2 rifles, same model/cal, wildly different ballistics

[b1dude]

Bottom Line:
Question for discussion....is it reasonable that manufacturing tolerances could be SO DIFFERENT between two identical rifles (make/model/cal) to provide 170 fps difference? Both rifles have the same amount of useage....and using identical ammunition? I think my experience says yes...just looking to see if others have had similiar experiences OR have any additional thoughts to add..

Discussion:
Newb posting here. Long time lurker and love these forums.
Scenario:I purchased two identical ( I thought) Savage Hog Hunter rifles about 7 yrs ago for my sons. We've slowly been growing in getting to know the rifles, basic shooting technique, and reloading for the .308 cartridge. We enjoyed some time together over the recent holiday on an amazing silhouette range with targets out to 500m. Some of the longest range shooting we had done with their rifles to-date. Suffice to say with some team spotting, regular hits out to 600 yds were the reward. But....I did notice some minor differences.....so....
I don't use a chronograph much due to access/cost/logistics of setup etc. However, I did get some range time with an entry-level Caldwell Chrono a few years back and worked up a nice deer load using one of the two rifles so my sons could have a decent deer cartridge to use for a fall deer hunt. I was working on the assumption the load would work decently for either rifle.
On our recent steel shoot, after Santa finally delivering a magnetospeed chrono I decided check our previous worked-up loads so we could really get into the ballistics and variables. What I found was completely unexpected.
1. We used two types of ammo, over the counter Hornady Match 168gr ELD-M and my Varget-based reloads of 168gr ELD-Ms. Turned out my Varget-based loads were almost carbon copies of the over the counter ammo, only with lower single digit SDs in both rifles...great.
2. HOWEVER, The two identical (make/model/cal) rifles produced muzzle velocity numbers with an avg difference of 170 fps...on both types of ammo, commercial or reload.

Any thoughts on this or does this simply prove, once again, an accurate/effective load in one rifle could may not be so accurate/effective in another?

 

[BoomFlop]

It proves that even though they are both 308 Win, they are not identical. Different reamers probably cut both chambers. Different barrels, high probability they were not from the same batch.

Far too many variables. Are both of your children identical even though they came from the same parents? Virtually the same thing.

Work up a great load for each of them and enjoy the time spent with your family.

Have a great day!
Steve

 

[b1dude]

Thanks...I totally agree. But wow.... 2550 fps avg in rifle #1 and 2730 fps in another...that's A BUNCH.
First big take way..... a chronograph is a priceless piece of equipment even if a person doesn't reload.

Interesting problem this also highlights for me...being the family gunsmith/reloader.
All of our hunters carry 308s so we can share ammo in a pinch.
I've done enough reloading at this point to figure out that max loads are rarely the most accurate and usually aren't worth the hassle.
This is now another great reason to stay away from max loads even if accuracy takes me there...lower fps nodes are the place to be if there is a chance of sharing ammo with the other rifles in the family/hunting party.

 

[asd9055]

In my limited personal experience happend only one time. I am not a gunsmith, but first things I would check would be the fired case capacity from each rifle (I'd weight 10 each) and distance to lands. That would compare the chambers at least.

 

[MagnumManiac]

Ok here's the thing. I build rifles, usually I use Match grade reamers, this is my preference to have minimum spec chambers…if I build 2 identical comp rifles and chamber the barrels to be within .001" of each other, which I diligently try to do on every build, the final ballistics and max pressures of those 2 barrels are NOT always close.
All barrels I use are lapped, chambers are polished/smoothed as best of my ability.
This does not guarantee the rifles will shoot the same, same goes for actions.
I have 5 300WM factory rifles that I use for hunting, I have more that I do not shoot, that's another story!
None of them, 2 are Rem 700's, the other 3 are Kimber 8400, Win Model 70 and Kimber 8400 Super America, have the same loads or velocities. I also shot my Kimber 8400 Select in this group and it was again different in ballistics against the others.
My fastest of them all is the Kimber 8400 Classic, it's 89-100fps faster even with factory ammo, it has the same 4 groove barrel as the other Kimber's.
So, this is somewhat normally what you see. Even with identical model rifles.

Cheers.

 

[b1dude]

In my limited personal experience happend only one time. I am not a gunsmith, but first things I would check would be the fired case capacity from each rifle (I'd weight 10 each) and distance to lands. That would compare the chambers at least.

