Well, my port cover spring on my BRN-10B broke the short leg off, so now my port door flops around. Does anyone know offhand if these use the same port door spring as an AR-15, or an AR-10, or ??? It doesn't appear to be anything proprietary, but better to ask first. I really don't want to shoot it with the port door flopping, as I don't want to risk damaging anything else.

Yes, same spring as the  AR15's.

Thank you! Now to pick one up and get it installed.

Yes, I agree with PHMDLJR - it's the same spring as an AR-15. A shame it broke, the springs Brownells used do seem brittle. And they're very long, you can get a nasty cut poking around in that port. I snipped that short leg a little shorter to make it less likely.

I did the same on mine.

IM your address and I'll drop a couple in the mail for you.

If you figure out a way to replace that spring without having to pull the whole damn barrel, I’d be interested to hear about it.

Remove the tiny C-shaped retaining ring from the front of the rod, and slide the rod out towards the rear of the receiver.

ETA: Apparently this method won't work on these rifles...

The hole the rod slides into at the rear is not a through hole on these. So you can’t slide out to the rear like you can on other rifles.

Thanks! IM sent.

Mace is correct; the barrel has to come off to get to it on this one best as I can tell. Guess adding the ability to just remove the C-clip to change it out was one of the improvements to the design over time?

Latest on the BRN-10B port door spring situation- A) I found a way to fix it without having to pull the barrel after all. Take off the handguards, then take off the gas tube. Once that is removed, turn the slip ring and the gas tube hole to the left until it lines up with the port door axle, then you can wiggle the axle forward just enough to get the axle through the gas tube hole and remove/replace the port door spring. Then slide it back into place, return the slip ring and gas tube hole to its upright position, reinstall the gas tube and pin, reinstall the handguards and you should be good to go. That way, you do not have to remove the barrel. This is a good thing, because in their infinite wisdom, it appears that whomever built these for Brownells installed the barrels dry (no anti-seize on the barrel threads). Dry enough that the barrel nut snapped off teeth trying to get it to turn. Thanks, Brownells. Anyway, if anyone else runs into the same issue on theirs, hopefully this thread will help?

which brings me to B) a friend of mine was wanting the rifle for a while now, and is talking about rebarreling it to 6.5 anyway. So I sold it to him, and am in the process of gathering a really nice Thompson M1 kit to do a semi-auto SBR with it to go with my MP-34 semi-auto SBR. So we both were happy with the deal. I still have my BRN-601, so I am still in the retro game (and LOVE the way that one is turning out. GREAT shooter) and am slowly getting it dialed in to where I want it detail-wise. When I get it finished I will post a thread for it.

Thanks Joker! Floppy door fixed with a new spring and your technique. Easy peasy, no bbl removal. I may jack your idea, but I'll always give you credit
Pooka

Glad to be able to contribute something useful back to the forum here.