Did the rifle have a floor plate originally? The blind magazine version of the 700 action uses a mag box that is screwed to the underside of the action. Also the magazine depth of the blind box may be too deep allowing the spring to squirm around as pressure is applied. Try affixing the bottom leg of the mag spring to a shim to keep it from moving as the mag is loaded. In the past I've used a thin piece of metal with a loop for the bend of the bottom leg to catch on and a slight cut to position the cut end of the spring. The idea is to keep the spring centered in the mag well through out its travel.. I doesn't sound as if the lips need opening as it feeds sometimes.. mode likely the follower is diving because it is moving in the mag well.
You impress me once again.
This rifle was a BDL-style, SPS rifle with the hinged floorplate. I sold the SPS stock and put the action into a Classic stock that I had previously.
As you have all pointed out the rounds are nosediving as the bolt slides over them. This makes them go forward a bit and then down enough for bolt head to pass over base.
That is a great idea about fixing something to the base of the spring. I considered that the flat spring was sliding around in the magazine inlet.......but never did anything about it.
The box is the SPS box, I think that I sold the older ADL style box that I had....can't remember. If I can find it then I will look for a screw to stake it to the receiver. I know that the box is just loose in the inletting right now. I made effort to keep the receiver off of the box when I epoxy bedded the action. I bet that there is a gap.....
As of right now, I KNOW that it will cycle 3 rounds so for this deer season it will only be loaded with 3. If I can fix it between now and then, then I will.