For no particular reason I want a short-barrel pistol caliber rifle.





I already have a registered short-barrel Rock River in 5.56 and there is a short-barrel Rock River 9mm conversion kit available locally.





My question is: is there there any reason I should not use the conversion kit and instead pay extra for a dedicated SBR lower?

 

Another 200 stamp?

Do you mind paying the extra tax?  I've considered registering another SBR lower just so I don't have to switch uppers / mag blocks around.  With a dedicated 9mm SBR lower,  you can either get one of the dedicated lowers (like those that use Glock mags, for example) or you can use one of the types of mag blocks that you need tools to remove (Hahn top loader, I think?)  

For myself, I use a single SBR lower, and a quick-to-change Hy-Tech mag block.  I have a pistol lower that I keep my "spare" upper on, but almost never shoot it.  More than anything, it just gives me a legitimate "home" for my extra upper, avoiding the whole "constructive possession" thing.

I don't mind getting another tax stamp, but I've heard ordering from RRA takes a loooooong time. So if there's no reason a dedicated lower is preferable to a mag block conversion I'd go with the latter. If the dedicated lower is more reliable then I'll go with that instead.

I would look at the CMMG 9mm dedicated lower, it's much cheaper than the Rock River if you just buy the Stripped lower ($175) and you might be able to get on SBR'ed from the factory.

I have both, a RRA with their mag well conversion and a CMMG dedicated lower. I just like 9mm and wanted to try both.

The dedicated CMMG lower is great. It runs the cheap CProducts and ProMags without a hiccup. The bolt hold open works perfect and mags drop like a rock when you put that button.  I have a couple of RRA lowers and uppers but I went with the CMMG lower on the 9mm as it was so much cheaper than the RRA. I have no idea why RRA gets almost $100 more for theirs.

The conversion lowers are more "sensitive". I started with a RRA block in an RRA lower. It took some time to get it working perfectly. Then I took the Mag Well adapter out of the RRA and stuck it in my spare Cav Arms lower and it ran like a dream! It too will run the lower priced mags without drama.

So either way will work. Its all in what you want and how you expect to use the firearm. I have a long running relationship with 9mm carbines, I have four or five different flavors, two of them the AR's.

You are a step ahead since you already have the SBR lower, but I had been considering a CMMG 9mm lower, but after my FFL made it more difficult to order through him I just ordered a $90 Spikes lower.  It is still waiting in the safe until I send off for the tax stamp, but I decided I wanted the option to switch uppers in the future if I need to.  If you already have a SBR lower you can use it for anything that doesn't require a mag block and keep the 9mm upper dedicated to the 9mm lower.

Okay, so what about someone (me) who doesn't own any NFA lowers and wants to go with a 9mm AR as a pistol to start (then pay the $200 at a later date)? I would like to have a 9mm with one of the Glock lowers since my psitols are Glocks, but I also like the ability to go with a 5.56 pistol if I decide on a conversion setup. Which route seems to be the more common nowadays? (Yes, I know, get both).

I built a dedicated 9mm marked SBR rifle in 5.5" and I shoot it the least out of all my SBR's..  It runs 100% but I were doing it again, I would go the above route and just use the hy-tech and uzi mags..

I vote "both".  Get an adapter and 9mm SBR upper now to see if you like the 9mm setup.  If you don't, you can sell the upper and adapter without any FFL hassles.  If you DO like it, get a Double Diamond Dedicated lower and SBR it.

Yea what he said.

Someone on here posted pictures of one(although they are a sot and converted it to full auto). It is the way I'd go if I was going with a 9mm. I agree with the getting the upper with a mag block first. If you like it get the double diamond lower(they will even engrave it for you) and register it as a second sbr.

I was going to built the 9mm sbr but I ended up getting a beretta cx4 instead as it took the same mags as my 92fs pistol. Thing is kinda fun but I think I'm going to sell it and put the money to something else. My 5.56 sbr is just more fun.

NOT MINE - borrowed the pic from the M4 ressurection thread part 2:

Does the double diamond have the last round hold bolt open feature?

Yes

That is sweet looking! It makes me want a 9mm carbine to go with my other two AR-15s

N/M. Just realized you were the same poster.