I currently have a 9mm SBR AR that utilizes a mag block. I went this route thinking that the magblock was only held in by the mag release as that is a quick in and out. So different calibers was my plan. The bolt catch also has to be removed. So the ease of caliber swaps is more of a pain having to pop a roll pin as well.
I ended up buying a Colt mag specific lower and in my laziness I have not gotten finger printed nor have I felt like doing the paperwork on it.
Now with recent developments and being introduced to a pistol brace I actually like I'll just build the new 9mm as a pistol.

So I have a standard lower (when I remove the mag block) that has a stamp. I just need to notify the ATF of the configuration change.
PSA has a .300 blackout upper for silly cheap and I have a Tirant9 that is rated for .300 subs.
Then there is the 12" .50 Beowulf upper that would just be silly fun.
Then there is just the good old fashioned 5.56

Kind of leaning towards the 5.56 as I have tons of loaded ammo, brass, and components. Sounds better than getting into a new caliber.

What do you want to do? 300 BO is probably more effective for an SBR, but its definitely more expensive than 5.56.

quote:

Originally posted by Dwill104:
What do you want to do? 300 BO is probably more effective for an SBR, but its definitely more expensive than 5.56.

Novelty at this point. I built the original 9mm just for fun and an easier gun for my wife to handle in the house.
The new 9mm will do the same.

Since you have a TiRant 9 I would consider 300 blk.


I was in a similar situation, but the existing silencer that I owned is 5.56 only. My rationale for going with 5.56 was:

1. Use existing ammo, mags, components.
2. Getting the quiet of subsonic 300 blk would require a purchase of additional silencer, with similar ballistics to 9mm but at much greater ammo cost. So why not just go 9mm.
3. Heavy 5.56 supersonic ammo loaded with the 70gr Barnes TSX bullet is pretty close in energy to supersonic 125gr 300 blackout, with better sectional density and ballistic coefficient.



In your case I would probably go with 300 blk because you can suppress it with what you already have.


I wouldn't go 50 Beowulf unless you have a real intended use for it. That would probably be expensive enough it would wind up gathering dust in the safe.

I have never fired my AR pistol without a silencer attached, and I don't really want to. So I probably wouldn't go 5.56 in your shoes unless you plan to add another can or you have one that you didn't mention.

.300 BO

You have a can for it.
It is optimized for short barrel.
Uses regular AR mags, bolt etc
Good variety of factory loads.

Yeah, I vote 300 BLK as well. Sure, it is another caliber, but one that uses 5.56 brass, stupid easy to find .30 cal bullets and all your same mags. Optimized for a short barrel and much better terminal performance than 5.56 out of a short barrel.

I thinking hard about building a 300 BLK pistol.

In your case, .300 BLK seems to be a good choice.

But in general, optimal caliber would also depend partly on how S of a B you're planning to put on your R.

For example, if you're wanting a 7.5" barrel, I'd go 9mm over 5.56. 11.5" barrel? Reverse that.

So...both. 300 first

quote:

Originally posted by RogueJSK:
For example, if you're wanting a 7.5" barrel, I'd go 9mm over 5.56. 11.5" barrel? Reverse that.

I have not much experience with SBR's (only have one), but my experience with a 12" 5.56 has been excellent. Accurate, well balanced, and very little velocity loss. Going shorter might require a different approach.
 

Since its a novelty, I'm going to vote .50 Beowulf.

Have a PSA .300 Blk with a 8.5" barrel. Suppressed it's really quiet.

.300 Blackout ESPECIALLY if you reload your own ammunition. Reloading makes the .300 Blk as cheap to shoot as any other .30 caliber round.

I vote for 300 Blackout too. It's perfect in a 8.5 inch to 10 inch barrel.

.50 Beowulf

I stayed with the 556,got 458 socom in regular rifle,that bitch kicks like a mule,the big bore ammo will also drain your wallet,and blacken your shoulder quickly.

quote:

Originally posted by 45 Cal:
I stayed with the 556,got 458 socom in regular rifle,that bitch kicks like a mule,the big bore ammo will also drain your wallet,and blacken your shoulder quickly.

Strange.

I shot a 458 Socom for the first time a few weeks ago. My impression was that the recoil was very soft.

All of them ... eventually.

I've got 22LR, 5.7x28, 223/5.56, 7.62x39, 300AAC/BO & 9mm

I shoot the 22, 223 and 300AAC/BO the most.

I've got other guns I enjoy the x39 & 9mm more.

I rarely shoot the 5.7 even though I've got 800+ rounds of it ... no reason really. It's a nice round and it not any more expensive than anything else ... I've got the barrel cut down to 8", I've got SBR and Pistol lowers (as well as an M16) and I've got bolts for full and semi but the upper just sit on the shelf.