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

[sea trout]

Hello fine folks!
Thanks for letting me ask 2 questions in one night!...thats what a snow day will do to ya!!!!!

Ok so, my wifes huntin rifle for 12ish years was a ruger m77 compact in 7-08. She loved it! It was like an extention of her arm, she was fantastic in operating it!
Then last year she grew more towards wanting a 308. At the same time she handled a new Kimber hunter 84 in a local gun store. She wanted that. So being common poor folk we had to sell the m77 compact 7-08 to buy the Kimber hunter 84 308. So last year we did.
The New Kimber was cool at first with ASTOUNDING 100 yard clover leafs with 165 gr ammo! But now she doesn't like it. She doesn't like the 1 1/4 inch longer leanth of pull and she doesn't like the magazine feed as much as her m77 compact.
So now we are in the market again for another 308 in either m77 compact or m70 compact.
The m77 compact 308 has 16.5 inch barrel and is exactly the same dynamics as her 7-08 that she was a perfect fit with.
The m70 compact 308 has a 20 inch barrel and is only .3lbs heavier than her old 7-08.

The reason for our history read is because the obvious choice is the m77 compact which she already is perfect with. EXCEPT!! We have plans for more western hunting in the future. I have already been and in the future I'd like us to go together often!

So our ? is after the long read is.....How much difference, if any, does shooting 308 out of 16.5 inch barrel vs 20 inch barrel make at 400 yards?
Is 16.5 inches enough to get full powder burn from 308?
Does the shorter barrel affect down range energy?

If y'all have answers to these questions please chime in!!!!
If you have any further suggestions or advice after reading our history situation please chime in!!!
For the exception of advising us to get some other brand rifle or other caliber. Thanks but we are ONLY interested in m77 or m70.

I hope my questions make sense and thanks for any and all answers! Like I've said in the past we only have a waffle house talk gun knowledge and y'all are ALWAYS so helpful when we have had questions over the years!!!

Sorry the post was so long

 

[Bluemoose]

.308

Are you shooting factory ammo or handloads? With the right load a 16.5 barrel at 400 yards should not be a problem. Just find or load the ammo the gun likes.

 

[sea trout]

Are you shooting factory ammo or handloads? With the right load a 16.5 barrel at 400 yards should not be a problem. Just find or load the ammo the gun likes.

Thanks very much! As of now we do not and have never handloaded. We don't have the equipment or experience.
However we are not opposedd to buying premium shelf ammo. I understand that may still not be as good as handloading.
Thanks!

 

[BriarPatch99]

There is a chart somewhere that gives the average loss per inch for .308 Winchester is about 22/23 fps ....

 

[sea trout]

There is a chart somewhere that gives the average loss per inch for .308 Winchester is about 22/23 fps ....

Per ??????
The difference between 20 and 16.5=3 .5?
Or per inch?

Thanks Briarpatch!!!!!!

 

[ryanh487]

You lose less than 100 fps of muzzle velocity. Won't change the lethality at 400 yards at all, and you should be practicing at that range to know where you bullet will impact regardless of what the charts say.

 

[BriarPatch99]

Yes ... "Average loss per inch" ...

Given the same load ... about 80 fps less for the shorter barrel or...

About 2.5"/2.6" lower @ 400 yds. calculated .... that is using 165 grain Hornady SST @ 2650 fps(20" barrel)

 

[NCMTNHunter]

As far as accuracy goes if you take away human error shorter barrels are typically more accurate because they are more rigid and less vibration. They can appear to be less accurate because they don’t balance as well and can be harder to steady in real life situations.

Velocity is a whole other bottle of wax. One thing I learned real quick once I started shooting through a chronograph is that in real life calculating velocity is right up there with catching unicorns. What I mean is that published velocity is rarely what you actually get and it varies greatly from gun to gun and brand to brand. For example my favorite 25-06 load has published velocity of 3000fps. My wife’s rem 700 fires that load at 3100fps and my browning abolt shoots the exact same load at 2850fps with the same barrel lengths.

I say all that to say that in the real world the ruger with the 16.5” barrel may well have the same muzzle velocity as the m70 with a 20”. Or in an extreme case the 16.5 could be 300fps or so slower. I would go with the rifle that fits her best and that she can hold steady and not worry much about barrel length. You are already having to compensate for drop at 400 yards anyways.

 

[Jester896]

the Ruger... if it is a Hawkeye... has a 1:10 twist and the M70 has 1:12. It wouldn't surprise me if they didn't perform close to the same. Not sure if BriarPatch99 used that in his calculations.

 

[BriarPatch99]

No ... Only the calculated velocity ....

As said above ... no two identical rifles will shoot the same ... Only real world measurements can really be compared ...

My 22" 25/06 will beat my 24" . 25/06 by 150 fps ... same load

 

[sea trout]

Thanks Y'all!
All makes sense!!

