So I am newer to reloading and sizing brass. I bought a .243 and wanted a little more brass to make up some different loads. A buddy of mine use to have a 308 and gave me about 60 empty brass cases.

SO I did some looking on the internet and saw I could use dies to size the 308 down to 243. I took the 308 brass and ran it through my 7mm-08 die first and then through my 243 die. I only did this with about 5 cases to see if it could be done.

Well it worked, but my question is when doing this is there a safety factor I need to know about or is this just a bad idea all around or am I ok and on the right path.

Any and all advice is welcome. Thanks for help and advice

With this radical a neck sizing you have to worry about neck clearance. When a case is sized down, the neck thickness increases.
If the brass in the neck gets too thick, there will not be room enough to expand and release the bullet, which could lead to a catastrophic failure of the cartridge case. Measure a fired .243 case from your rifle against a resized .308 case with a bullet seated in it. If the resized .308 case is as large or close to as large as the fired case, the neck thickness is too great and needs to be reduced.

Neck thickness can be reduced either by reaming or by outside neck turning.

I have sized hundreds of 308 cases down to 243 including 308 military brass. I would suggest you take a couple of your sized 308 cases and load them with a starting load of powder. Fire those rounds then take the fired cases and try to insert a 243 cal bullet in the neck. If the bullet falls into the case you are good to go. If the bullet cannot be pushed into the case with your fingers you need to ream the necks

Sure, you can resize 308 to 243, but I see no reason to do this. 243 brass is so common and is readily available even as used brass.

Neck thickness will likely be an issue in most chambers. Good advice above to find out.

Yes, sizing down from .30 to .24 will make for thicker necks, but the likelihood of it being a problem is overblown. Commercial rifle chambers have necks that are rather generous. Often, the neck-down process simply results in brass that is a better fit for the chamber.

In instances where the neck is actually too thick for the chamber then you will experience hard chambering. If you cram a too-thick neck into the chamber the extra resistance to the bullet's initial movement will raise pressures, perhaps unacceptably. Checking to see if a bullet will easily re-enter a fired case will certainly tell you whether your necks are too thick, but you can tell the same thing by whether there is chambering resistance (however, chambering resistance can also be caused by other dimensional mismatches, so don't make any assumptions.)

The length of the converted brass is usually more of an issue, and I'm surprised that no one has mentioned it. When you neck down the .308 brass the displaced brass will not only make the neck thicker but will make it longer. It is almost certain that you will need to trim your necked-down brass back to spec length. Too-long brass causes a similar problem as too-thick brass in that if it engages the leade of the rifling the act of chambering it will crimp it against the bullet resulting in a similar pressure increase as a too-thick neck.

Bottom line: I have reduced hundreds of .308 cases to .243 and only with a few certain types of military brass have I found that the necks required thinning.

I prefer to use the larger brass and then cut for thickness.
I prefer to do it for all of my rifles that need it.
several of them don't.
some of them were cut so that no matter what they had to be cut for proper thickness.

the easy way to find out.
take one of your fired cases and measure the OD of the neck.
size one of your made cases so the ID is whatever your expander ball makes it and measure the OD of that case.
if your more than ,005 under the fired case you got nothing to worry about.
if your over you gotta cut, and if your close [within the .005] seat a bullet and measure again.

Neck thickness need not be a problem just another step in the process..I would check for a donut at the shoulder junction first of all, but there are not I would either inside neck ream or outside neck ream, both work but some argument exist on that..Ive done both and both worked just fine..but inside with get that donut if its there..

I know some inside neck ream all their cases, some also outside neck ream all their cases, this is a big mistake, it wasn't ment to used (abused) in this manor.

and you can't put it back.
most rifles have large enough neck clearance they would definitely benefit from more brass in there filling it up.

Check OAL again after sizing. Going down in caliber makes for longer necks and going up gives shorter necks as a rule. Be Well. Packy