Most AR-15 barrel spin rates are 1:7, most bolt action .308s are 1:10. Is there a reason why .308s are slower spinning?
- Russ in NOVALv 78 months agoFavorite Answer
Optimal spin rates are determined by the bullet's weight, diameter, length, and velocity. A larger diameter bullet tends to have a slower twist rate. Longer, heavier and slower bullets have a faster twist rate.
Also .308 caliber rifles that are optimized for 150 gr bullets tend to be 1:12, where those optimized for 168-177 gr bullets use 1:10.
Here is a good general read on twist rates: https://www.chuckhawks.com/rifling_twist_rate.htm
- curmudgeon55Lv 78 months ago
M16, early AR15 had 1:14 twist, then 1:12 twist rate- 1:10 a bit later in Viet Nam conflict. slower twist gave the tumbling, crippling wounds when bullet hit - and bullet was 45 to 50 grains at start, full metal jacket on varmit bullet. 52 grain and 55 grain bullets had great range, penetration but less tumbling, lesser wound channel. 1:9 became more common, SS109 bullet and the 855 were heavier bullets and now the 62 grain semi round nose gets the 1:7 twist. In AR15 that is used for deer hunting the heavier bullets are required- some specify only the 60 grain or heavier soft points allowed, locally the 55 grains is minimum preferred by DNR to be used. There ar many AR15 barrels with the 1:10 twist and some ordered with 1:14 for the lightest, quickest blowing bullets to be used on varmits or be used on targets at moderate range, tightest groups since fastest moving. .308 standard is /was 1:12, 30-06 was 1:10 twist -- bullets expected to be used on .308 were the 10 gram to 12 gram bullets, 30-06 was expected to use some times the older 220 grain/ 15 gram round nose bullet that would need more spin, tighter twist for something like moose. 125 grain bullets loaded for .308, rarely for 30-06 in factory. .308 'heavy' factory was the 200 grain small bear and moose load, wouldn't stabilize 225 +- bullets as tighter twist 30-06 that came from the 30-03 with its standard load of 220 grain round nose, 'Light bullet' was 180 grain special same as old Krag 30 caliber USA. 30-06 used the 10 gram spitzer 3 years after original issue of 03, that is/was where the final 06 name came from. tight twist was kept, older heavy bullets were still used at times, in stock. .308 rifles 1950s as civilian 7.62x51 Nato variants used the old 30-30 rifleing twist standard since expected bullets would be about same grain weight as 30-30 used. Military NATO bullets were expected to be 10 gram to 12 gram with possible machine gun 13 gram bullets, full jacketed while the civilian ammunition for hunting would be similar weight bullets with soft points. Tight twist can 'scrape' some jackets, slow twist with proper bullet will have less 'drag' on bullet. .
- Anonymous8 months ago
Its for accuracy over range
- daniel gLv 78 months ago
Probably for the difference between .22 and .30.
My Remington in 22-250, same bullet as .223 has a twist like 1:13.
Actually shoots flatter, but a heavier bullet looses its accuracy in a shorter range.
Still a tac driver at 500 yards under a 155 Gn bullet. (55 Gn.)
Your .22 start wobble around 600 yards, 6MM more like 1000.
.308 starts falling apart at 1300 yards.
- How do you think about the answers? You can sign in to vote the answer.
- USAFisnumber1Lv 78 months ago
The best spin is dependent on the length and weight of the bullet and how fast it goes down the barrel. They pick the best spin rate for the most common ammo used. Having a faster twist rate than necessary is better than having a slower twist rate. Get the rate too slow and the accuracy really sucks.
- BBeanLv 78 months ago
A LOT of AR-15 barrels are 1:7 but can you really say MOST of them? To answer your question with a somewhat scientific reason in layman`s terms and more than I care to explain ...but think surface speed that hardly a few talk about. The symbol π x dia. gives circumference which is how surface speed is calculated. So, If a circumference of a .308 dia. were spinning at a 1:7 twist @ (2600-2800fps) the surface speed would be excessive to the point of a self destruct bullet. All the spin a bullet needs is to stabilize to point of target...no more. So, the rule of larger diameter....slower the spin. NOTE: Layman`s terms are applied here with a simple math formula which I am not going to attempt to relay to you. Just enough to whet your appetite if you are truly interested.
- RickLv 78 months ago
bigger caliber has a slower muzzle velocity .........................
- Anonymous8 months ago
Larger caliber means MORE MASS and 1:10 in a BOLT ACTION is acceptable...and more accurate.