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Posted: 12/31/2005 12:44:40 PM EDT
I've heard before that a tighter twist rate in a barrel will reduce velocity, how much velocity would one lose going from a 1/9 twist to a 1/7 twist? Or is the velocity loss more of a theoretical thing that is unnoticeable in practical use?
Link Posted: 12/31/2005 12:50:00 PM EDT
[Last Edit: 12/31/2005 12:50:43 PM EDT by remedy]
You have it backwards. The tighter the twist the faster the bullet spins and quicker it stabilizes. It doesn't necessarily increase velocity, just stability of the bullet. Barrel length and powder charge of the cartridge increase velocity.

Think about what the twists are there for. They make the bullet spin. More of them = more spin.



- rem
Link Posted: 12/31/2005 1:10:49 PM EDT
I'm not refering to the rotational velocity, I'm referring to the muzzle velocity. I know that a faster twist results in more RPM's, what I am referring to is the amount of velocity lost when one goes to a tighter twist rate. Because a tighter twist rate would offer more resistance to the bullet as it goes down the barrel. Just as an extreme example, what do you think would happen if the twist rate were something like 1/.025'', the inside of the barrel would look like it was threaded, and the bullet would be torn apart if you tried to force it through the barrel at high velocity. The diiference between 1/9 & 1/7 is much less, but I think the same principles apply. So how much velocity do you lose by going to the tighter of the two?
Link Posted: 12/31/2005 1:14:12 PM EDT

Originally Posted By PanzerMK7:
I've heard before that a tighter twist rate in a barrel will reduce velocity, how much velocity would one lose going from a 1/9 twist to a 1/7 twist? Or is the velocity loss more of a theoretical thing that is unnoticeable in practical use?



very little, most likely within the ES and not enough to make any decision on one being better than other one this merit alone.
Link Posted: 12/31/2005 1:18:18 PM EDT
That's pretty much what I thought, but my curiousity got the better of me, has anyone ever done any empirical tests with match ammo to find out how much it really amounts to. I'd like to see the info, even if it is just to satisfy my idle curiousity. You can never know too much about your rifles
Link Posted: 12/31/2005 1:53:51 PM EDT
[Last Edit: 1/1/2006 11:27:38 AM EDT by Molon]

Originally Posted By PanzerMK7:
That's pretty much what I thought, but my curiousity got the better of me, has anyone ever done any empirical tests with match ammo to find out how much it really amounts to. I'd like to see the info, even if it is just to satisfy my idle curiousity. You can never know too much about your rifles hr


How's this? Hornady 55 grain TAP, lot #020147.

10 rounds fired from a Colt 16.1" light weight barrel with a 1:7" twist chronographed at 2871 fps with an ambient temperature of 79 degrees F.

10 rounds of the same lot of ammo fired from a Colt 16.1" M4 barrel with a 1:9" twist chronograped at 2872 fps with an ambient temperature of 77 degrees F.

Now on two other occaisions using the same lot of ammo fired from the same Colt 16.1" M4 barrel with a 1:9" twist I chronographed 10 shot strings with velocites of 2890 fps at 79 degrees, and 2892 fps at 77 degrees.

As you can see in these examples there is a greater difference in velocity in two different strings of the same ammo fired from the same barrel than there is between two different barrels with different twists with the same ammo.

I've chronographed thousands of .223 and 5.56 rounds and have yet to see a statistically significant difference in velocity in relation to barrel twist. Maybe in extremely controlled conditions you will see a difference, but in a practical setting I have yet to see it.
Link Posted: 12/31/2005 2:04:07 PM EDT
Great answer, now I have to come up with another essentially pointless question for you guys to answer, I'll be in touch
Link Posted: 12/31/2005 2:11:52 PM EDT

Originally Posted By PanzerMK7:
Great answer, now I have to come up with another essentially pointless question for you guys to answer, I'll be in touch hr


Not a pointless question. I used to wonder the same thing until I did some chronograph testing.
Link Posted: 1/1/2006 11:11:03 AM EDT
addendum

I spent a couple of hours going through Hatcher's Notebook, Understanding Ballistics, The NRA Firearms Fact Book,, and Hatcher's Textbook of Pistols and Revolvers, and I couldn't find anything that documents that twist rate has a significant effect on muzzle velocity.....still looking though.
Link Posted: 1/1/2006 2:00:44 PM EDT
......i doubt if you would ever realize any "velocity" differences related to twists.....it has more to do with trajectory.......example.....most barrels are 1 in 9 twist....you are probably using 55 gr bullets most or all the time.....if you look at the old original ar`s....they liked 1 in 7 bbls....but testing and advancement proved a "tighter" twist resulted in somewhat better accuracy with std 55 gr ammo......if you look around....you will see some "varmint" bbls are built in 1 in7....or more common 1 in 8 twist......that is assuming you will be using a heavier bullet....say 69 grain....this will give you a more flatter bullet travel....at somewhat farther distance......there are bullets of about 80 grain out there....a little heavy for my liking....in 556 cal....but some guys with a technical knowledge and reason to use them can make them work.....rule of thumb would be that if you need to use a 556 for long range....a heavy bullet....out of the proper bbl (length and twist)..would achieve the results you are looking for........keep in mind...marines qualify at 600 yrds with std rifles and ammo....far as i know...that is 20 in bbl and 55 gr ammo.....far as m4`s or m249`s....???....but the differences in usage are negligable......when we "sharpshooters" get into it....we are getting fussy about such things that are in reality a "moot" issue for most shooters....but if you are going to get serious about a specific target...(prarie dogs...groundhogs.....coyotye...whatever)...it`s sure a good thing we can vary our equipment....to our specific liking......so...a general rule....if you are doing "routine or general" shooting with a ar....55 gr and 1 in 9 twist is fine......light weight targets at long range...(say 300 yrds)...55 or 60 grain with 1in 8 should....again should...give a somewhat better result......groundies at three hundred.....(or more).....69 gr out of 1in 8....would most likely be the best choice....for tighter "group"..hits................and so it goes
Link Posted: 1/1/2006 4:32:02 PM EDT
How's this? Hornady 55 grain TAP, lot #020147.

10 rounds fired from a Colt 16.1" light weight barrel with a 1:7" twist chronographed at 2871 fps with an ambient temperature of 79 degrees F.

10 rounds of the same lot of ammo fired from a Colt 16.1" M4 barrel with a 1:9" twist chronograped at 2872 fps with an ambient temperature of 77 degrees F.

Now on two other occaisions using the same lot of ammo fired from the same Colt 16.1" M4 barrel with a 1:9" twist I chronographed 10 shot strings with velocites of 2890 fps at 79 degrees, and 2892 fps at 77 degrees.

As you can see in these examples there is a greater difference in velocity in two different strings of the same ammo fired from the same barrel than there is between two different barrels with different twists with the same ammo.

I've chronographed thousands of .223 and 5.56 rounds and have yet to see a statistically significant difference in velocity in relation to barrel twist. Maybe in extremely controlled conditions you will see a difference, but in a practical setting I have yet to see it.



There, solid data, no BS. Excellent!

Congrats!
Link Posted: 1/1/2006 5:57:34 PM EDT
[Last Edit: 1/1/2006 5:59:16 PM EDT by SOSNBA]
Velocity loss...barrel erossion...i just think that those with 1:9 rifling are searching for some form of rationale as to why they didnt get 1:7". So far none of these issues have been proven to be true. What do you think the next issue that they'll try to raise with 1:7"?
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