Your issue is one of the most common to be found on reloading forums. And it is likely due to your having a over length case.
Overlength, that is not from base to case neck, but overlength from base to shoulder.
The set up instructions given with sizing dies assume a lot of things. And seldom following the instructions do you ever get the brass sized to the correct length. If possible you would like to set back the case shoulder about .003” from the fully expanded condition it comes out from the rifle. In no circumstance do you ever want to size it more than .006” as you are likely to get a case head separation on the next firing. There have been instances where I followed the instructions and I got a crush fit in the rifle chamber because the sizing die did not set the shoulder back enough. In those instances I have had to grind material off the bottom of a sizing die to get sufficient case sizing. Just take the instructions that the factory sends with their dies, and toss out the part that tells you to size to the shellholder. Or shellholder plus a ¼ turn. You will find that such guidance is inaccurate at best, rubbish on the average.
To properly size cases to a correct length you will need a new piece of equipment that has not been mentioned: case gages. I really like the Wilson type case gage. You size your round and drop it in the gage. This gage measures the distance between shoulder and base. It is a "go" and "no Go" gage. And it is a true measurement, as I have dropped my chamber headspace gages in my wilson gages and found perfect agreement between them. You want to size your case between “go” and “no go”, and for my rifles, I size everything to gage minimum.
If you want to try an experiment, size the case following the factory sizing die instructions. Then measure the sized case length with your case gage. If the case is between the “go” and “no go” of the gage, go buy a lottery ticket. Because it is your lucky day and you are going to be a millionaire. What you will probably see is a case that is over length. On a few occasions you will see an underlength case.
This web site is really useful for showing how to use case gages. I recommend looking at the pictures, and it explains the special case gages needed for the belted cartridges.
I have a belted magnum and had to buy the special Wilson case gage to figure out just how much to set the shoulder back. Headspace in belted magnums is measured off the belt. The distance from base to shoulder is not controlled and is all over the place for different manufacturers and their different reamers. This is also one of the reasons belted magnums have short case lives. People don't control the base to shoulder measurement, they over size these cases, and they get case head separations.
I have a hard time believing, but there are fans of belted magnums. That is in the same league as being a fan of drum brakes.
I do like my manual choke though.
http://www.realguns.com/Commentary/comar46.htm
The midsection of the Wilson gage is cut big. It only measures headspace. What the Wilson type gage and the other functionally equivalent gages do not measure is "fatness". This is an important measurement for gas guns and should be controlled. You do not want any resistance to bolt closure due to overlong cases or over fat cases. It is a safety consideration for gas rifles with free floating firing pins, it is a reliability consideration for the other few actions.
For those who want more sophistication than Wilson case gages, Sinclair makes gages that will measure the fired length of a case, and allow you to set the shoulder back from that dimension. I do have these Sinclair comparometers, found them useful when case gages are not available, but they are more complicated than a “go”, “no- go” gage.
