Scorch: thanks for that. I was on the chat with people last night, and one of the had a receiver with exactly the same series as mine! The consensus there was as you said: the 243 rounds are too small for the well. Also, it seems these PH's had the aluminium well, which demands to be kept very clean and lubricated in order to feed well.
Question - when you mentioned installing rails to guide the shoulders, do we want to guide it all the way down to the bottom of the well?
Interestingly, the 7x57Mauser and the 243 share many similar dimensions, up to the shoulder/neck where the 243's shoulder starts a full 4 mm before the 7x57. The neck is 1.25mm narrower than the 7x57 too.
What is your experience with the spring? Could the spring on my gun be weak and need replacement?
I noted that my follower had been cut in the front part, down to 3" in length. There's a whopping 10mm gap from the front of the follower, up to the front wall of the well. I noticed that the tip of the rounds seem to catch in the breech, when pushed into the chamber. The exposed soft point on the bullet starts to get a bit roughed up after a while...
Since I to space from the back, what do you think of cutting the base/foot of the spring to sit the follower almost flush with the front wall of the well? Then I'd have space to fit the spacer in the back too.
Thanks all for the comments so far!