In connection with an upcoming carbine course, I have been getting questions about whether it will affect accuracy if one uses a 55 gr. bullets in a barrel with a 1-in-7 (“1:7”) twist, as opposed to ammunition with heavier bullets. These are good questions, and exactly the kinds of issues this course was designed to address. But since people are now in the process of purchasing ammunition for the course, and some of the ammunition using 62 gr. bullets is currently more available than 55 gr. ball, I will answer the question in advance of the course.
For purposes of a course, and for all PRACTICAL purposes, there will be no detectible accuracy difference between using 55 grain and 62 grain ammunition in barrels with a 1:7 twist. Several of my rifles have 1:7 twist barrels, and I have primarily used 55 gr. loads for more than 30 years. I use 55 gr. ammunition for practice and training simply because in the past it was readily available and less expensive than 62 gr. loads. It has worked just fine, and I am sure it will work just fine in your rifle.
I would not be at all surprised to learn there are “measurable” differences in accuracy between 55 and 62 grain bullets fired from a 1:7 twist barrel. But I never concerned myself with that difference, because “measurable,” does not mean “practical,” or even “noticeable.” Any such differences in accuracy are so small they are entirely swallowed up by the inaccuracies introduced by the operator, the quality of the of the ammunition in either weight, the quality of the barrel itself, the sights being used, and factors like whether one is in a stable position or moving.
In terms of history, I believe the military at one time had some issues with tracer ammunition fired from rifles with a 1:7 twist. It was not the weight of the projectile that was the issue – it was the length of the projectile. In fact, it is the length and shape of the projectiles that make a 1:7 twist more suitable to stabilize longer, heavier projectiles, not simply the weight.
As an aside, I have heard about (but never seen) light, fast varmint bullets shedding their jackets mid-air and “coming apart” when fired from a 1:7 barrel. A source I trust reports seeing this happen one time, but he could not duplicate it. In any event, there is a lot of difference between fast, thin-jacketed 40 gr. bullets and 55 gr. ball.
So, in terms of one’s performance in a course, there will be zero difference between firing 55 gr. and 62 gr. bullets from rifles with a 1:7 twist. None. 90% of the shooting is from a standing or kneeling positions, from improvised position, or while moving. One will be shooting well above average if they are putting rounds into 4 inches at 30 yards under those conditions. If you don’t believe me, try it.
Regular and frequent handling and shooting one’s rifle is much, much more important to the issue of overall competence than details like bullet weight. In my experience, it is people who do not shoot their rifles much who get all wrapped up in technical minutiae, because they have insufficient field experience from which to gauge what is important and what is not. Buying the ammunition that is available and affordable makes it possible to go out and shoot, which is important to one’s ability and confidence. Sitting at the computer researching lab data and “expert opinions” contributes nothing real.
-- Peter Georgiades
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