S&W M&P15 lower multi caliber concerns...

[NathanB]

No you should be fine as long as it’s within specs.

When you file a form 1 to build an sbr and the process to register your firearm kicks off you have to designate it a caliber in the form. My sbr is registered as a 5.56. You may slap on any upper you’d like to it as long as you are easily able to convert it back to the registered configuration.

[raxar]

You can get a 50 bmg upper for AR's, you should be fine.

[Emptymag]

So my question was not about legalities and was more safety oriented. No possible damage to myself or others strapping any ar15 upper on an m&p lower?

-Zach

I don't own a single AR that has a matching brand upper and lower.

No... I have one... all seven are mixmasters, but one of them has an Anderson upper receiver (but other brand barrel, BCG, etc) and Anderson lower.

[arjohnson]

So my question was not about legalities and was more safety oriented. No possible damage to myself or others strapping any ar15 upper on an m&p lower?

-Zach

You can put any caliber upper on your lower you desire, if it wasn't the case the lower would be marked for specific caliber.

When was the last time you seen any pistol receiver marked with the caliber? They are always marked on the barrel as are most rifles as well.

As for NFA, the lower must be registered with a specific caliber, that's the reason for multi caliber marked lowers, the caliber markings are on the barrel.

[danhr]

I'm almost certain that the only time the marking is of concern is with NFA items which need to be marked.
That's probably the reason "multi-caliber" markings came about in the first place. (for NFA - "multi" covers all bases and allows them to be swapped at will)

Can you shed a little light on NFA and multi-caliber marked lowers? From what I learned when doing my SBR 300 BO build, I read from numerous sources that using "multi-caliber" on a form 1 is almost an instant denial. And low and behold, the LGS who did my paperwork on my form 1tries using multi caliber on it the first time around and it got denied.

[arjohnson]

Can you shed a little light on NFA and multi-caliber marked lowers? From what I learned when doing my SBR 300 BO build, I read from numerous sources that using "multi-caliber" on a form 1 is almost an instant denial. And low and behold, the LGS who did my paperwork on my form 1tries using multi caliber on it the first time around and it got denied.

When filing a form 1 you must specify caliber on the paperwork.

[Emptymag]

Can you shed a little light on NFA and multi-caliber marked lowers? From what I learned when doing my SBR 300 BO build, I read from numerous sources that using "multi-caliber" on a form 1 is almost an instant denial. And low and behold, the LGS who did my paperwork on my form 1tries using multi caliber on it the first time around and it got denied.

Apparently I was wrong. I guess. I don't know much about NFA stuff.

I thought that what you submitted on paper needed to match what's stamped on the lower and that "Multi-caliber" on the lower made it "flexible".

I shouldn't have commented without being 100% certain.

[esh21167]

As far as safety, the modularity of the AR allows any upper to function, as others have said. Now, it's up to you to make sure you use the right ammo in the right upper. Might sound stupid and elementary, but things have happened, especially between 5.56 and 300 BO.

[zachomega]

Thank you all! I really appreciate all of the info!

-Zach

[Xringshooter]

Apparently I was wrong. I guess. I don't know much about NFA stuff.

I thought that what you submitted on paper needed to match what's stamped on the lower and that "Multi-caliber" on the lower made it "flexible".

I shouldn't have commented without being 100% certain.

When building an NFA item, be it a SBR, SBS or suppressor, the Form 1 requires a caliber to be specified on the application for the application to be approved, "Multi-Caliber" will either get it denied or sent back to be redone. Once approved you can make it any caliber you want as there is no requirement in the regs (that I could find) that says it HAS to be the caliber you initially put on the form. I'll talk to the NFA Branch next week to see what exactly they say.

The ONLY markings that are required on the firearm (the lower for an AR) is the makers Name, City and State, nothing else is required.

Even if the lower is marked with a caliber, it doesn't mean anything, just like buying an AR receiver that comes with a pistol buffer or rifle buffer/buttstock already attached, it is still logged in as a receiver. It is a receiver up to the point that a barreled upper is attached. Then it is a pistol or a rifle and falls under those laws/regulations.

I'm seeing more lowers come in from various places that have no caliber designation on the lower.

Zachomega - the lower has really no stress on it when the firearm is fired so if the lower is from a reputable source (and seeing that just about all forged lowers are made by a handful of companies they should all be ok) you shouldn't have any problems with any of the different calibers that can be fed by it.