AR Sponsor
Posted: 6/2/2009 9:46:33 AM EDT
|
What is the difference in practical accuracy with:
1/7 twist with 55 gr. and 62 gr. bullets 1/9 twist with 55 gr. and 62 gr. bullets Thanks |
|
Quoted:
Thank you for the reply. Sorry for the (apparently) stupid question.  Not a stupid question, just that barrel twist and bullet weight questions tend to get asked often. While there are great 75-77 gr bullets out there that work very well in 1:7 twist barrels, most of the cheap plinking ammo is in the 50-62 gr range which is perfect for the 1:9 twist barrels. I prefer a 1:9 barrel because it is better for my uses. |
|
bushmaster 14.7 hbar 1/9
bushmaster 20inch hbar 1/9 LMT defender 2000 M4 14.5 1/7 LMT M4 upper on bushy lower 14.5 1/7 shoots M193 same I even shoot the win usa 45gr with nice groups with the 1/7 twist Ive also shot BH blue box 75gr/77gr in 1/9 twist out to 220y accuracy is good |
|
Quoted:
There will be more difference between two barrels than the difference the two twists make. With the excellent bullets we have available in the 75 to 77 grain range, I see no reason to have a 9in twist barrel. What shooting distances are you talking about? You surely know that the trajectory of, say, a 69gr SMK vs a 77gr SMK is pretty much identical up to about 500 yards. Wind deflection between the two is about 1" @ 200 yards. By 400 yards, it's opened up to a staggering 5". Now, the fact is that the vast majority of people here don't shoot their carbines anywhere near those distances. For all practical purposes the 1:9 is perfectly fine. In fact, the 1:7 is actually a faster twist than ideal for the sub-80 grain stuff. The reason the military uses 1:7 is to stabilize the very long tracer bullets. |
|
Quoted:
What is the difference in practical accuracy with: 1/7 twist with 55 gr. and 62 gr. bullets 1/9 twist with 55 gr. and 62 gr. bullets Thanks For those bullets, I'd rather have the 1:9. There's no advantage to spinning them faster (with the 1:7) and the effect will be a slight velocity loss, and more heat and wear in the barrel. |
|
The length of the round is what determines the required twist for rifling, BUT, since the bullets are pretty much the same profile, the weight will vary proportionally to the length, besides the ammo box is marked with bullet weight NOT length.
In "Theory" 1:9 should be best for 50-62gr and 1:7 best for 62-77gr. To much twist doesn't usually make the bullet unstable, I seen the expert say to much twist just makes it take a little longer to settle down into gyroscopic stability. Yes, NOT enough twist will NOT stabilize the bullet and it can hit the target sideways and keyhole and be inaccurate. BUT, lots of 1:7 owner will tell you, they shoot 55gr out their 1:7, long and short range and its perfectly accurate, just as good as 1:9. Seen 1:9 owners claim to shoot larger than 62gr accurately, and it almost always seem that's it Hornady ammo they claim that does it, so the quality of the bullet probably plays a factor in getting less then ideal twist to stabilize a bullet that usually it would NOT. I got a 1:9 barrel because I don't think I'll ever shoot 77gr, and I want to shoot .22lr conversion. I'm sure lots of 1:7 owners will tell you they get perfect accuracy with 55gr, you can't go wrong with that twist. Can 1:7 shoot .22lr accurately? I think someone has posted they do a .22lr conversion with their 1:7 and it was fine, I'd like to hear more than one though? |
|
How many times do you see the post asking if one's 9 in twist will stabalize the hornady 75 tap? Some will, some won't.....Why set yourself up for a limit?
