No it isn't, it is the best case for it, you just have to do more work before you use it. I did this up for my web page Savage Lever Actions of North America
Resizing 307 Win to 303 Savage
This has been a warm topic as of late, with Bertrams and Norma going for at least a $1 a piece. People have been asking what is the best case to resize from. After plenty or reading of various reloading manuals, and some talking with a few others that resize. I have come to the conclusion that the 307 Winchester is the best candidate.
The reason I have come to this descision is due to the case size and the case head, people have suggested using the 220 Swift brass, but that won’t work either. Case is right for resizing but the case head is the same size as a 250 Savage case head, which means you will get quite a few problems of the case not extracting properly(know this from personal experience) 250 will not always extract using a 303 Savage bolt, had an H that had been converted to 250 and it was not very good on positive extraction. You could use the 30-30 in a pinch but it is pretty much a 1 shot affair, the cases tend to split. John Barness (Mule Deer) uses the 30-40 Krag brass, but in my eyes the case is too thin, but just my opinion, John has been at this a bit longer than I have.
The first thing you need to do is get your 307 cases, I recommend Winchester brass from Midway, as they are really reasonable on cost, I think 30.00 for 100 pieces. Get yourself your 303 Dies, Lees are good, I am using redding dies now, I use my Lee resizing die just to resize the Winchester cases now. Once you have your brass and your dies you can go ahead and get started. Mind you there is some work involved, but when you are finished I think you will be satisfied. If you have an O press this may go a bit easier.
Second is to Lube all of your cases, then run them thru the sizing die, do it in three steps. Go down about 1/3 of the way on the case then lube again, go down another 1/3 of the way then lube again, finally run the case down home. You will notice that your cases look like very small belted mag cases when you are done, this will be taken care of in the next step.
Step1
Step 2
Step 3
Once all of your brass is resized you will need to remove the belt from the case, I suggest a strong drill a junk, a handful of 30 cal bullets and a good file. I lightly seat the bullet in to at least 10 cases. Place the bullet end into the chuck on the drill and remove the belt with the file. You need to bring it down to .440. Do this with all 100.
Trimming
Before trimming and After
Finally you need to trim them down to spec in case length. Helps if you have a power case trimmer, trust me on this one. You need to trim it back to 2.05. When you are finished, put those used bullets back to use, put the bullets into 10 cartridges at a time and ensure they cycle thru your action. I didn’t got to the range and had 4 out of the 100 that would chamber. I am on my 3 reload for the 1st 20 I did. Haven’t had any problems. Someone suggested annealing the case where you know the shoulder is going to be, I don’t but it may make life a bit easier.
Finished Product