308/307 cartridge

[William T. Watts]

In therory a 308Win and 307Win share the same demensions except for the rimmed case the 307 cartridge has, both rounds use the 308Win reloading dies. I've been kicking around the idea I could use 308 cases and feed them by hand into the chamber, there isn't a problem with extraction so this should be doable. I turned the rims on a 1st fired and unfired 307Win case so they would fit in a 308Win L.E.Wilson cartridge case gage and found the fired case was .017" longer than the minimum length of new unfired case. In therory they should have been no more than .004" difference but weren't. Obiviously The new cases I had fired thru my 307 rifle and resized back to minimum were taking a beating from the firing and resizing process, I have since readjusted my resizing die to bump the shoulder back only .001- .002 and life of the cases should be extended. For those of you that have calibers that have rimmed cases it may be time to examine turn down your resizing die until it touches the shell holder, then turn down an additional 1/4 turn to ensure your cases are fully sized. The idea of using 308 cases in a 307 isn't quite as practical as I had first thought, further the chambering of my Winchester Big Bore rifle isn't as precision as I first thought. Non the less this has been interesting and can be done, for the time being as long as new cases are available I'm putting this one to rest!! William

[Tom Matiska]

The exterior dimensions of the 307 and 308 are the same. I thought the 307 to have thicker case walls more similar to the mil spec brass than its civilian cousin. That plus seating depth are why the 307/308 data differs.

Just to be clear to others, we are not talking about using factory 308's or using 308 data in a 307 chamber, just using the 308 brass as a rimless subsitute.


As you seemed to have figured, the rim isn't just an extraction feature, but is also how the round head spaces. You observed how it was working the brass, but I'm wondering if the headspace was effecting accuracy also?

Have you tried more groups with the dies backed off from the shell holder?

[William T. Watts]

Tom you are correct about using 307 load data in 308 cases, external dimensions of the 307 Winchester and 308 Remington case (Factory) is very close but not the same. The 307 case is heavier by 8-10 grns and most of the weight is in the wider and somewhat thicker rim, additionally internally the web is thicker/heavier in the 307 factory case. I used new factory 307 Winchester cases with the rim turned down enough the case will enter a 308 L.E. Wilson cartridge case gage, those cases almost reached the max step on the gage. In my first post I mentioned the chamber in my 307 rifle is .017" longer than a 308 chamber. The two chambers are different enough to abandon any thought of using 308 cases even for feeding by hand, plus the wider rim is needed for proper feeding in a 307 rifle because the follower will not align a loaded round with the chamber. I've been loading ammunition for 45 years, I've also completed the Gunsmith program at Trinidad St. Jr. College, I did nothing to put myself at risk, if you try this you may. Don't do it!! William