short/light bullets for the diameter. A long heavy bullet could rivet the base and you will have the possibility of a problem if the neck is too tight.

.002" clearance around the loaded neck is about as small as you should go. As Wood mentioned, for a hunting gun .003-.004" probably makes more sense for reliability.

It is true that some (BR shooters) use smaller neck clearances - all the way down to what they call "fitted necks": the case is not resized, as the neck can not expand and will thus still hold bullet after firing. This is a pretty specialized method though, and I doubt it makes sense to apply to general purpose shooting. (Most BR shooters size their cases these days, fitted necks are not that commonly used).

- mike