 

[fishtail]

Although I can’t find what the length of the test barrel was, Remington’s 308 factory ammo with the 150gr CoreLokt has a published velocity of 2820fps. In my 18 1/2” carbine its average muzzle velocity is 2630fps.

 

[sea trout]

Thanks for the input y'all!
We don't really understand the performance difference with different velocities but we're learnin! Is it away's safe to assume that the faster fps is always gonna deliver more desirable long range eneregy and better coefficiants?
And we really don't understand what bullet coefficient is really. Performance? mushrooming? Energy?

I think we'd prefer to end up with m77 compact with 16.5 inch barrel.
Just don't want to make the mistake of loosing TOO much 400 yard energy!
I shoot my 22inch barrel 30-06 and my brother shoots his 24inch barrel 308 at 400 yards with the scopes that are mounted on our rifles fairly easy with memory of where to hold with different winds.
Shooting just 100 more yards for us at 500 yards it becomes a mathematical equation for for us to shoot!
I'M TRYING TO AVOID THAT SCENARIO WITH 16.5 INCH BARREL 308. Were looking for 95 percent east coast woods and food plot gun. 5 percent future western hunting.
Thanks for y'all help gentleman!!!!!!!!!!!!!!

 

[fishtail]

With the recent availability and popularity of an AR10 in 308, you would think some ammo manufacturers would have already introduced some “Custom AR hunting ammunition”.
Simply by using faster burning powders designed with suitable projectiles.

 

[Klondike]

Twist

I am a huge fan of the .308 caliber and currently own two rifles and load for both.

A little off topic but I simply don't get rifles in 308 with a 1 in 12 twist. 1 in 10 in my view is much better for stabilizing the wide range of bullet offerings

Also agree with the OP's wife - I don't like the Kimber Hunter at all. That safety and rifle feels like a toy and I am not buying the mid position on that safety at all

 

[sea trout]

I am a huge fan of the .308 caliber and currently own two rifles and load for both.

A little off topic but I simply don't get rifles in 308 with a 1 in 12 twist. 1 in 10 in my view is much better for stabilizing the wide range of bullet offerings

Also agree with the OP's wife - I don't like the Kimber Hunter at all. That safety and rifle feels like a toy and I am not buying the mid position on that safety at all

Not off topic at all Klondike thanks a lot! Twist rate and performance is very important and your answer is great for our consideration thanks!!!

 

[sea trout]

With the recent availability and popularity of an AR10 in 308, you would think some ammo manufacturers would have already introduced some “Custom AR hunting ammunition”.
Simply by using faster burning powders designed with suitable projectiles.

Don't most of those have 16.5 inch barrels? And I think special forces are using Armalites and LMT's to shoot over 400 yards?????!!!

 

[BriarPatch99]

Seatrout ... Ballistic Coefficient is the ability of a bullet to overcome resistances of the air it is traveling though ...

Given two bullets of the same weight ... same muzzle velocity ... the one with the higher B.C. travels farther and flatter ... But ...this only has enough difference at longer ranges ...

Muzzle velocity will play a bigger role at 400 yrs or less ... than will B.C. The only real way to compare barrel length is to take a rifle and cut the barrel inch by inch and shoot it over a chronograph ... anything less is a "guess" because no two guns are the same.

Even barrel twist mentioned above does not play the same game ... Take two identical barrels in a 1/10" twist . .. one may act as a 1/9" the other may act as a 1/11" . .. or they can both act as close the same ... but no guarantee ... barrels made today are closer to the named twist than barrels of yesteryear...

Also what ever powder that will give the highest velocity in a 22" . ..Will also give the highest velocity in a 16.5" barrel . .. this has been proven many times over.

 

[JustUs4All]

Yep. Briarpatch99 is right on as usual.
Ballistic Coefficient is a measure of how streamlined the projectile is.
The twist rate of the rifling in a barrel is important in that different rates are required to stabilize a projectile in flight, like a thrown football.
The burn rate of powder is an indication of how quickly the powder will burn when the cartridge fires. Theoretically, a quicker powder could be used in a shorter barrel to get more out of a particular load.
The most important thing to your wife is how comfortable she is with the weapon. Sounds to me like she has lots of experience with and a strong preference for the Ruger m77 compact. Get her another one in .308 and don't look back.
Good quality factory hunting rounds will let her take game out to 400 yards with little trouble if she can hold on the target and it sounds like she can.
Y'all have fun hunting.

 

[BriarPatch99]

The theory that using a faster powder in a shorter barrel will give more velocity has been disproved many many times over ...

The powder that gives the highest velocity will give the highest in a short or long barrel ...

Powder burns in the first few inches of a barrel ... the expanding gasses that remain after the burn ... will continue to push on bullet until it exists the barrel .... With a long barrel it has more time to push over a short barrel ... that is the reason long barrel give more velocity ...