[dburkhead]

Okay, that makes sense. Then it's not a "push feed"/"controlled feed" issue so much as a case of on my action, on all rounds except the 4th (2nd on the left) the round is fairly far forward in the chamber before the rear pops free of the rails and up onto the bolt face. On the others, all of them waited until fairly far forward before springing free of the rails. The 4th round in mine, came up from the rails fairly early, however (I have no idea why) and was held by the extractor and the "lip" on the other side of the bolt and so went straight forward rather than angling across like the other rounds do, which is why that round didn't "stick" like the second round did. (And there was no lump of metal on the left side so the odd numbered rounds didn't have anything to hang on.)

In any case, it's working now.

Next step is a barrel.