Joined

·
5,282 Posts

1 - 9 of 9 Posts

Joined

·
5,282 Posts

Joined

·
17,055 Posts

Hornady 40gr V-Max bullet, BC=.200, Velocity=3740 fps

Hornady 50gr V-Max bullet, BC=.242, Velocity=3495 fps

Norma Soft Point 53gr, BC=.237, Velocity=3215 fps

Norma Orix 55gr Bullet, BC=.185, Velocity=3117 fps

Norma TAC 22 is 22 LR ammo ... not 223 Rem. 40gr LRN bullet, BC=.15, Velocity=1066 fps (Standard Velocity)

Here's a link for Norma TAC ammo: TAC

Joined

·
17,055 Posts

BTW, 77gr bullets are intended for very fast twist rates ... typically 1:7. Shooting them in a rifle with a slower twist rate will likely end up with unstable bullets.

Joined

·
17,055 Posts

Bullets get their spin from a combination of velocity and twist rate. The heavier (longer) the bullet, the faster it must be spun to maintain downrange stability and accuracy. Also .... the longer the barrel, the higher the velocity. Chamber pressure and case capacity limit how fast you can drive a bullet so when you use a bullet that is much heavier than the gun was designed for ... the only choice is to reduce the powder charge or else you will blow the gun up from excessive chamber pressure.

I have never chronographed 77gr bullets from my AR556 but I have chronographed 55gr and 62gr. As an example, a 55gr factory load will produce 3240 fps in a 24" barrel .... about 3100 fps in a standard 20" AR-15 barrel but only 2750 fps in my AR556 with a 16.1" barrel.

The bullet's spin rate is directly related to velocity and twist rate. The formula is RPM=12/Twist Rate times velocity in fps, times 60. Another example .... a 1:12 twist in a 24" bolt action rifle shooting a 55gr bullet @ 3240 fps will produce a bullet spin rate of 12/12=1x3240x60=194,400 RPM. Same cartridge in a 16.1" AR556 1:8 TR .... 12/8=1.5x2750x60=247,500 RPM.

A 77gr factory load produces about 2750 fps from a 24" barrel. Using the same 85% as above, that same cartridge would only produce about .85x2750=2337 fps in an AR556. A 1:7 TR with a velocity of 2750 fps will produce a spin rate of 282,857 RPM whereas in an AR556 1:8 TR and a slower velocity of 2337 fps will produce a bullet spin rate of 210,330. For downrange bullet stability, the minimum spin rate at the muzzle must be 250,000 RPM or faster. This is just NOT achievable in a shorter barrel with a slower twist rate. BTW, 77gr bullets are intended for long range target shooting where a much higher BC will allow the bullet to maintain stability for at least 1000 yards. This is just not a practical application for an AR556, which was designed for "close up and personal" type of shooting .... 100 yards or so. If you want a gun with long range accuracy, an AR-15 or bolt action rifle with a 1:7 TR is just the ticket.

What I have found with my AR556 ... it is actually the most accurate with 55gr bullets than with anything heavier. I can squeak in groups just under 1" at 100 yards whereas 62gr bullets best groups are about 1.25".

Joined

·
5,282 Posts

1 - 9 of 9 Posts

Join the discussion

Ruger Forum

A forum community dedicated to Ruger firearm owners and enthusiasts. Come join the discussion about optics, hunting, gunsmithing, styles, reviews, accessories, classifieds, and more!

Full Forum Listing
Explore Our Forums

Recommended Communities

Join now to ask and comment!