A guy offered me a model 35 today on a trade. I don't want a 40, I'm looking for a 9mm. How difficult is it to put a 9mm barrel to make it a 34. Is the slide or breech face the same? This would be my first glock.
All I can offer is some of my experiences with the 34 I shoot in IDPA, etc. A great pistol by the way! You didn't mention the intended use but the 35 is a pussycat to shoot, obviously the 34 is even more so. You might want to look @ the inside of the slide on a 34 to compare it to a 35. even compared to a G17 the 34 has pretty extensive milling done to the interior of the slide to Hog out a lot of weight. Suspect this has a lot to do with being able to cycle all "power factors" of 9 mm ammo.
I know of several locals who have put Lone Wolf 9MM barrels in their 40 Glocks. The 40 extractor grips the 9MM case fine even though the 40 has a larger breachface. Personally, I'd be ok with this set-up for competition but not for carry. BTW, the slide is milled on the 34/35 to maintain the same slide mass as the 17/22 for functional reasons.
For what it is worth (not much) my understanding is it depends on who made the bbl as to if the extracter needs to be changed. Check with the Company and go with what they say. Other than bbl and maybe the extractor mags is all that is required.