.22 is a great option - I highly recommend the S&W M&P 15-22. It works exactly the same as an AR so the experience you get on it will carry over. There are a lot of people on here throwing out all kinds of suggestions. I suggest picking a platform you really want to focus on learning. You can work on fundamentals with any gun but you'll spend a ton of money real fast if you buy a bolt gun and then decide you want a semi auto, etc. For me, I focus on the AR. It's what I use at work and IMO it is the most viable and versatile weapon I have access to in the civilian sector so I want to be very good with it. Sure I want a bolt gun eventually, but I'm still working on my ARs for now. As far as the rifle itself - I know AP's stripped lowers have worked very well for many people and I know many people like to build their first AR. It's very doable but I still don't recommend it. Do your research and invest in a quality rifle that will have high resale value. Even if you research everything on the rifle, you will find stuff you don't like on the first one. If it comes down to it, you can sell it and buy or build a new one. I have no personal experience with the Sig. I know people who like the Sig and people who don't. My only personal experience is seeing two malfunction repeatedly at the last range I went to. Could it have been something other than the rifle? Definitely. But there are likely other rifles out there that could have kept going. I'm in the minority, but I won't spend any of my money on a gun that may have an occasional malfunction - I will pay the extra money for a brand with a better reputation. Colt or BCM are the two big ones that I recommend. Colt will likely be a it cheaper, but BCM will have better features. Both will run like hell.

.22 is a great option - I highly recommend the S&W M&P 15-22. It works exactly the same as an AR so the experience you get on it will carry over. There are a lot of people on here throwing out all kinds of suggestions. I suggest picking a platform you really want to focus on learning. You can work on fundamentals with any gun but you'll spend a ton of money real fast if you buy a bolt gun and then decide you want a semi auto, etc. For me, I focus on the AR. It's what I use at work and IMO it is the most viable and versatile weapon I have access to in the civilian sector so I want to be very good with it. Sure I want a bolt gun eventually, but I'm still working on my ARs for now. As far as the rifle itself - I know AP's stripped lowers have worked very well for many people and I know many people like to build their first AR. It's very doable but I still don't recommend it. Do your research and invest in a quality rifle that will have high resale value. Even if you research everything on the rifle, you will find stuff you don't like on the first one. If it comes down to it, you can sell it and buy or build a new one. I have no personal experience with the Sig. I know people who like the Sig and people who don't. My only personal experience is seeing two malfunction repeatedly at the last range I went to. Could it have been something other than the rifle? Definitely. But there are likely other rifles out there that could have kept going. I'm in the minority, but I won't spend any of my money on a gun that may have an occasional malfunction - I will pay the extra money for a brand with a better reputation. Colt or BCM are the two big ones that I recommend. Colt will likely be a it cheaper, but BCM will have better features. Both will run like hell.

That's a great suggestion 

you could always pick up a Colt OEM2 and put the handguard, trigger guard and buttstock of your choice on it.  It would allow you to set it up the way you want from the get go.