DON'T GO GRINDING OR POLISHING ON ANYTHING UNTIL YOU CLOSE THE ACTION ON AN EMPTY CHAMBER AND PULL THE TRIGGER, LOOK AT THE HAMMER AND MAKE SURE IT IS NOW RESTING AGAINST THE FRAME.
You shouldn't have to slam the action closed.
If the hammer fell to the frame, then you know your cases are too long (shoulder to rim) you will have to turn your die down a little bit to correct that (1/8th turn at a time).
Size 1 case (@ 1/8 down), put in the chamber, close, pull hammer, and trigger to see if it falls to the frame now.
If it doesn't turn the die down another 1/8th turn and repeat until the hammer will reliably drop.
You don't want to set it up to headspace on the rim, if you do you'll just end up going thru cases faster.
These cases stretch enough if sized correctly, you don't want to add anything more to it.
If you enjoy shooting this cal., I suggest a light bullet and small powder charge to save your cases. I have to trim every 2 firings w/ hunting loads.
BTW Once you have your die setup, you can use 30-30 brass. You'll have to fireform them, but they are a little cheaper than 7-30 brass. You'll also have to keep them seperate and load them different than the 7-30 cases.
BTW2 DON'T BUY FEDERAL 30-30 CASES, IF YOU DO DECIDE TO GO THIS ROUTE. I had to pull bullets and throw 120 cases away when the necks started splitting. I was told remington and winchester will not do that. I haven't bought new brass yet, so I can not confirm that. Good Luck.