Getting 7 saum to feed in an fcp receiver?

Ok. I rebarreled my model 10 fcp 308 to 7 saum. Took some doing, but that's another thread.
Anyway, I bought a magazine meant to feed wsm based cartridges. The differences between the wsm and saum is in the order of .003-.004" in length, base, and shoulder diameter. Seemed insignificant enough that it should work, right? Wrong.
I can load the rounds into the magazine fairly easily, but when I go to feed and chamber a round the bullet hits the feed ramp in the receiver before the brass, and the cartridge is reluctant to tilt upward at this point so I can feel resistance in the bolt. With a little more effort I can get it to tilt up and continue to feed, but if I push too hard/fast the brass jumps out of the retainers in the magazine and jams up between the bolt face n receiver.
What am I doing wrong? Is this a magazine issue or a feed ramp issue, or yes?
I've already ground down the feed ramp in an attempt to fix the issue but if I go too far there'll be no material left for the recoil lug!
Do I just need to tweak the cartridge retainers to allow the round to tilt up further, easier?
Any help appreciated..

It's a "You're trying to use a magazine with a cartridge that it was never designed to be compatible with" issue. And yes, you'll need to tweak the feed lips on the magazine to allow the front of the cartridge to site a little higher. Believe it or not, stacked magazines are the hardest and most expensive part to design on a firearm.

I have done this with LRP rifles 3 times each feed great all you have to do is work the feed lips. We have used two 260 mags and one 6.5 creed the mags that came with the rifles. I also use a unaltered 308 mag we have and it works without doing anything just fine also. The amount of work on the feed lips was small just a little lift on each side didn't take much at all. the 7mm saum is awesome have fun with it.

Well, based on the cartridge dimensions and their differences, it looked like I needed to squeeze the base end of the feed lips by about .020 and the shoulder end by about .004. I did it and it feed pretty decent but two hours later the problem was back. Do I just have to keep tweaking them over time?

I did get out to shoot it, though, and you're right, it's a joy to shoot! And accurate. Only took 2 shots to get on target. This is the results of shots 2,3,4,5, and 6 down a brand new McGowan barrel, with a full barrel cleaning between shots 3 and 4, with Nosler accubond trophy grade factory ammo - .430MOA:
http://i813.photobucket.com/albums/z...pseba83213.jpg
And the rifle, before the trigger guard and butt stock bag:
http://i813.photobucket.com/albums/z...ps2bdd4d1c.jpg
http://i813.photobucket.com/albums/z...psd63765a7.jpg