A friend of mine has a stripped Doublestar lower that he would like to make into a dedicated 9mm rifle. He is committed to buying a quality upper, possibly a Colt 6951, that takes Colt style magazines. What is needed for the lower receiver in order to make it into a dedicated AR 9mm rifle? I know he will need a Hahn Receiver block (which one), quality lower parts kit, and 9mm buffer. What else am I missing? Thanks

I'm far from the expert here, but just to throw it out there, he can get a dedicated 9mm lower from PSA for (I believe) less than the cost of a Hahn mag block.

I tried telling him that, but he prefers the block just in case he decides he does not like the 9mm platform

I don't know if Colt bolts are ramped or not, but make sure you know before buying an LPK. Determines whether or not you need a 9mm hammer.

Gotcha, even though he could still have the Doublestar lower and the PSA lower for less than the cost of adding the block, it isn't worth arguing with people who know what they want.  I guess my best advice would be to narrow down which upper he plans to buy and then go into the industry forum here for that manufacturer (if they're on here) to see what bcg/hammer combo have worked with those specific upper.

Starting with a stripped lower here's what I did on my 9mm build:

Standard LPK
ADCO ramped 9mm bolt
9mm Barrel 16" at first, now an 8" SBR suppressed
Standard upper, railed flat top with Trijicon RMR

Started with a Hahn removable mag block.  Had nothing but problems with it.  Failures to feed mostly, some FTE's

Switched to a dedicated Hahn mag block.  Goes in under the bolt catch.  No problems since.  Oh yeah, and I use all Metalform mags.

If he's committed to buying a Colt upper, just tell him to buy all Colt parts.

Otherwise, since you already have a lower, the following will work just as well at substantially less cost.
DPMS LPK
KNS pins (cheap insurance against broken hammer pins)
KAK block and 5 oz carbine or 7.5 oz dedicated buffer
Spinta 9mm bolt (regular or blem)
Spinta barrel (they have 4.5, 7.5, 8, 8.5, and 16")
PSA metalform mags (or metalform mags from any other source you prefer at a higher price than PSA)
There is nothing else 9mm specific that is absolutely necessary, but you can get a 9mm specific upper (carry handle or flat top) if you prefer.  RRA stuff is also good.

For colt style setups, the cost difference between regular lowers with mag blocks or dedicated lowers is not as significant as it used to be.  You can get a KAK block for $100 and a regular lower for as little as $50.  The PSA colt style lower is $120.  For that little of a difference, I personally prefer the flexibility a regular lower affords.

How do KNS pins keep the hammer pin from breaking?  I know they stop the pin from rotating but how does that protect them from shock that causes breakage?  It seems like a properly ramped bolt and hammer combination is what's required to save the pins.

Only two things I'd add to that list:

A MagLULA loader for Colt SMG mags -- hand loading those mags is a literal pain.
Tons of 9mm ammo, because they are so much fun!

Also I have two dedicated 9mm lowers and a Hahn block. The Hahn runs just as reliably as the dedicated ones and I wish the feed ramps on my dedicated ones (from CMMG) were as finely machined as the ones on the Hahn.

I've also used three different buffer systems, all of which worked well. I have the Maxim Defense PDW stock on one with the proprietary JP Silent Captured Spring. On another I have a carbine tube with the regular 9mm JP SCS. I also found that an A5 tube with an A5H4 buffer worked just fine (which is great because all the lowers I'd use the Hahn block with have A5 systems.) All of this was tested with the JP 9mm bolt.