I'm starting to work towards building a 9mm SBR and was planning on just using a standard AR lower with a mag block.  I actually already have a stripped spikes tactical lower engraved for my trust that I was getting ready to submit a form 1 for, but starting to wonder if I should go a different route before I submit the form 1 and pay to SBR it.  I also already have two PSA metal form colt style 9mm mags.  I noticed that a lot of the mag blocks such as spikes tactical and RRA are as much or almost as much as a NFA or Joe Bobs Spartan billet lowers for Glock mags. Starting to wonder if I should go that rout instead.  I did see that PSA has a $59 mag block that is kinda tempting.  

I'm wanting to get some feedback and also check to make sure I'm understanding the pro's and cons of each option correctly.  Either way I go I'm going to want to end up with last round BHO.

Standard AR Lower with mag block and standard AR upper:

Pros:
-I already have an extra lower(It can be re-purposed for a future build if I don't use it)
-BHO on last round with most mag blocks I've seen(If I'm understanding correctly the Spikes, RRA and PSA all have BHO, please correct me if I'm wrong on that as that would be a big deciding factor)
-Doesn't require a 9mm specific upper that does the BHO and cost more than a standard AR upper receiver.
-I can use the lower for other SBR uppers if I take the mag block out.(I do already have a 300 BLK SBR lower tho)
-I can use an inexpensive standard stripped AR upper for my build.

Cons:
-requires a mag block that cost lot if I go for a Spikes or RRA mag block.

Glock type lower and 9mm specific upper receiver:

Pros:
-Uses Glock mags.(I have a Glock 17)
-Purpose built billet lower and upper receivers
-If I already have to spend $150ish for an upper receiver with BHO I might as well spend the extra $50 to get a side charging upper like this one:(Not sure if this is really a pro or con)
http://www.joeboboutfitters.com/Matrix_Arms_9MM_Side_Charging_Upper_p/ma-9mm-sidecharge.htm

Cons:
-Requires use of higher cost upper that incorporates BHO in the upper to have the BHO work.
-I've heard about people having to modify Glock mags for them to work in a Glock type 9mm AR?  Is this true?

I'm wanting to decide which is the better way to go before I either submit a form 1 for my lower or buy a Glock type lower and form 1 that.  I'm leaning towards using a standard lower and the PSA mag block but wanting to get some input before deciding for sure.

Thanks for any thoughts and input.

Edit: Option 3..buy a CZ Scorpion Evo 3 and SBR that.  Doesn't look much more expensive and would allow for use of a side folding stock

I have an SBR lower so I'm going with the mag block route. My reasoning is my SBR setup is currently a 300 blackout, and by doing a 9mm upper and mag block I can shoot more since 9mm ammo is affordable. I also don't own a Glock, so I don't need a Glock mag lower.

I have only around 50 rounds through my NFA lower with non LRBHO upper, but some observations... I like that it feeds directly into the chamber with no ramp... slanted Glock mag may not be as aesthetically pleasing as the vertical Colt/UZI stick mag...the mag release on the NFA is a little farther forward than standard AR... my pistol mags drag a little and may need to be slightly clearanced to drop free, the 33 rounders drop free... the NFA modular bolt and spring/buffer with their complete lower are well matched to my 900 fps 147 hand loads...pretty good factory trigger... safety is a little tight...good cheek weld with the B5 stock...

I'm using one of these:  https://www.matrix-arms.com/product/9mm-stripped-lower-colt/




Works the same as a standard AR lower with Colt mag block, but is more astheticly pleasing since the magwell is sized to better match the slimmer mag.  It's more expensive than a regular AR lower, but when you factor the cost of a regular lower plus a quality mag block, it's not far off.




It also has a little lifter part that allows LRBHO while using a standard AR bolt catch.  Uses any upper and matches well with inexpensive forged ones vs. the more uniquely designed billet lowers.








 

Thanks.  at $150 most of the mag blocks I'm seeing are that much.  Unless I can find a less expensive mag block I might go with that option.  Does the LRBHO work well?

For me, the "pro" of being able to use the SBR'd lower for non-9mm use was the deciding factor on not having a dedicated lower.  Using Glock mags is interesting, but I'm not sure it's a "pro".  Kind of a wash really.  That said, now that I have 3 SBR lowers, the dedicated lowers look more attractive, but not enough to SBR a 4th...... yet....

So far I have 1 SBR lower that I use for a short 300 BLK and .22 LR upper.  I'm going to SBR another lower weather I go for a dedicated 9mm lower or a standard lower.  I figure having 3 short uppers and only one SBR lower could get old fast, lol.  

Trying to decide if I'd regret getting a Pro-Mag or PSA mag block.  Does anyone with experience with either have input or recommendations on another low cost mag block?

I'll let you know within the week or so once I get the last few parts and put it all together.  



But yea, I've read it works fine.



 

I have a PSA mag block in a PSA lower. The fit was a little loose so I used blue loctite. There's a little mag wobble but nothing excessive. The drop freely too.

Does the bolt hold open work?

there is another choice.
the dedicated colt lower.
125 on sale
http://blackcreekprecision.com/parts-accessories/lower-receivers/gpi-dedicated-9mm-forged-lower-receiver-assembly.html

no mag block needed, cheaper than the glock one.
even the slim line one is 189.

it appears to have last round hold open, but not sure. it doesn't call it out specifically.

 
Okay, got the receiver and had some time to play with it.






First off, you might not be able to use this out of the box.  Both myself and a friend of mine bought these lowers and built them up in the past week.  We both used different upper parts, but the same bolt (New Frontier Modular).






We quickly found that the upper and lower could not be closed due to interference between the bolt and the feed ramp on the lower.  Ultimately, we both had to re-profile the feed ramp, lowering the race-way surface and rounding the top-outer corners of the walls on it to clear with the bolt.  Luckily, the feed ramp is a removable part, making it easy to work on and replaceable if you mess up.  Some time with a file and a dremmel to polish it out and then a hit of cold bluing got everything together and working fine.  My friend's also had an issue where the ejector finger in his lower was too thick to fit in the recess in the bolt, so we had to pop that out and thin it down a bit as well, but mine was fine there.






Now, maybe our experience is unique to the NFA modular bolt, and others will clear the Matrix feed ramp, however I can't confirm.  It wasn't too difficult to re-profile them to work, but is a bit disconcerting to have to grind away at a new part.  But, such is the case at times with the non-standard world of 9mm AR franken-builds.  



 


As for the LRBHO, this works great on these lowers and my Metalform mags.


 

Yes, but it was a little sticky at first. Just kept working it and wiping the excess off.

I thought the same way as you when I was planning out my build for a 9mm SBR. Awhile ago, there was not the amount of companies making 9mm specific lowers and most everyone went with magwell adaptors. All I read on these fourms is the problems that people were having with them. The pinned Colt 9mm lower was the best there was but prices were high. I held out when I read an article that Rock River was going to produce a 9mm specific lower. Same size magwell as a .223 but broached for a 9mm mag only. I built mine off that and have never had an issue. I still see the same issues and arguments over adaptors...some things never change.

Anyone have any experience with the PSA G9.  Found one at a local shop.