What a Difference Bullet Weight Makes!

[Bailey Guns]

I've had an old Colt lightweight upper for about 25 or 30 years. Never had it on a lower so I've never shot it. It's in great shape and looks like it's seen minimal use. The barrel isn't marked with a twist rate but I assumed it was 1/12 due to it's age.

So I finally decided to put it together as sort of a "retro" project. I put it on an H&R lower I got from PSA. A few days ago I took it out to test fire it and, using 62gr SS109 ammo, shot about 5 rounds from 25 yds just to check the sights. Pretty much perfect...shoots exactly to point of aim. I moved up to about 10 yds and fired 7 rounds in rapid succession. I was amazed at the amount of keyholing I saw on the 10 yd target. I've personally never seen anything like that. Strangely enough the 4 or 5 rounds I fired from 25 looked fairly normal...maybe a very slight sign of keyholing but I had to look very close. Fluke I guess...because I don't think a tumbling bullet will stabilize between 10 and 25 yds.

I didn't have any lighter ammo handy. Today I got some 55gr FMJ stuff...Tula steel-cased and generic Remington. Wow! What a difference 7 grs makes. Turns out, with the right ammo, this is one of the best shooting ARs I have. Especially with the cheap Russian stuff. I loaded what I thought was 20 rounds (actually 11 rounds of Tula and 10 Remington...math is hard) and fired several groups from 15 and 25 yds, all standing. All of the 3-shot groups except 1 have 2 rounds touching. This could be my favorite rifle.




62gr SS109


55gr

[Great-Kazoo]

those 1:12 twist bbls really like 52-55 gr ammo

[Bailey Guns]

^^ I can vouch for that.

[beast556]

My rifles that have 1-12 twist shoot 62gr stuff exactly how that paper plate looks. With 52gr otm hand loads they are lazer guns.

[00tec]

My 1:12 loves 40gr, but I'm in a different platform with a much longer barrel

[Oscar77]

If you are curious, take a tight fitting bore brush with a rotating rod (like say a dewey) and run it down the bore.
Take note of the revolutions and how long it took and you will find your twist..........

PS: It looks old, so look all the way around the barrel (not just on top where they usually are marked) and back to under the FSB...........
You might just see a "12" ............. I doubt that is so old, it's a "14."

[Jim B]

Wow! What a difference 7 grs makes.

My 1:9 twist is really accurate with 68 or 69 grain bullets, but won't stabilize 75 grains.

Try 40 grains and see how it does.

[Bailey Guns]

If you are curious, take a tight fitting bore brush with a rotating rod (like say a dewey) and run it down the bore.
Take note of the revolutions and how long it took and you will find your twist..........

PS: It looks old, so look all the way around the barrel (not just on top where they usually are marked) and back to under the FSB...........
You might just see a "12" ............. I doubt that is so old, it's a "14."

The barrel is marked "C MP Chrome Bore". According to what I've read that would make it a 1/12. There are no other markings anywhere.

[Oscar77]

The barrel is marked "C MP Chrome Bore". According to what I've read that would make it a 1/12. There are no other markings anywhere.

Oh........Sorry.
I misread your "......The barrel isn't marked with a twist rate ......." to mean there weren't any markings at all.
Yes, if it has that ( "C MP Chrome Bore" ) then, yes...........I'd go with 1/12".
(The VERY early ones had simply "12" marked on them. The EVEN EARLIER ones and extremely rare ones were simply marked "14." For their respective twists).

Just yesterday, I worked my A1 upper out to a 200yd zero and played around.
Man, those things are light!

[Bailey Guns]

Oh........Sorry.
I misread your "......The barrel isn't marked with a twist rate ......." to mean there weren't any markings at all.
Yes, if it has that ( "C MP Chrome Bore" ) then, yes...........I'd go with 1/12".
(The VERY early ones had simply "12" marked on them. The EVEN EARLIER ones and extremely rare ones were simply marked "14." For their respective twists).

Just yesterday, I worked my A1 upper out to a 200yd zero and played around.
Man, those things are light!

Yeah, I wasn't very clear. And it is really light. 5lbs 6oz empty. I really like it.