Early Colt 603 FSB or ?

[Ruffian]

In 1980 I purchased an AR-15 barreled Upper to run as a dedicated 22LR with an Atchisson kit. I purchased it from a company called "Old Sarge" who advertised in Shotgun News. I received a SP1 upper (with a .312 forward pivot pin hole) and a barrel (probably not Colt) marked 5.56 on top behind the flash suppressor. the barrel is devoid of any other markings. the FSB appears to be an early forged unit with the forging flash ground off. The feature I'm curious about is a hole on the front of the FSB. I have never seen one of these and I can't imagine it's use. the upper does function just fine with 5.56 as well as 22 LR with the Atchisson kit. In truth I've never used it much and may build it into a retro style AR. If anyone has any knowledge on why this hole is there, I'd be interested to know.

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[Renegade04]

It does appear to be an early forged FSB with flashing removed. I have never seen one with a hole in the front. Not sure of what the purpose is myself. Is it a .625" or .750" FSB?

[scottryan]

The barrel has a .625 fsb

Can you post a picture of the barrel markings?

[dmsdog]

I had a similar FSB from Old Sarge in Natalia Tx. Still have his catalog. He once told me it was to allow easier insertion of the gas tube.... Many years ago....

[GRA556]

My guess is that somebody drilled that for some reason and may not have come from the factory that way.

[Ruffian]

Here are some more pics. The upper came with a re-weld m16 bolt and a chrome bolt head and a legit looking Colt SP1 upper. The barrel has 5.56 marked on it and does not appear to have a chrome bore. You can see that finish is inside the curious hole which started this thread.

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[Ruffian]

I had a similar FSB from Old Sarge in Natalia Tx. Still have his catalog. He once told me it was to allow easier insertion of the gas tube.... Many years ago....

It was many years ago! I'm trying to decide to use this FSB on my retro clone build. I've decided to use the barrel. I always wondered where "Old Sarge" got these parts. I would think an SP1 upper was hard to come by in 1980. My only thought was decommissioned police SWAT rifles or some other type of LE agency (maybe upgrading to M16A1s).

[Ruffian]

It's the skinny .625".

[ClassIIIGunsmith]

that front hole might be part of the guns fail safe. blow the gas tube before you damage the rest of the action in a constant f/a fire. maybe the reason for the front circle disk cut on the gas tube looking kinda like a weak spot on the gas tube

[scottryan]

I think that is some type of Colt barrel from their prototype shop.