I'm toying with something ridiculous.  Since I already know how to built a modern AR, and have done so, it's no longer fun, interesting or challenging.  For my next build, the plan is to scrounge, scavenge, scrimp, and actually buy some of the cheapest cast off but still suitable parts.  Aside from obvious quality and possibly wear issues, what other pitfalls should I look for.

Will an A1 barrel fit into a flat top upper with out modification?  Will an A1 upper match up to an A2/A3 upper?  What about bolts and carriers and charging handles and stuff, and more stuff?

The result of this project will undoubtedly be a Frankengun, so I hope the purists aren't offended by my question.

Also, I'm thinking of cutting an old 20" bbl to 16".  Are the threads easy to recut?  Is that a gunsmith job, or can anyone with a brain and a lathe do it?

Thanks for any and all wisdom, opinions, rumours, etc.

Quoted: Will an A1 barrel fit into a flat top upper with out modification?

Yep.

Will an A1 upper match up to an A2/A3 upper?

What about bolts and carriers and charging handles and stuff, and more stuff?

There have been no major design changes to bolts, carriers, or charging handles affecting interoperability.

The result of this project will undoubtedly be a Frankengun, so I hope the purists aren't offended by my question.

Nope.  I guess you missed all the retro/frankengun threads here?

Also, I'm thinking of cutting an old 20" bbl to 16".  Are the threads easy to recut?

For a gunsmith, with a lathe, and re-crowing experience?  Yes.  Cake.

For you?  (since you asked this question, I will assume you have NOT done a couple hundred) then No.

Is that a gunsmith job, or can anyone with a brain and a lathe do it?

I would leave it to a gunsmith, or an experienced machinist.  Even a good machinist, who can turn the OD, and cut the threads and shoulder no problem, might not have enough experience to cut a quality crown that wont hurt accuracy.

Also keep in mind, if you chop a 20" to 16", you have to open up your gas port or the gun will not cycle.

ok disrespecting my own post....i think i meant matching an a1 upper to a later lower.

i admit i missed the retrofranken threads.  i tries some searches, but mine only turned up one hit for frankengun.  i know generally that genetic splicing is frowned on by many

Thanks a bunch for the info, falarak!!!

yes the uppers will fit onto lowers.

its an ar15

Quoted: yes the uppers will fit onto lowers. its an ar15

But be aware, the color of your early type upper will not match your new production lower.  So be ready to either live w/a two tone AR or getting out the refinishing equipment if you can't.

As noted above, all assemblies of ARs are interchangable between old and new. I've even seen pics of an Air Force EM with what is obviously an early M16 lower, fitted w/a current M4 upper assy and collapsible stock

2-tones are cool.

YEAH I'm thinking probably multitone.  Hopefully my parts will all be mismatched enough so I don't have to refinish anything