9mm Upper build question.

AR/M4 AR Pistols & Pistol Caliber ARs
1

I have bought a 9mm RRA upper that has had the barrel removed, a barrel and a YHM Specter FF handguard. Now looking at these pieces, especially the barrel, it looks like I’m missing a pin to keep the barrel from rotating or sliding out…not sure the purpose or if it’s even the case.

Anyone understand what I’m talking about? Is there a online tutorial or manual anywhere?

Thanks everyone!

P.S.
SBR pics will be posted once this all comes together!:cool:
I have my 5.56 complete, now I need my 9mm upper on stand-by!!:wink:

2

The barrel nut (from your handguard) is what keeps the barrel from “sliding out.”

The barrel should have an index pin which fits into a corresponding slot at TDC (top dead center) of the threaded boss at the front of the upper.

If your barrel does not have such a pin, it is defective. It may be a simple matter of putting a replacement in their yourself, but I’ve never done it personally, so I don’t know.

if you’re talking about the handguard itself rotating, the YHM should come with two anti-rotation screws to prevent that.

3

It’s not RR, but I built a 9mm AR, and the barrel (Spikes) does NOT have an index pin, which makes sense because it has no feed lips or gas hole, so it doesnt matter how it is indexed.
Hope this helps.

4

I have had several 9mm barrels, from a variety of makers including Colt, KAC, and TROS, and each had an index pin.

I don’t think I’d want a barrel without one.

5

Yes, I’m speaking of the index pin now that everyone has explained it. Can I buy a index pin or am I screwed?

Thanks everyone for all the replies!:slight_smile:

6

Is there a hole in the barrel? If not, one would need to be drilled, and it should be pretty easy to put the appropriately sizd pin in there.

Per the other poster, I guess some people making 9mm barrels are omitting the index pin.

If the barrel is torqued up properly, I guess there isn’t much chance of the barrel rotating, but I can’t see any benefit to leaving out the index pin, other than to make the barrel a little cheaper to make.

7

Yes, there is a hole where a pin looks like it could sit in and that is why I brought up the question of if one was needed. Where could I buy one?

I’m looking too at maybe selling and getting a TROS 5" 3 lug as that is really what I want.

8

Please explain why you would need one for a 9mm build

9

I’m sure the barrels without one will probably work just fine.

With quality barrels out there with an index pin, I’ll just stick with them.

10

Sorry, I can’t help with that.

Maybe someone else knows, or maybe you don’t really need it at all.

11

BTT, for needed reply.:confused:

12

ok, can you take a pic? is there a hole in the barrel, or a space in the threads?
If there is a hole in the barrel, do you have groves for taper pins for a standard A2 front sight?

Ultimately, you do not need an index pin, because a 9mm barrel A) does not have a barrel extension with feed lips,
and B) does not have a gas port to properly index/line up

So why worry about it? I have a barrel made by a solid company, it has no index pin because it does not need one for any reason, if i had or you have an A2 front sight, then you might find it useful, but not necessary. It doesnt matter how i spin my barrel, it will still work.

13

I know that my S&W 76, which screws the barrel to the receiver with a barrel nut much like the AR, does not index the barrel to the receiver, and as a result, the barrel can rotate in the receiver. I put witness marks on mine and there was around 30 deg rotation after around 1000 rounds. That obviously isn’t good for its accuracy, and it’s one of the reasons why the Swedish K is a better SMG.

Now, the S&W 76 barrel nut is hand-tight as opposed to 30-80 ft*lbs, so maybe this is not anything to worry about in an AR without an index pin. I don’t have the expertise to make that call.

But personally, if I couold put the pin in there without too much trouble, I’d do it.

14

Tighten it to 75lbs and forget about it, I have not seen any rotation in mine, and I piss through the rounds.

Still want to know whether there is a space in the threads, or a hole in the barrel.

15

Thanks for the torque setting…I’ll give it a try.:cool:

And yes, the is both the hole in the barrel and slot in the threads to accept a index pin “IF” I had one.