I took my new (to me) colt SP1, dated 1968, to the range.  I used 55 grain ammo and was able to put my shots, with iron sights, to group about the size of a softball at 100 yds(lol real technical!).  Then, I zeroed the rifle on that ammo.  Then I switched to 62 grain ammo.  It was some lake city green tip I had.  To my surprise, at that range, I grouped the same size except that the group shifted a few inches right and a couple inches high, but still the size of a softball.  
My question is:  If I can group the 62 gr as good as the 55 at that range, wouldnt the 62 be far a deadlier round considering that its hardly stabilized?  In other words, would the 62 be much more likely to tumble considering the slow twist rate?

Incorrect understanding of terminal ballistics.  Old mythology.

Twist rate has no effect on terminal ballistics.

Just because a bullet is heavier weight does not mean it will have superior terminal ballistics.

M855 aka "green tip" type ammo is not going to be reliably stabilized in a 1:12 twist barrel and should not be used for any critical application.

If terminal ballistics are important to you, FMJ ammo is not the correct choice regardless of type.

Hi and thanks for that response.  What I mean to say, is that since the 62 and 55 grain seem to be comparable in accuracy at 100yds, wouldnt the 62, being less stable and longer in length, tumble much easier than a 55 grain at that range?  I'm not concerened with terminal ballistics, ie force, but the destructive power of the tumble.  Is this not correct?

What you are looking for with any of the FMJ ammo’s is fragmentation.  For the 55 grain ammo this will depend on WHICH 55 grain fmj you’re using and what speed it's travelling when it hits the intended target.  The 62 gr operates in the same way.  Without fragmenting fmj ammo sucks for terminal effects.  


Please check out the "The AR15.Com Ammo Oracle" it will help with the confusion.    

No, that's not correct.  The slight difference in aerodynamic stability from differences in twist rate are meaningless compared to the density of body tissue or other solids or liquids for bullet impact.  It's an old myth that twist rate has any effect on bullet "tumbling" or other terminal ballistics effects.

"Terminal ballistics" is the totality what the bullet does upon impact with a target not just "force" as you say.

Are you certain that your rifle is a 1/12 twist? If it is I'm surprised that you weren't seeing keyholing with M855.  A change in air temperature or other factor might be enough to lose stability, so not recommended that you rely on it.

I'm surprised that you weren't seeing keyholing with M855. A change in air temperature or other factor might be enough to lose stability, so not recommended that you rely on it.

The few 1-12's I've had that would shoot M-855 pretty decent inside 100 yds would definately start keyholing by 135yds.  Had way more that wouldn't shoot M-855 than those that would and would also not recommend it.  Only time I use it in a 1-12 is when it's what's left in the bag and I need one more retro fix before riding home or just grabbed the wrong mag.

I use M193 type ammo almost exclusively.

I wouldn't use M855 for social purposes unless I had nothing else but light varmint ammo.

Heavier is better for ammo generally, but not so with M855 vs M193.

Plenty of good SD ammo for .223/5.56. Heavy OTM and soft points.

OP, look into some 55 or 60 grain SP.

http://www.midwayusa.com/product/662754/black-hills-remanufactured-ammunition-223-remington-60-grain-soft-point-box-of-50