I have a Rem 260 mountain rifle and lots of 308 brass, that I would like to use. I don't have a ball mic, a neck turner, or a reamer. I don't want to do a chamber cast.

I do have a digital caliper. Here is what I know.

Measuring the outside of the neck on several cases in several spots.

New loaded factory ammo 0.290
Once fired case 0.299
Loaded resized 308 case 0.297

I don't know how much it has to expand to release the bullet without raising pressure, or how much I can expect it to contract from the chamber wall.

From what I know, can I be sure these resized 308 cases are safe or unsafe? Not enough data?

Thanks

The maximum neck diameter of a loaded round listed by SAAMI is what you should go by. Not sure what they list. But online .2969" & .297" are posted as maximum. By your measurements, your neck diameter seems to be OK. But sizing down may form a donut at the neck/shoulder junction. This donut can be a problem when seating a bullet, making the neck to large to release the bullet freely on firing, raising pressures. Also, watch out for bullets with a pressure ring that can add as much as .0008" to the neck diameter. Never force a round into the chamber.

quote:

I don't know how much it has to expand to release the bullet without raising pressure,

All you need to do is compare the diameter of your loaded/reformed rounds to the normal fired diameter. If the fired neck size is larger than the loaded neck size you are good to go. It's almost a certainty you will be okay.

I apologize for being so vehmently in disagreement with a poster above, but SAAMI specs don't mean squat. It is the ACTUAL measurements of your chamber and brass that count, as it is not uncommon to find either or both outside of SAAMI specs.

Your fired brass, which has some degree of springback after firing, measures .299". Your loaded reformed .308 cases measure .297". This means that you have somewhere north of .002" (.001" per side) of clearance when the cartridge is fired between the bullet and the inside of the case neck.

Try placing a bullet inside a case neck that is .002" larger than the bullet and you'll find zero resistance to sliding it in and out. This should answer your questions.

Another exceedingly simply way to determine if your necks are too thick is to place a bullet inside the neck of a fired, unsized case by hand. If it goes at all, even if with some resistance (remember the springback) then you have fully adequate neck clearance.

As you've found with your detailed measurements, most factory chamber necks are on the generous side, thus the slight thickening of the neck walls when forming from a larger caliber typically results in a better (closer) fit.

Sometimes neck walls will require thinning, but I've found very few instances where this is required, even when necking .308 nickeled cases down to .243. This is NOT to say that ammunition cannot benefit from judicious neck turning (thinning) in that simply being small enough doesn't ensure being concentric and consistent in neck wall thickness.

You probably had about .002 of springback, so your neck diameter is probably about .301. That is good if your resized brass is .297.

In making 22PPC cases from 7.62 x 39 cases I tested neck clearances down to what I measured to .001" but did not go any smaller. There were no pressure problems detected.
Stonecreeks evaluation and explanation seem point on, but you will never know till you cautiously try it in your own rifle. Be brave but careful. roger

Thanks folks. I'll try them this weekend and see how they shoot. I loaded them last year, but was not sure about them being safe.

I do have some 260 cases from the factory ammo I started with, but I lost two in the woods when I shot a deer. 38 cases annoys me. If it were my land, I'd go find them.

This rifle hasn't shot well, so far, but I haven't tried much of anything. I have not even really scrubbed the bore.

Precision Shooting Reloading Guide recommends loaded cartridges necks be .002 smaller than KNOWN chamber neck diameter.

I have a 260ai, not excesively tight chamber. It doesn't run well w/ necked down 308 mattch brass w/o neck reaming. To make the brass, I have to neck down in steps or I lose about half the cases. I first neck to 7mm, then down to 6.5. I know some guys can do it in one pass, I just can't seem to. IMO, not worth the effort. Trade the brass off for some 260, 7-08 or 243 & go from there.

i've never had to do anything to 260 brass necking it down from 308