? about powder burn for 308 in 16.5 vs 20 inch barrels

[JustUs4All]

So much for that theory. Not to worry. I will have another tomorrow.

 

[Jester896]

Even barrel twist mentioned above does not play the same game ...

a 1:10 16.5" barrel won't perform very close to a 1:12 in 20"? I might need a lesson in twist rate and velocity.

 

[sea trout]

If you want a m77 compact in 7-08 I have one I'd sell. It is a Ruger M77 Hawkeye Laminate Compact. It has been bedded, free floated and given a trigger job by a Brian Ott. It shoots Win brass, Win primers, 41gr of Varget with 140gr accubonds at less than 1/2" and is topped with Leupold vx3i 2.5-8x36. I don't like the short Bbl and when shot a doe at last light was blinded by the muzzle flash.

Thanks man we're gonna be on the look out for one in 308.
I know your rifle is a fine one though!!

 

[BriarPatch99]

a 1:10 16.5" barrel won't perform very close to a 1:12 in 20"? I might need a lesson in twist rate and velocity.

Give this a read ... http://bulletin.accurateshooter.com/2015/02/does-barrel-twist-rate-affect-muzzle-velocity-litz-test/

Oh ...also read the comments down below it ... some more details in those too...

 

[Jester896]

funny...it would seem that I am a bit backwards...which is not far fetched for me
although the test were done with identical length barrels I still wonder what differences the 1:10 in a shorter barrel with the slower velocity and faster possible stabilization of the bullet may be as far as the 1:12 longer barrel higher velocity will have on the accuracy at 4-500 yards...probably negligible. the end result is still going to be a dead deer.

 

[BriarPatch99]

Jester ... Calculating using the 165 grain SST ...

@400 yards the bullet would be at 113,400 RPM with the 20" 1 /12"

@400 the 1/10" would be at 131,220 RPM ...

I'll do a little digging to see what RPM it takes to be stable ...

 

[BriarPatch99]

Calculated using a Hornady 165 grain SST...
@400 yards RPM is 113,400 for 1/12" 20" barrel
@400 the 1/10" is 131,200 RPM

SG number is 1.49 for 1/12" which is marginally stable (1.5 minimum)

SG number is 2.44 which is fully stable for 1/10"

 

[sea trout]

Calculated using a Hornady 165 grain SST...
@400 yards RPM is 113,400 for 1/12" 20" barrel
@400 the 1/10" is 131,200 RPM

SG number is 1.49 for 1/12" which is marginally stable (1.5 minimum)

SG number is 2.44 which is fully stable for 1/10"

Briarpatch could you explain to a commoner what this means performance wise for 16.5inch m77 barrel 1 in 10 vs 20inch m70 1 in12 barrel?
Thanks dude! Interesting stuff

 

[BriarPatch99]

The 20" barrel (1/12")will be barely stable at 400 yards (with a 165 grain Hornady with a muzzle velocity of 2650) ...

The 16.5" with a 1/10" ( MV 2578) will be fully stable with same bullet ....

Please keep in mind that these are "Calculated" using estimated velocities ... a rifle in your hands may be faster ... or slower ... the only way to know for sure is to shoot across a chronograph...

 

[sea trout]

The 20" barrel (1/12")will be barely stable at 400 yards (with a 165 grain Hornady with a muzzle velocity of 2650) ...

The 16.5" with a 1/10" ( MV 2578) will be fully stable with same bullet ....

Please keep in mind that these are "Calculated" using estimated velocities ... a rifle in your hands may be faster ... or slower ... the only way to know for sure is to shoot across a chronograph...

WOW!! I understand like your saying there's always variables. But in a summery this new to me knowledge completely capsizes my original concern for 400 yard hunting!!
That's why I appreciate y'all so much! Thanks GON forum

 

[BriarPatch99]

Seatrout ... Here is a good read on velocity loss with the .308...

https://rifleshooter.com/2014/12/30...ato-barrel-length-versus-velocity-28-to-16-5/

 

[NE GA Pappy]

factory ammo will vary more round / round than the difference between the 16.5 and 20.0. you can see 100 fps between factory ammo round/round or more

 

[Jester896]

Calculated using a Hornady 165 grain SST...
@400 yards RPM is 113,400 for 1/12" 20" barrel
@400 the 1/10" is 131,200 RPM

SG number is 1.49 for 1/12" which is marginally stable (1.5 minimum)

SG number is 2.44 which is fully stable for 1/10"

Thanks!

 

[Chase4556]

WOW!! I understand like your saying there's always variables. But in a summery this new to me knowledge completely capsizes my original concern for 400 yard hunting!!
That's why I appreciate y'all so much! Thanks GON forum

Everyone else already hit it, the 16.5 will do just fine for ya at 400yds.

My biggest suggestion, put a scope on it that you can dial in elevation, or with a reticle that allows you to have an accurate holdover(first focal plane scope). With this, you can come up with a dope chart and dial in your elevation adjustments for those possible out west hunts. No more guessing your holdover, with practice you will know exactly how much you need to adjust to shoot dead on at 200, 300, 400, 500, ect ect and make a good ethical kill.

 

[BriarPatch99]

factory ammo will vary more round / round than the difference between the 16.5 and 20.0. you can see 100 fps between factory ammo round/round or more

Factory ammo sure can & does vary....
but it won't pick or choose which length barrel it varies in...

So the variances affect both barrels ... but as a general rule the shorter barrel will also yield less velocity on top of variation ...

Stacking "tolerances" can be Not good especially when the 20" barrel is already at marginal stabilization ...

 

[NE GA Pappy]

Factory ammo sure can & does vary....
but it won't pick or choose which length barrel it varies in...

So the variances affect both barrels ... but as a general rule the shorter barrel will also yield less velocity on top of variation ...

Stacking "tolerances" can be Not good especially when the 20" barrel is already at marginal stabilization ...

Agree. What I was trying to point out is that over a box of factory ammo, the variation he will see in FPS will be more than the difference of the 3.5" of barrel length.

I have seen FPS +/- 120 in a box of factory ammo, shot thru the same gun, on the same day, under the same conditions.

What I am trying to convey... get whichever gun she really likes, and don't sweat the nominal difference in FPS. That deer will be just as dead being hit with a bullet traveling at 2100FPS as it will being hit with the same bullet traveling 2020FPS

 

[BriarPatch99]

Hopeful that I have answered some of the questions that arose during this thread ... I am sure that the numbers bored some folks along the way ... maybe some of the links I shared will be useful to some folks ...

Seatrout ... go get that gal a rifle and tell us about it ....

 

[sea trout]

Hopeful that I have answered some of the questions that arose during this thread ... I am sure that the numbers bored some folks along the way ... maybe some of the links I shared will be useful to some folks ...

Seatrout ... go get that gal a rifle and tell us about it ....

WORKIN ON IT MAN THANKS!!!

The m77 compact we are familiar with is the hawkeye. I'm assuming the 1 in 10 inch twist is on that 308 hawkeye barrel.
We also found an m77 for sale today but this one is the mark ll. It's called a youth model. But it shares the exact same dimensions as the hawkeye compact.
I cant find any information if the mark ll 16.5 inch barrel shares the same 1 in 10 twist like the hawkeye barrel has.
I'll still be looking for this info but if y'all know or know where I can find out please chime in!
Thanks y'all!

 

[Jester896]

I wasn't bored.

I found some info from a post on The Firing Line from '08 that said it was 1:10 then too.

 

[sea trout]

Awesome Jester thanks!!!!!!!!!!!