So I am looking to get a BCG for another AR build. I saw over on AIM surplus that they had a BCG w/ C158 bolts for $99 but are out of stock. However, they did have a BCG with 9310 bolt for $89 in stock. I did some reading and it seems that C158 is a good steel (and the mil-spec), but 9310 is just as good.

Does anyone agree or disagree with this?

Also, does anyone have anything good or bad to say about AIM surplus BCGs in general ?

The only bolt I've ever broken was C158.

I have both.  I have no preference between them.  If the yield and tensile strength are at or above mil spec I don't care what alloy they're made out of.  I have 3 AIM BCGs.  They seem to work as well as anything else.

I have the 9310 bolt very good bolt. Actually 9310 can be hardened to be about 5% stronger than C158, if the process is done correctly. The reason they use 9310 is it can be bought in much smaller qualities than C158 can.

OP go for the AIM 9310, mine has been 100% reliable.

Double Post

Does shooting more 556 instead of 223 influence the choice of which steel is better for a bolt material?

Not for me. I'll spend enough money on ammo to break the bolt that buying a new one will be negligible.

Remember, Mil-Spec is a minimum.

Looks like I will be getting the 9310. Thanks y'all!

It looks like AIM has a NiB 9310 bolt on sale for $99 so I ordered that one.

9310 is supposed to be stronger IF the heat treat is done correctly.

It is somewhat more difficult to heat treat correctly compaired to C158.

The odds of getting a bad heat treat on a 9310 bolt is higher than a C158.

I've seen that mentioned, where does it come from?  They're both oil quenched alloys, and 9130 is listed with a higher range of critical temperature than C158.  I've heat treated a lot of different steels so I understand some of them act differently even when following the correct recipe, but generally speaking alloys that are heated in the same range and quenched in the same medium are not harder to do correctly than one or the other.

You'll hear all sorts of bullshit nonsense about mil spec vs 9310 but some of the most recognized names are using 9310 and without issue.  I've got a $200 Noveske (RCA) nitride BCG with a 9310 and a Rainier Arms Match sporting the same.  All are flawless in design and durability so fear not the 9310 bolt.................

So I ordered my BCG Friday and it took AIM until Tuesday to ship it out. I click on the tracking number and then I got the dreaded news; UPS MI.



I know it was free shipping and all, but I would rather not have to wait 2 weeks to get my order. I don't order from Midway or brownells very often because they either charge you a little for lousy UPS MI service or rake you over the coals with standard UPS or Fedex service. This might one of my only orders from AIM.

I don't know way more companies use Priority Mail. Its cheap and I have only once ever had a package be late and that was only by one day. A BCG would fit nicely into a small flat rate box.

This is why I get most things from Amazon with Prime service.