Can someone smarter than me explain this? 308

[Denny2277]

OK here's the story. My neighbor and me were out shooting a few rounds down range and checking our speeds with a chrony. I have a 1/10 twist Savage Pecision Carbine 20" and he has a 1/12 twist Remington SPS tactical 20". I had my stand by load of 168gr A-Maxes, 210M Fed's, Nosler brass and 46grs of Varget. I was getting and average speed of 2730fps. I had about five rounds left and I told him to give them a try in his Remmy. He got an average speed of 2640. That's a 90fps difference between the two rifle's with simular hardware. If anything I would think the 1/12 twist would be faster. The only thing I can come up with is the longer thought of his chamber must have cut down on the pressure.
Any of y'all ever have anything like this happen? Anyone want to take a gander at what my be giving us the different speeds?

 

[remaction]

Re: Can someone smarter than me explain this? 308

Using the same chronograph, right? I would think that differences (all slight differences) in throat, chamber reamer (and how your brass fit), bore diameter, number of grooves, clean bore, etc. could all factor in. This highlights why we must work up loads for our individual rifles.

 

[67rschev]

Re: Can someone smarter than me explain this? 308

Some barrels are fast .... Some are slow . Finish work , steel , chamber cut ect . I have an Obermeyer that will warp 168's at 2840 at 45.5 . All though they are smks and 2 inches longer

 

[Temp9]

Re: Can someone smarter than me explain this? 308

Measure the case capacity of the fired cases. I bet the faster rifle produces fired brass with a lower capacity (tighter chamber).

 

[leadbullet]

Re: Can someone smarter than me explain this? 308

round count in a barrel can also make a big difference.

 

[boltgunluvr]

Re: Can someone smarter than me explain this? 308

Classic example of how each firearm is unique and the necessity of reloading for the person that demands accuracy.

 

[jkonzal]

Re: Can someone smarter than me explain this? 308

I would say the difference comes from chamber size/length and bore diameter. I would not let my precision loaded ammo be fired in another gun either.
boltgunluver: ditto!!!

 

[Denny2277]

Re: Can someone smarter than me explain this? 308

How can I find out which reamer was used on my barrel? I'm getting great accuracy and I'll aways take a extra 90fps.

 

[Niles Coyote]

Re: Can someone smarter than me explain this? 308

Why... I don’t know but I see the same in my two factory Remington 308’s compared to my FN, Winchester and my custom barreled 308.

Cant advise what reamer savage used on yours.

 

[Grumulkin]

Re: Can someone smarter than me explain this? 308

On another forum some time ago, 10 supposedly identical new rifles were shot with identical ammunition. With some loads there was as much as around a 200 fps velocity difference.

 

[sinister]

Re: Can someone smarter than me explain this? 308

Remington throats are typically longer -- shots from those rifles will get slightly more gas blowby at the throat, and it will take just a bit longer for the bullets to start engraving, thus having a different (lower) pressure and velocity curve.

Other barrels may have a tighter bore (say .298 inches) and all shots from those will have higher pressure and velocity.

 

[BuzzBoss915]

Re: Can someone smarter than me explain this? 308

I'm surprised that somebody would be surprised about different level of performance, one gun to the next. Could be 90fps or 9fps but chambers and barrels will always be unique. No mention of accuracy? That will also be different and unsurprising. BB

 

[Denny2277]

Re: Can someone smarter than me explain this? 308

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BuzzBoss915</div><div class="ubbcode-body">I'm surprised that somebody would be surprised about different level of performance, one gun to the next. Could be 90fps or 9fps but chambers and barrels will always be unique. No mention of accuracy? That will also be different and unsurprising. BB </div></div>
I'm suprised that there isn't more emphasis on reamers. 90fps is the equivalent of two to three inches of barrel. All you hear about is load data, barrel length, and twist rates. Sounds to me like which ever reamer is used makes a big difference also.

 

[sinister]

Re: Can someone smarter than me explain this? 308

Remington reams their barrels so that you can chamber long, heavy round nose bullets. It's to accommodate the lowest common denominator ammo buyer.

 

[5R Milspec]

Re: Can someone smarter than me explain this? 308

It could be the fact that one barrel is more of a true 1:12 and the other not.Like many have said there are all kinds of small factors to count that would cause this to happen.

 

[Niles Coyote]

Re: Can someone smarter than me explain this? 308

While the reamer will make a difference, judging on Remington factory barrels that have been re-chambered with a match reamer by quality smiths on this site suggest that the overall barrel quality may play a bigger role in velocity. I have yet to hear of a re-chambered Remington barrel gaining 90fps, I have heard of no noticeable difference but better accuracy...

Also, FWIW, the difference between my 20” Tactical and my 26” Varmint is 90-100 fps when the same load is used in both and the same load in my 21.75 rock creek barrel is the same as the 26” Remington to as much as 15fps faster.

 

[Greg Langelius *]

Re: Can someone smarter than me explain this? 308

Bore length and twist are not the whole story. Bore land/groove diameter also affect velocity, bigtime.

Greg

 

[cmh2007]

Re: Can someone smarter than me explain this? 308

my new barrel is .5 inch less than my old barrel, and across the same chronograph the new Obmeyer is 150fps faster with less powder charge than the old barrel, and fouls almost none. it is just one of those things, each barrel is different

 

[Deadshot2]

Re: Can someone smarter than me explain this? 308

Simple word. It's called friction. Some barrels are smoother than others. Even a few micro-inches of finish difference in the bore can make a difference. Also, there are bound to be different bore diameters. OK, all .308 barrels start out with the same reamer but how deep is the rifling? How much wear was there on the button or hook that cut the rifling when it cut yours?

This is another one of the numerous reasons that a load that's fantastic in one rifle can be not so good in even the exact same make, model, rifle, even within a few serial numbers of each other.

Twist rate is probably the least of the factors that caused this speed variation. Different friction characteristics was most likely the largest.