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55grn fmj in a 7 twist tube - how crappy will this be?
- Thread starter JR Hartman
- Start date
Accuracy will depend on the quality of the gun and quality of the bullets you use. Cheap thin jacket bullets may blow up with the extra twist. This is not because of their weight, it is because of their quality. Just not made to take the spin. Berger Target bullets have a thicker jacket to keep them from doing this.
The problem with high spin is that an unbalanced bullet will wobble. It rotates about its center of gravity instead of geometric center. Again a quality bullet will be balanced and not a significant issue unless you plan 100 meter benchrest competition.
The other more likely source of inaccuracy is the length of the throat in the chamber. If you buy a 7 twist gun, it should be made to take long bullets. That longer throat may be trouble for a very short bullet. Unlike the bullet wobble issue, this effect can be major. I have a 6mm gun which shoots 0.2" groups at 100 meters with 68 grain bullets, but it turns into a 2-3" shotgun pattern group with 62 grain bullets. I believe the throat length is the problem.
The problem with high spin is that an unbalanced bullet will wobble. It rotates about its center of gravity instead of geometric center. Again a quality bullet will be balanced and not a significant issue unless you plan 100 meter benchrest competition.
The other more likely source of inaccuracy is the length of the throat in the chamber. If you buy a 7 twist gun, it should be made to take long bullets. That longer throat may be trouble for a very short bullet. Unlike the bullet wobble issue, this effect can be major. I have a 6mm gun which shoots 0.2" groups at 100 meters with 68 grain bullets, but it turns into a 2-3" shotgun pattern group with 62 grain bullets. I believe the throat length is the problem.
I regularly practice with 55 grain bullets out of a 1 in 7" gun. Accuracy is nowhere as good as with 69's or 77's, but it is still decent. I have never had problems even with cheap 55 fmj's - and I drive them quite fast.
If you are interested in the gory details of the effect of spin on accuracy, here is an article by Dan Lilja, the barrel maker. If less interested in the gory details, then skip down to the example table of accuracy impacts near the bottom. This of course assumes the rest of the gun/barrel combination is of very high quality, and capable of shooting in the .1's with a slow twist. And this of course ignores the impact of a long throat on a short bullet.
