P-64 not chambering

[snailman153624]

Although the mag differences may be an issue for some, I doubt it has much to do with the OP's problem, as they had successfully fired 200 rounds before running into problems.

[fully machined]

That blue nose ammo looks like some old Glaser safety slug. Has the ammo you are trying out been loaded and unloaded, chambered and unchambered? Some of it looks like it has.