How many parts can you replace on a gun before it is a different gun?

I was going to say as long as the receiver is there, it's technically the same gun...

There are a couple rifles (Argentine 1891, some Mosin Nagants, some Winchester 94's) that technically are pre 1899, and thus don't legally require an FFL to sell and ship. I got an Argentine shipped straight to the house like that.

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This could apply to any model like ARs, etc. Is the gun the receiver or frame, or the entire thing?

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AR's (and other recievers like shotgun etc), my understanding is, once a rifle, always a rifle (or shotgun). If you put a stock on it, it's now a rifle, and you can't legally swap to a shorter barrel and a brace or Shockwave grip.

The pistols and shockwaves technically never were long guns, so that doesn't apply... but don't put a stock on them or it will.

Now, functionally, you can change the stock, float the barrel, tap and mount a scope, and transform an old gun into something completely different in experience. But I think, if that was a sporterized 1891 that originally was pre-1899 for example, then it's still considered exempt from FFL.

So what kind of firearm was the 107 year old specimen? Just curious because I like to hear about old guns.