So I bought a 20" A1 upper off of the EE a while back.  Slapped it on a new lower, put all the parts together and was really proud of myself.  Then I got to wondering about loads for her.

The add for the upper said Fulton A1 upper with 20" 1/12 colt barrel.  I have no reason to disbelive this, especially since the seller rocked to deal with.  

My question is this.  Is there anyway I can confirm the 1/12 twist?  I pulled the guards off, and the only markings are about 4 inches behind the FH, on the bottom of the barrel, and the are "C MP B"

No where else is there any other marking.  The barrel also has M4 feed ramps which I found kind of odd, but what the hell do I know.

Any ideas?

Got a cleaning rod, jag and patch? Start the patch in the bore and mark the rod's position+orientation.  Insert until one turn is made.  Mark again.  Measure.

More precise:
Take a paper plate, poke a small hole in the center and run it back to the cleaning rod's grip end.  Start the patch in the bore and make a mark on the rod.  Make another mark on the plate to indicate up.  Without touching the plate, run the rod into the bore for one turn.  Make another mark on the rod.  Measure.

It might be a Bushmaster with the "B" and I would bet it is chromed "C".  MP is mag particle.

FSB which i assume is original to the barrel has a C on it as well.  


Ok, lets check the upper too, Only mark is an A that looks like it is broken in half right above the horizontal line.

C MP B=

Colt Magnaflux Proofed chrome Bore

These barrels were used on new production M16A1 rifles in 1972 and 1973 and were 1/12 twist.

Quoted: C MP B= Colt Magnaflux Proofed chrome Bore These barrels were used on new production M16A1 rifles in 1972 and 1973 and were 1/12 twist.

My barrel has the exact same markings, and I agree on it being a Colt Magnafluxed Chrome Bore.  But WRT twist, I thought/assumed it was 1/12 based on stuff I read until I actually checked it manually as suggested above.  Turned out to be 1/7 twist.  So it seems there is some variety out there.

To the OP I'd say check it manually, it could make a big difference in what ammo you decide to buy for it (although the 55 grain you have probably been using is fine in pretty much any twist).

Nunya

Quoted: Quoted: C MP B= Colt Magnaflux Proofed chrome Bore These barrels were used on new production M16A1 rifles in 1972 and 1973 and were 1/12 twist. My barrel has the exact same markings, and I agree on it being a Colt Magnafluxed Chrome Bore.  But WRT twist, I thought/assumed it was 1/12 based on stuff I read until I actually checked it manually as suggested above.  Turned out to be 1/7 twist.  So it seems there is some variety out there. To the OP I'd say check it manually, it could make a big difference in what ammo you decide to buy for it (although the 55 grain you have probably been using is fine in pretty much any twist). Nunya

Edited correction:

You forgot the P, its Colt Magnaflux Proofed chrome Bore.

The C MP B marking was long out of use by the time Colt started producing 1/7 twist barrels in 1982.  Starting in 1974 Colt marked their rifles barrels C MP CHROME BORE.  Carbine barrels were also marked C MP CHROME BORE.  In 1982 1/7 twist rifle barrels went into production and were marked C MP 5.56 NATO 1/7.  Carbine barrels were still marked C MP CHROME BORE even though the twist was changed for a small time before switching to the modern C MP 5.56 NATO 1/7 marking.

Thanks.

You guys rock...now to manually check.