Advice on early Colt upper

[EBR Works]

I have a large hole Colt SP1 upper that I purchased a few years ago. The prior owner said he thought it was from about 1970 and that it only had a few hundred rounds through it. I have a complete Stag A2 lower with bullet button coming next week to assemble a retro-look AR. What is the twist on this barrel? Is this upper chambered in 5.56 NATO or .223 Rem? Thanks.

[Blue]

If it's large hole make sure you get the adapter for it too so it'll work with your lower.

[EBR Works]

If it's large hole make sure you get the adapter for it too so it'll work with your lower.

Thanks for the heads up - I do have the adapter.

[Bagger]

nice looking Sp1 upper

[DB2]

What is the twist on this barrel? Is this upper 5.56 NATO or .223? Thanks.

1:12 twist. good for 55 grain. If it is a SP1 and not a M16A1 barrel it will be .223.

Is that a Colt scope, or a knockoff?

[EBR Works]

1:12 twist. good for 55 grain. If it is a SP1 and not a M16A1 barrel it will be .223.

Is that a Colt scope, or a knockoff?

Definitely a SP1 not a M16A1 - so .223 it is.

Prior owner said that it was an original Colt scope. Here's a pic. Thanks for the info!

[Army]

Not from the early 70's. Colt started the big hole...IIRC...with the Sporter marked receivers in the mid/late '80's.

As there was no other maker back in the early 70's, Colt did not need to alter theirs exclusively.