I'm trying to figure out how to convert a 550 series lower to function with a semi-auto upper. My 551 clone was done by the late Estes Adams, so any directions or parts sources that you have are appreciated.

What do you have, a full auto capable lower?

It won't function in automatic or burst on a 551 upper that was converted from a 556 if the blocking bar was left in place. You need three things to make it F/A; the blocking bar milled off, the proper F/A carrier (note 556 carriers are F/A capable, export Swiss semi carriers are not) and of course the lower.

If you have a full auto lower and are asking what to remove....the selector can stay but you remove the timing pawls. I've got two F/A lowers converted to semi.

I'll gooogle the pics and post below.

edit - too many to post but good overview here: https://www.sigarms556.com/thr...and-semi-auto.16146/

Thank you. To be clear, I'm trying to convert the lower to semi. Whatever I think of the NFA, I'm not going to break the law.

my reply wasn't intended to be instructs to get around NFA but rather the conditions that need to be in place for that lower to do what you're trying to avoid.

So, you may not need to modify it but totally understand if you desire to do so. If your blocking bar is intact (import semi-auto Swiss guns or any 556 upper as delivered by the respective factories), then you're not going to have a f/a gun by putting a capable lower on it.

the blocking bar in the right side of the upper keeps things in check.