Sorry that my post was so incoherent. I was asking the question that almost everyone else in this thread is asking. Why do you want to replace the piston upper ?
Hopefully that was easier for you to read.
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I understand the common parts angle but if you put a new top end on the Sig bottom why would you even considering keeping bastard upper?
It does shoot well and is reliable. I might be able to mate it with a lower that I put together one day. I guess I don't like to sell guns if I can help it.
Oak
I myself would prefer to have a complete BCM, and/or a complete SIG, rather than bastardizing them un-necessarily. SIGs are good carbines, so I'd just build a BCM, and have two.
Nothing wrong with your SIG, don't get caught-up in name-brand recognition. I own a BCM, but I actually regret selling my S&W to get it.
I would hang on to it. You could buy a BCM upper and build a lower to go with it later.
Thanks.
That's my plan now. I'll stash parts as I find them, then put something together. Should be a good learning experience.
Oak