Installing a tightbore
 

Quick question: while disassembling my MP5, I didn't take care to notice the orientation of the old inner barrel or the hop up rubber. Like the old one, my 6.03's got a couple of notches and a hole, and I'm not certain how to line them up, or how exactly the hop up rubber fits onto the barrel (should it be covering the hole or just behind it, etc).

Can anybody instruct me or post a link with some relevant pics? I've been looking at this all afternoon and am surprised that no one's discussed this yet.

The rubber hopup sleeve will have a nub running down the length of the inside which fits onto the barrels's notch (about 1/2 to 3/4 of an inch long). You should slide the sleeve on until it stops. It should be covering the hole and quite a bit more (2x to 3x the length of the hole)

Aha, I hadn't noticed that nub. OK, so I've fit the rubber on. Now how do I position the assembled barrel and attached hop-up rubber into the hop up/gearbox/upper receiver portion? Does it matter how I shove it in there?

I believe that "notch" on the barrel faces down... I had my mp5 apart a few days ago, and I think thats how it went, id check but its at the shop

Barrel notch faces down with rubber sleeve nub also at the bottom fitting into the cut out that is on the bottom... Hope this helps

Make sure you rub some silicon oil on the outside of the rubber, or else you might either tear it when installing it, or it won't be installed correctly and you'll get bad airleaking issues.

Uh, nope, can't say it does. But what the heck, I'll try it anyway... posting results in a few minutes.

Roger that. I'm still having trouble getting it to stay on the barrel properly while easing it in there. Huge pain in the neck so far.

EDIT: Actually, so far I'm having more success without lube. Taking care not to tear it, of course.