Difference between "Multi-cal lower" and 223/556 lower

[srqsteve]

What is the difference between a lower designated as "Multi-cal" to one that is for 223/556?

[Odessaman]

Not a darn thing but the inscription. Manufacturers didn't want to lose potential sales from customers who want to build a .300 Blackout or some other rifle that uses the standard 223/556 lower, but didn't want their custom job inscribed with 223/556.

Now, if you have a lower marked "Multi-cal" and you apply for a tax stamp for an SBR, ATF wants you to put the intended caliber in the application, not Multi-cal. So if you plan to slap a 10" upper in 5.56 on a Multi-cal lower, you need to put 5.56 on the Form 1.

[srqsteve]

Thanks; that's what I thought but wasn't sure.

[flcracker]

... if you plan to slap a 10" upper in 5.56 on a Multi-cal lower, you need to put 5.56 on the Form 1.

Am I correct in my understanding that as long as you retain possession of the upper that matches the information on the approved Form 1 and it can be readily re-installed on the lower, then you can swap out any other upper you wish?

[Odessaman]

... if you plan to slap a 10" upper in 5.56 on a Multi-cal lower, you need to put 5.56 on the Form 1.

Am I correct in my understanding that as long as you retain possession of the upper that matches the information on the approved Form 1 and it can be readily re-installed on the lower, then you can swap out any other upper you wish?

Yes, that's correct - once you get the stamp and get the lower properly engraved, you can swap back and forth among other compatible uppers.

[imnotryan]

Yeah most if not all new lowers sold no longer include any caliber indication on them.

It was never anything more than a "the upper assembly on this is supposed to be..."