Ok, both rifles are bolt action, I shot one round with the 303 powder with a 180gr bullet out of my 308. No signs of case being cracked or damaged or fouling in the barrel, primer stayed in place. Will i experiment some more? Maybe.
Barrel fouling doesn't really tell you anything as far as safety goes. If your load it too light or underpressure, you can get excessive fouling.
Generally the higher the pressures, the cleaner the burn.
Look for fouling around the neck of the brass case. If it seems really dirty after only one firing, that can mean there is not enough pressure to seal the brass into the throat and you're getting blowback. Blowback means gas pressure is able to leak past the seal between the brass and the chamber, and this can cause accelerated wear to your throat. You will get some blowback anyway, but it usually takes a few firing for it to really show on the brass unless you're running a semi. Realistically, I don't think this is anything to worry about, you're likely fine in that respect.
If you got cracked or damaged cases, well, you've got bigger problems.
What you want to look at is the primer and case head. You can judge how much pressure the load is generating by how warped and flattened the primer gets.
If the primer still has noticable rounded edges like it originally did before firing it, you're fine.
If the primer appears to have gotten flattened, that shows pressure. If it's a little bit flat but still shows the roundedness, you're going to be ok.
If the primer looks perfectly flat, and there is no more rounding on the edges, then you are over pressure and in the danger area.
Another thing to look for is markings on the case head, where the name and info is written. If you can see features of your bolt face , like the ejector, imprinted on the brass, that's another high pressure sign that you are in the danger area.