Frankenguns - pros and cons?

[Weetabix]

So, I've accumulated a few AR's. I like this about this one and that about that one.

I can see the potential pros of swapping parts to have the best of each. And, in theory, it should be OK. But as we all know, in theory, theory and practice are the same, but in practice, they are not.

Are there any major cons to swapping pieces here and there? What do I need to watch out for?

[evan price]

If everything is in spec interchangeable parts interchange.

[Weetabix]

Hearing no objections, I shall proceed.

[Rich Jordan]

The only issue I've run into with swaps is with one slightly out of spec lower (bought during the 2008 panic) and some very tight fits between upper and lower receivers, which might have been more finish related.

[Kommander]

The only thing to worry about is tolerance stacking but that's more of a theoretical issue than something you need to be concerned about. The only thing I would really worry about is keeping the bolts matched with the rifles. Remember that most AR parts, regardless of their supposed manufacturer, come from just a few factories.

[Weetabix]

I was thinking more of this upper with that lower, move this furniture over there, swap triggers, etc. I haven't really done anything inside an upper, so I was probably going to leave those alone.