Colt AR Barrel

magicmanml2

Okay, I bought a used 18" barrel for my new build. paid $99 for it. It is a what I call a pencil barrel. not m4 profile and not beefy enough for HBAR I think.

Only stamp on the barrel is C MP C. It is on the bottom of the barrel about 2 to 3 inches from the muzzle. It does not have a caliber stamp anywhere. From searching, I believe it is a Colt barrel with a chrome chamber. Is that correct? also internet says its 1 in 12 twist. Is that correct for an 18" barrel?

tsr1965

You have an early SP-1 barrel. Very thin, and designed for the 55-60 grain projectiles in maximum weight. Anything over that it will not stabilize.

Best

Txs

Also check for stampings on the bottom, back near the chamber area.

EDIT:

Colt also outsources AR/M16 barrel production. A letter code identifying the manufacturer would be found stamped here.

armilite

Seeing you say you bought it used more likely it originally was a 20 inch barrel that someone shortened. Don't recall of any 18 inch barrels back then.

nmyers

Armilite makes a good point. Actually, I don't believe that Colt ever made an 18" barrel.

In the last couple of years, a distributor has been selling "Colt M16 parts kits" that included barrels used in FA mode. It could be that someone tried to "accurize" this barrel by shortening it.

Neal

EDIT: The SP1 was 1:12

magicmanml2

Here is the best pic i could get.

the barrel near the threads is beveled out, is that possible to do if you cut a barrel down?

armilite

Are you sure you measured it right. You'd have to measure it with the bolt in the receiver lugs.[;)]

magicmanml2

okay so evidently not, lol. So from the muzzle to the barrel extension is 20-3/8". So I guess I have a 20" barrel. Thanks for the info above, early SP1 barrel. whats the twist rate? I bought it to make more of a varmint gun for alittle more reach. I hope that was a good idea......

armilite

quote:Originally posted by magicmanml2
okay so evidently not, lol. So from the muzzle to the barrel extension is 20-3/8". So I guess I have a 20" barrel. Thanks for the info above, early SP1 barrel. whats the twist rate? I bought it to make more of a varmint gun for alittle more reach. I hope that was a good idea......


Probably a 1 in 12.

Riomouse911

With the wide variety of AR twist rates (1-12, 1-9. 1-8, 1-7 etc...) A simple and easy way to measure barrel twist if it isn't marked: On your cleaning rod put on your patch puller and put in a tight fitting patch. Put it in the barrel 2-3 inches, and put a mark on the rod at the mouth of the chamber. As you push it in watch closely until the rod makes a complete circle, and mark the rod again at the mouth of the chamber. Pull it out, measure between the marks, and you'll have a very good idea of the barrel's 1" in ?? twist rate.

sandwarrior

Very early SP-1's and M16's (pre-M16A1's) were 1-14". They would stabilize the 55 gr. projectiles, but not the matching tracer bullets.

The only way to know for sure is to measure it. You need a good bearing handled cleaning rod, a jag and a tape/measure of some sort. It helps to have a marker of some kind.

Take just the upper and place it where you can hold/clamp it. Put a patch on a jag on the cleaning rod and insert that into the barrel (past the chamber) Once it's in there, measure the distance from the barrel to the handle (or any point readily identifiable on the cleaning rod). Also note the position of top or a mark that is identifiable. For as the rod turns you will need to come back to the same positon. Push the patch up the barrel until the rod rotates one turn. When you then measure that distance and subtract it from the original measurement it will tell you the rotation of the barrel.

ex: place the jag in the barrel, measure distance to handle and a mark made top dead center. Pushing the rod up the barrel, the rod rotates and when the mark you noted comes top dead center again the distance to the handle is 12" less than your initial measurement. The rifle twist is then 1-12". If your barrel is what I think it is, it should be 1-14".