1966 SP1 Ammo Question

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Got some conflicting information on 223/556 ammo in my AR15. I have a very early 1966 Colt SP1 100% original marked 223 Caliber. I presumed I cannot shoot 556 ammo in my rifle based on recent articles stating I could blow it up. A guy @ my gun club told me that is not an issue with the early SP1's, he said the SP1 barrels are the same barrels that are on the Military M16's and can shoot both without any issues. Any guidance from NES would certainly be appreciated. The last thing I want to do is destroy a 53 year old rifle. This gun is in mint condition I want to try and keep it that way.
 
Most likey chambered in 5.56 NATO. Look at the barrel down near the muzzle. It should be stamped with the chambering. Sometimes the barrel stamping is faint.
 
Then don't shoot it ? /s

Every SP1 is rollmarked with 223 - you're fine. What do you plan on shooting through it?
I have been shooting it with 223 and 556. I stopped shooting 556 because of all the negative publicity on 223 rifles. I would would like to feel safe in shooting 556 if I get a deal on the ammo.
 
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I think that colt was building Sp1s on the same line that the M16's were being produced on. My guess is that they were all chambered for 5.56, but the civilian guns were marked .223 because at that time, that's all that was available on the commercial market. Shit, in the late 60's, they hadn't even finished developing 5.56, let alone surplussing any of it. Most of the problems they had in the field with the early m16s were ammo related. Vietnam was the ultimate field trial for that cartridge and rifle. You could probably shoot m193 ball with no problems, but stay away from M855. 1-12 twist barrels make heavier bullets do weird shit. If it were me, I'd probably just stick to 55gr .223.
 
I think that colt was building Sp1s on the same line that the M16's were being produced on. My guess is that they were all chambered for 5.56, but the civilian guns were marked .223 because at that time, that's all that was available on the commercial market. Shit, in the late 60's, they hadn't even finished developing 5.56, let alone surplussing any of it. Most of the problems they had in the field with the early m16s were ammo related. Vietnam was the ultimate field trial for that cartridge and rifle. You could probably shoot m193 ball with no problems, but stay away from M855. 1-12 twist barrels make heavier bullets do weird shit. If it were me, I'd probably just stick to 55gr .223.

I would agree with the above.

I have a Daewoo K2 with a 1/12 twist barrel and actually had the opportunity to speak with a fellow who was related to one of the designers of the K2 for South Korea. He went back and forth with his relative (in Korean) and the gist of it is that the 1/12 barrels are optimized for 55 grain projectiles. Heavier ones experience excessive wobble and become unpredictable.
 
Pretty much what he said above too. I have an SP1 made in 1972 and I shoot a 52gr HPBT with 25.5 varget through it. I wouldn't shoot anything heavier than a 55gr/62gr just due to the barrel twist, and no reason to anyways.
 
Based on what has been noted above it sounds like as long as I stick with a bullet no larger then 62gns 223/556 is not an issue. Thanks for your responses.
 
Many years ago I was shooting the SS109 ammo thru my SP1 which I believe was a 62gn bullet, cant remember if it was 223 or 556.
 
I have been shooting it with 223 and 556. I stopped shooting 556 because of all the negative publicity on 223 rifles. I would would like to feel safe in shooting 556 if I get a deal on the ammo.
What type of 5.56 have you been shooting?
M193, M855 , M196 , M856 ?
Take what ever your shooting color the tip of the bullet from case mouth to tip. Now chamber a round and extract it (do not fire it) see if you can see land marks on the bullet.
If you do you know you have a old school short lead 223 chamber. See way way back there was a time 52 grains was about the max anyone was shooting 223. Then mil 5.56 comes along with bullets with heavier bullets and ogive that hits the lands of the old school 223 shorter throats and leads.
Then that creates and dangerous high pressure , that can have some really bad end results.
Now Im not old enough to know any better but im thinking colt would not bother cutting different chambers 223 vs 5.56 mil spec? Im also going to gander that 5.56 nato ammo was not generally available in the early days of semi auto AR production.
 
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I have a very early 1966 Colt SP1 100% original marked 223 Caliber.

(Snip)

The last thing I want to do is destroy a 53 year old rifle. This gun is in mint condition I want to try and keep it that way.

I love that early AR-15s are now C&R eligible.
 
What type of 5.56 have you been shooting?
M193, M855 , M196 , M856 ?
Take what ever your shooting color the tip of the bullet from case mouth to tip. Now chamber a round and extract it (do not fire it) see if you can see land marks on the bullet.
If you do you know you have a old school short lead 223 chamber. See way way back there was a time 52 grains was about the max anyone was shooting 223. Then mil 5.56 comes along with bullets with heavier bullets and ogive that hits the lands of the old school 223 shorter throats and leads.
Then that creates and dangerous high pressure , that can have some really bad end results.
Now Im not old enough to know any better but im thinking colt would not bother cutting different chambers 223 vs 5.56 mil spec? Im also going to gander that 5.56 nato ammo was not generally available in the early days of semi auto AR production.
I used to shoot the ss109. Have not fired any of it in years.
 
I love that early AR-15s are now C&R eligible.
Yes, my 4 digit 1966 is a C&R rifle. Alot of fun to shoot, has the old peep rear sight. What is weird you can flip up the rear sight only to find another peep sight that is exactly the same, why? On the 1967 I believe you had a choice, 1st choice was 0 to 300 yds, flip the sight and you had 300 yds and greater. When I shoot targets I use our 100 yard range and the rifle is very accurate.
 
Yes, my 4 digit 1966 is a C&R rifle. Alot of fun to shoot, has the old peep rear sight. What is weird you can flip up the rear sight only to find another peep sight that is exactly the same, why? On the 1967 I believe you had a choice, 1st choice was 0 to 300 yds, flip the sight and you had 300 yds and greater. When I shoot targets I use our 100 yard range and the rifle is very accurate.

One’s a battlesight. It’ll be unmarked, and it’s the one you usually use. The one marked L is for shots past 300m.
 
As you suggested mac1911 I colored an American Eagle 62gn 556 bullet that I use in my mini14, cycled the bullet a couple of times with no sign of touching the lands so I guess I am good to go. I did a little research last night and it appears colt used the same M16 barrel on its civilian AR15 1st generation SP1.
 
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