Interesting.... I would expect to find a volume difference between the two chambers...yes?
If I do...that tells me the tighter chamber was reamed with a more "worn" reamer in production...yes?
I would think the next deduction path says the tighter chamber should produce higher fps.
Lastly...if I can detect no difference in chamber/reamer impacts then the primary difference is in the rifled section of each barrel and I let accuracy results drive the loadings for each rifle and deal with it*.

Is that the line of thinking?

* Or I have a perfectly good reason to launch a re-barrel project for a .257 cal build I've been dreaming about if I have data that says I got a bad rifle from a bad "batch"

 

[asd9055]

Interesting.... I would expect to find a volume difference between the two chambers...yes?
If I do...that tells me the tighter chamber was reamed with a more "worn" reamer in production...yes?
I would think the next deduction path says the tighter chamber should produce higher fps.
Lastly...if I can detect no difference in chamber/reamer impacts then the primary difference is in the rifled section of each barrel and I let accuracy results drive the loadings for each rifle and deal with it*.

Is that the line of thinking?

* Or I have a perfectly good reason to launch a re-barrel project for a .257 cal build I've been dreaming about if I have data that says I got a bad rifle from a bad "batch"

@MagnumManiac in post above has a lot of experience. I take what he says seriously.
My comments were on how I would approach it as a scientist/engineer. Also, I'd make sure that the rifles are fitted in the stock the same way with same pressure points.
My experience: Out of 5 WBY MK V in 300 WBY, I only found one that would not shoot my standard load with 200 AB or 200 PT and RL22. But my experience is limited. I took my friends load for a 30.06 used it straight in my M70 30.06, got the same group. Maybe I got luck again. Next I'd be testing my 338 LM against my friends...will see if my luck holds out...
P.S. I only tests loads if they are way under Max! Safety ALWAYS FIRST!

 

[LongBomber]

There are too many variables in a rifle barrel and chamber to think that two nearly identical rifles will shoot close to the same velocity or accuracy with 1 load.
Chamber's, throat length, bore and groove dia, surface of the bore etc. All change the pressure developed which changes velocity.

 

[asd9055]

There are too many variables in a rifle barrel and chamber to think that two nearly identical rifles will shoot close to the same velocity or accuracy with 1 load.
Chamber's, throat length, bore and groove dia, surface of the bore etc. All change the pressure developed which changes velocity.

For long range and ELR I am sure you are right! But I have real rifle experience at 200 yards that several rifles, different vintage shoot same load! That's my facts, my rifles!

 

[MagnumManiac]

This is the other thing regarding differences between rifles of the same make and cartridge.
When the barrels are being rifled, several are done at a time with different cutters, broaches or hammer forging mandrels.
The same goes for the chamber reamer. They have several running at a time and yes, they dull over use. Everytime one is sharpened it cuts a little smaller until it is no longer usable.
When I worked in a gun shop turning lathes and doing repair work, we had a dozen brand new rifles come in that had no rifling in the bores. Almost 3 dozen rifles were shipped this way to different stores.
Bad poo happens.

Cheers.

 

[LongBomber]

For long range and ELR I am sure you are right! But I have real rifle experience at 200 yards that several rifles, different vintage shoot same load! That's my facts, my rifles!

I have multiples of a few calibers, none shoot the same speed with the same load. Moderate loads in one of my 300mags will lock up the bolt in another.

 

[asd9055]

I have multiples of a few calibers, none shoot the same speed with the same load. Moderate loads in one of my 300mags will lock up the bolt in another.

I have no reason to doubt what you say

 

[b1dude]

Thanks to everyone for jumping in on this discussion! Just to maybe add some context to the question..... any two rifles will NEVER shoot EXACTLY the same...I would agree for the purpose of this discussion. That premise really wasn't the source of my question. The source of the question is at what point are differences so great that you should be concerned?

 

[xsn10s]

Thanks to everyone for jumping in on this discussion! Just to maybe add some context to the question..... any two rifles will NEVER shoot EXACTLY the same...I would agree for the purpose of this discussion. That premise really wasn't the source of my question. The source of the question is at what point are differences so great that you should be concerned?

As far as when should this be a concern two thoughts come to mind. First is when accuracy or velocity takes the rifle out of acceptable parameters for the task YOU need. And second if you see something that becomes a safety concern. Otherwise load up and enjoy.