nick, was it a mk II or III? it sounds like the barrel wasnt inserted all the way.
the breech must be in full contact with the rear bushing (that circle you attach with
the 7 rivets.) when viewed from the extractor port. it must look & fit like one solid piece. if there is the slightest gap between the two, the bolt will be stopped by the bushing before it can come into contact with the barrel, creating the exact problem you state. thats the problem with some templates. if they are off by even a little bit in crucial areas your screwd. but your problem is easy to fix. first, file down the 4 front rivits at the opposite end, near the barrel. then punch in the remainder so you can remove the front bushing. remove the barrel and clean both the outside bottom of the barrel and the part where it meets the rear bushing to make sure there are no bits of metal from the machining that would keep them from mating perfictly. then re insert bushing & barrel & while viewing the breech from the ejector slot. when it looks like one solid rounded piece and it can go no farther its done. your old holes, (if you can see them) should be slightly off now. thats good. find a way to hold the barrel assembly in place & tight while you drill 4 NEW HOLES right through the tubing & into the front bushing,then rivet.
note: carefull to make sure you dont hit the existing holes when you make your new ones or they will be too big & allow movement. then you can use some kind of filler,paste to fill in the 4 old holes in the tube & sand flush & no one willever know they were there after painting. ive used "instant muffeler solder" in the past its avail at most auto stores & seems to work well. but latley ive been trying "BONDO" with excellent results. i figure if its good enough for cars its good enough to hide cosmetic flaws on non stress parts. i even use it to "clean up" some of my welds after ive done all i could with the grinder & sanding. any way i hope this helps, because i suck at typing and it took over an hour to write this. when building stens in the future just ignore the front 4 holes and drill your own, clear through the tube and bushing, after fitting. p.s. instead of rivets i use machine set screws after drilling & tapping the holes. i find this gives a much nicer look in non stressful parts.
let me know how it goes! Rusty.