BTW, the 8in steel target we shoot at 400 yards would be a real pain with 5 inches more windage, probably based on a 10 mile an hour wind. The wind never goes down to 10 miles per hour here in kansas..... |
|
Quoted:
The length of the round is what determines the required twist for rifling, BUT, since the bullets are pretty much the same profile, the weight will vary proportionally to the length, besides the ammo box is marked with bullet weight NOT length. In "Theory" 1:9 should be best for 50-62gr and 1:7 best for 62-77gr. To much twist doesn't usually make the bullet unstable, I seen the expert say to much twist just makes it take a little longer to settle down into gyroscopic stability. Yes, NOT enough twist will NOT stabilize the bullet and it can hit the target sideways and keyhole and be inaccurate. BUT, lots of 1:7 owner will tell you, they shoot 55gr out their 1:7, long and short range and its perfectly accurate, just as good as 1:9. Seen 1:9 owners claim to shoot larger than 62gr accurately, and it almost always seem that's it Hornady ammo they claim that does it, so the quality of the bullet probably plays a factor in getting less then ideal twist to stabilize a bullet that usually it would NOT. I got a 1:9 barrel because I don't think I'll ever shoot 77gr, and I want to shoot .22lr conversion. I'm sure lots of 1:7 owners will tell you they get perfect accuracy with 55gr, you can't go wrong with that twist. Can 1:7 shoot .22lr accurately? I think someone has posted they do a .22lr conversion with their 1:7 and it was fine, I'd like to hear more than one though? 1:7 isn't necessary until you get into bullets that are really very long. Based on weights, 1:7 isn't needed until you're OVER 80 grains. That said, it's not going to have much of a practical effect on accuracy with lighter bullets, it'll shoot 55gr just fine. On the longer end of the bullet spectrum though, 1:9 should give great performance with bullets up to 69 grains, and for the heavier stuff that you can still load to mag length like 75 and 77 grains, you really only need 1:8. Here is a twist rate calculator. Measure the length of some projectiles and you'll see that the ideal twists are often much slower than people think. http://kwk.us/twist.html |
|
Quoted:
1:7 isn't necessary until you get into bullets that are really very long. Based on weights, 1:7 isn't needed until you're OVER 80 grains. That said, it's not going to have much of a practical effect on accuracy with lighter bullets, it'll shoot 55gr just fine. On the longer end of the bullet spectrum though, 1:9 should give great performance with bullets up to 69 grains, and for the heavier stuff that you can still load to mag length like 75 and 77 grains, you really only need 1:8. Here is a twist rate calculator. Measure the length of some projectiles and you'll see that the ideal twists are often much slower than people think. http://kwk.us/twist.html I forgot to mention, most people believe the only reason the military went with 1:7 was to shoot tracers, which are less dense because of the tracer charge they have to carry, thus are a much longer round than their counterpart by weight non-tracer bullets. What you've said, pretty much supports that idea, there are 1:8 barrels out there and they seem to be getting more popular. |
| Out of curiosity I measured one of my 75gr. A-Max bullets. They're roughly 1.115" long. Plugging that number into the calculator @ http://kwk.us/twist.html along with a MV of 2750 fps gives you an optimal twist of 8.3. That seems to fit with people who say their 1:9 barrels can stabilize these bullets. |
|
Quoted:
Out of curiosity I measured one of my 75gr. A-Max bullets. They're roughly 1.115" long. Plugging that number into the calculator @ http://kwk.us/twist.html along with a MV of 2750 fps gives you an optimal twist of 8.3. That seems to fit with people who say their 1:9 barrels can stabilize these bullets. Incorrect. You’ve failed to account for a number of factors, specifically (no pun intended) the lower specific gravity of the 75 grain A-MAX, which is approximately 9.6. Even the limited formula that you referenced shows you would need a twist rate of at least 1:7.8” for the 75 grain A-MAX and even then that will only give you a gyroscopic stability factor of 1.01. Also, the picture below shows what happens when you fire the 75 grain A-MAX from an AR-15 with a barrel that has a 1:9” twist. 
|
|
Quoted:
Quoted:
Out of curiosity I measured one of my 75gr. A-Max bullets. They're roughly 1.115" long. Plugging that number into the calculator @ http://kwk.us/twist.html along with a MV of 2750 fps gives you an optimal twist of 8.3. That seems to fit with people who say their 1:9 barrels can stabilize these bullets. Incorrect. You’ve failed to account for a number of factors, specifically (no pun intended) the lower specific gravity of the 75 grain A-MAX, which is approximately 9.6. Even the limited formula that you referenced shows you would need a twist rate of at least 1:7.8” for the 75 grain A-MAX and even then that will only give you a gyroscopic stability factor of 1.01. Also, the picture below shows what happens when you fire the 75 grain A-MAX from an AR-15 with a barrel that has a 1:9” twist. http://www.box.net/shared/static/0n9igns8zi.jpg I should have mentioned that only SOME people with 1:9 barrels report success with the 75 and 77 gr. stuff. I thought I mentioned that, but re-read my posts and it looks like I forgot to. Still, I never said that 1:9 WOULD shoot them well, just that some people have reported decent results. |
AR Sponsor