If its a wsm you need to bump the shoulder back a few thou .
Check out shortmag site ,
http://www.netrifle.com/forum1/default.asp
Heres what they have to say about the problem.
We get many posts from folks loading the short mags for the first time who are puzzled when their handloads will not chamber easily. Though there are a number of possible causes, in nearly every case the problem is caused by the sizing die not being set up properly.
Short answer - Here are two quotes from previous threads that will point you in the right direction:
"Many of us find we have to get all the sizing the die can provide. That means not only bumping the shell holder, but making sure all spring is removed from the press linkage. That means there should be resistance at the end of the stroke as the press cams over. - Moderator"
"This is a problem we solved here almost 3 years ago with the ShortMags. You need to 'bump' the brass case shoulder back about 2 to 3 thou. Get yourself the Stoney Point Head & Shoulder Gauge. Take measurements of the case and then set your FL sizing die to move the shoulder back about 0.002" to start and they should slip right in to your chamber. I wouldn't move the shoulder more than 4 or 5 thou though, you then get into working the brass too much. - Hunt-Nut"
Long answer - First, full-length re-size. Neck sizing does not work well with the short mags for most reloaders.
Second, you can NOT setup your sizing die in the same manner you are used to with standard cartridges. You will need to get virtually all the sizing your dies can provide. To do that, you will need to adjust your die down further than you are used to - down past contact with the shellholder. You will need to have it require extra force to get the press handle to "cam over" at the end of its stroke. This extra effort removes all the 'play' in your linkage and gives you the little extra bit of sizing the short mags need.
To do this right, getting enough sizing for easy chambering but without overworking the brass (if that's even possible with your press/die combo), you need to be setting the shoulder back approximately .002". One of the easiest and least expensive ways to check this is to use the Stoney Point device made for this purpose. It's called the Head 'n Shoulders Cartridge Headspace Gauge. Read about it at the Stoney Point site. It is not expensive - here's one good place to get it.
Once you have the gauge on hand, you can measure the shoulder position on your fired, unsized cases. After res-sizing, measuring again should show the shoulder moved back aboout .002". You may be surprised to find that if you have not turned your sizing die in further than normal, the shoulder may not be moved back at all. Turn the sizer die in in small increments, just a small fraction of a turn, until you get the target amount of shoulder set back. If your dies allow for more than .002" of set back, going further will shorten brass life so if .002" solves your chambering problem, stop there.
If you have followed these instructions and still have problems with difficult chambering of handloads (even though factory loads chamber fine) - then start a new thread in the reloading procedures section and note that you have already tried the procedure in the FAQ section. We will help you figure what else may be causing the problem.
"The very atmosphere of firearms anywhere and everywhere restrains evil interference- they deserve a place of honor with all that is good." - George Washington