Hi everyone, I am looking to find a Sig Lower to start building a rifle. I am new to this and fell in love with the Sig MCX 556 but am unable to find one in stock with a 16inch barrel. I know people make their own guns but no clue how to go about doing so.
Thanks in advance for any info.

There are too many proprietary parts in the MCX to build your own. In particular, barrels are out of stock, the lower receiver is proprietary and only found in complete rifles while the only source of most other parts is SIG.

It would be much more practical to build your own direct impingement AR 15 while waiting for complete MCXs to come back into stock.

Bill

Are you looking to drop an MCX upper onto an AR lower, or looking for a stripped MCX lower to build?

If the latter, that's a no-go. But you could just build an AR lower and drop an MCX upper onto it. Building an AR lower isn't difficult and even in these trying times, you should be able to pretty easily source everything you need, save the MXC upper.

It's been a while, but I asked this question of a guy at the SIG Academy Pro Shop, and I recall him telling me that any MCX upper you get has everything you need to adapt a standard AR lower to the MCX.

Thanks for the info, any suggestions on a good direct impingement lower?

FWIW, the lower is just the lower. It houses the fire control parts. The gas system (whether it be DI or piston) is housed in the upper receiver group.

Personally, I would look for a billet ambidextrous receiver of some stripe, though those will be in shorter supply these days. Having an ambi safety, at the very least, is a great convenience, and being able to hit the bolt release with your trigger finger (assuming you're a righty) is also nice. Plus, not having to install a trigger guard is a bonus, since I find that's the second most obnoxious part of building an AR lower, and the only one that, if done really wrong could actually result in damaging the receiver.