I have a bag of brass that a guy gave me years ago. It's all .30-40 Krag headstamp but the bag says "30-40 brass sized down to 303 Savage". I have an original 303 Sav shell and it looks like a dead ringer except the rim is thicker

That is possible in a stout press with a really good lube like Imperial Sizing Die Wax, but in my experience it's better to turn down the base and rim slightly. This doesn't require a lathe, just putting the case over .308 diameter rod in a drill motor, and using a fine file while the case is turning. As I recall, using the decapping rod from any .30 caliber Lee Loader as the rod is how I did it.

It's also possible to reform .220 Swift brass to work in the .303 Savage, without any "turning."

Did both when .303 Savage brass was scarce, but it takes a while! As I noted in Chapter 42 of the first Big Book of Gun Gack, "Loading the Savage Rifle Rounds," it took me about 20 minutes to make .30-40s into serviceable .303s. .220 Swift brass is easier (if you can find any), since it only needs to be necked up, trimmed and sized.

Thank you! Now I know if I get a 303 at some point I can turn these rims down and use this brass

303 brass is readily available.

Cool. And?

is there a question here ?

Just that your original statement had nothing to do with my question

Well, you can make 22-250s out of 30-06 machine gun empties But, it makes no sense.
Ditto 303 Sav from 30-40 Krag.
Aside from a lot of work, shooting one in a 30-40 Could be interesting since the headstamp is wrong and world is filled with dummies.

I've been a reloader for over 50 years and I've reformed so many cartridges from other parent cartridges over the years , I've never had an issue
its not for everyone .. but he can make 303 sav out of 30-40 ..it should be turned on a lathe .. A couple of decades ago 303 sav brass was crazy expensive and very hard to find.. I've made and formed brass for the 303 lots of time from 220swift, 307 win 30-40krag 303 brit .. maybe another one or two I've forgotten about .. now brass can be found and its reasonable really no need to reform brass for it at this time .. but you do what you gotta do when you want to shoot your rifle . its a good skill to learn ..
by the way Graf's and son has some in stock

Plab

Once again, I'm not reforming 30-40 into 303. I have a bag of brass that says "30-40 resized to 303 Savage". It's already done. By who I have no idea?

I have a box of 30-40's that were resized to 303 Savage. Bought them back when 303 Savage was practically impossible to find. They shoot fine. Whoever made them did trim the rim somewhat.

Let me see if I got this straight, Mike has a bag marked 303 made from 30-40, and wants to know if he should take a chance and load them. Seems like a yes, Mike, you can do that. Or, No mike, you should not do that, would be the two appropriate answers, and have the bases covered?

Back when the .303 brass was in such short supply there was a commercial provider that made quite an amount of brass from the 30-40. For the life of me I can't recall the name.

I bought some of the brass and still have a few pieces I believe. It worked OK but some of it did split after a few reloadings.

If it fits the .303 chamber OK I'd certainly use it.

Buffalo Arms? I know they used to sell some reform 303S brass.

Yes, that sounds right. Thanks!

I've bought a fair bit of brass from them over the past 5 years or so, but never any of thier reforms.

They had a fire sale on the PPU 303 brass a few years ago. It was well under $10 a bag, so I bought about 500 rounds. Picked up a bunch of Starline 375 Win there last fall, and some 250 and 300 Savage over the years. They seem to get obsolete stuff in stock more often than a lot of other online shops.