Multiple uppers on SBR lower

[Mike K]

I think this has been asked before, but I cannot find it.

I currently have an AR15 SRB with a Spikes Tactical 10.5” upper in 5.56. I have been looking at getting a 9” 300 Blackout upper. Is there any way I can use the current SBR lower with the new upper or do I have to go through the Form 1 process again? I thought I read that you can send a letter to the ATF updating information on your SBR, but I don’t know at what point you need to start with a new gun. I assume the caliber change (I want to keep my current 10.5 upper) will require its own lower, but just figured I would ask.

[David]

People say write a letter showing the changes, others say dont.

Personally, I wouldnt worry about it. Do what you want.

[motoman]

From my understanding it doesn't matter what upper you put on it because it's the lower that's registered and has the tax stamp.

[CJ]

General consensus is as long as you can return it to your original form 1 configuration you can swap out any upper onto the registered lower.

[Chopped54]

So if you sent in the Form 1 showing a 10.5" barrel, can you go shorter?

[CJ]

So if you sent in the Form 1 showing a 10.5" barrel, can you go shorter?

yes, once you have a SBR lower you can attach whatever you want to it. Just make sure you can return it to it's registered form if asked.

[Chopped54]

yes, once you have a SBR lower you can attach whatever you want to it. Just make sure you can return it to it's registered form if asked.

Cool, I had wondered....

[Mike K]

Thanks for the help, this is great news! So now that we have that out of the way, with a 300 BLK upper, I know it uses 5.56 mags, but do I need to get .300 blackout specific mags or do I need to change the follower? Not sure if it uses regular 5.56 mags loaded with .300 blackout or if I need something dedicated for the .300 blackout caliber.

[motoman]

.300 Blackout uses standard AR15 magazines. The .300 Blackout is a necked down .223 casing with a .30 caliber projectile.