When Jeff Cooper envisioned his Scout Rifle in the 1980’s he wanted a universal “Do-All” Rifle. Meaning that it could be just as effective hunting moose, as it is defending the homefront. He envisioned a rifle that was lightweight, with a short 18in barrel, with both an optic and iron sights, a quality trigger, and a good sling.
I very much like the idea of the Scout Rifle. In a world that has been taken over by the AR-15, of which I own several, somehow the nostalgia of a simple light and functional rifle that fires an intermediate cartridge, calls to me. Personally I enjoy doing things a little different. Looking at the intent behind Jeff Cooper’s Scout Rifle, I wonder if I can’t improve on the versatility of the rifle by doing things just slightly different.
The Cartridge/Chambering
The 308 Winchester is a not a new cartridge, in fact some would argue it is nearing it’s obsolescence. Introduced in 1952, it was the product of the military’s research and eventual adoption of the 7.62x51mm NATO cartridge. Since that time it has enjoyed worldwide popularity as both a hunting cartridge and a target cartridge. The myriad of 6mm and 6.5 Cartridges looks to have all but overtaken the 308 Win’s reign.
Non of that diminishes the fact it is still a very potent and versatile cartridge. The US Military adopted their first .30 Caliber Cartridge in the late 1800’s with the 30-40 Krag, and still use 30 Caliber to this day. It is arguable that the bullet selection for .308 Caliber is greater then for any other caliber on the market. Load data for the 308 Win spans from 110gr to 208gr.
Given that I own, several 30 Caliber rifles, I decided to stick with the caliber for this build. With the availability of 308 Win brass, and the fact I already load for it, I couldn’t find a strong argument to switch. However I still wanted to do something a little different with this build that is typically done. Just so it wasn’t a vanilla 308 Win bolt action rifle.
308 Win loads in a Rem 700 short action. The same action that is normally used for 223/5.56, or the 300 Blackout. I already own an AR-15 in 300 Blackout and enjoy shooting both subsonic and supersonic loads from the rifle. Due to the popularity of the 300 Blackout, there have been a few companies that have chambered bolt action rifles in 300 Blackout. These rifles are whisper quiet when suppressed and a ton of fun to shoot.
In a twist of irony, 300 Blackout subsonic ammunition typically outperforms 308 Win Subsonic. This is due to the faster 1:8 or 1:7 twist barrel used in 300 Blackout. This enables rifles to stabilize the 220gr bullets. Conversely 308 Win has a 1:10 twist barrel, as it was only ever designed to shoot bullets in the 155gr-178gr weight range, and was never intended to shoot subsonic ammunition.
Berger Bullet’s has a free online tool for estimating the stability of a bullet. We are at 5550 feet, so a 1:8 barrel may work for us. To be safe, and to possibly make use of even heavier cast bullets, a 1:7 twist is likely ideal.
Here inlies where we will depart from the convention, and take what would be a vanilla rifle build and add some chocolate chunks. Due to the popularity of 300 Blackout there are plenty of fast twist barrels on the market. This will enable us to exceed the subsonic performance of the 300 Blackout by using bullets in the 240-250gr range. All while giving us a rifle that will shoot full house 308 Win loads. In my book this potentially increases the functionality of the rifle.
Potential Complications
There are potential tradeoffs to everything we do. Switching to a faster twist barrel comes as some potential costs. For example, the engraving force may be increased leading to higher pressures the typical. Also faster twist rifling will spin all bullets faster. Switching from a 1:10 twist to a 1:7 twist will increase bullet spin by ~30% for a given velocity.
Conventional wisdom is that you only want to spin a bullet fast enough to stabilize it sufficiently. Over spinning a bullet can lead to the following;
- Increase spin drift, or the tendency for the bullet to drift towards the direction of spin.
- It can open up group sizes due to centrifugal forces magnifying of the natural minute imbalances in a bullet.
- It the most extreme cases bullets with thin jackets, such as those designed for varmint hunting, or target shooting can come apart mid air.
While most of the concerns are regarding accuracy, there is one concern that poses a particular safety issue. That is the bullet coming apart before it hits the target. If the bullet comes apart just as it leaves the muzzle, it can damage chronographs, or down range equipment. Of all the potential outcomes, this is the most undesirable one, it is also one we can evaluate for before we spend a dime.
Evaluating the Possibility of Disintegrating Bullets
Bullet Spin rate is a function of muzzle velocity and barrel twist. As we’ve noted increase the rate of twist from 1:10 to 1:7 will increase the spin rate of the bullet by 30%. That assumes the velocity is constant. Velocity, as we know, is a function of pressure, and barrel length. So for a given load, we know that a 16in barrel will yield a lower velocity then a 22in barrel. We can simulate this using QuickLOAD.
QuickLOAD estimates that a 155gr Barnes Match Burner will leave the barrel at 2,535 from a 16in barrel, from a 22in barrel it picks up roughly 200fps to 2732fps. In order to calculate the bullet’s rate of spin we use the Muzzle Velocity * Barrel Twist *60. Barrel twist is a ratio, how many inches equates to one revolution. To use in an equation we typically do 12 inches divided by the number of inches per revolution. In the case of a 1in7 twist barrel we would do 12/7 = 1.71.
In the case of our scenario we can use 2732fps * 1.2 *60 to come up with 196,704rpm. Comparing that to our 16in barrel the predicted velocity is 2535fps * 1.7 * 60 to get 258,570rpm. Comparing these two we can see the increase is 24%. That looks to be significant, so how do we know if that bullet will stay together.
Well, for this we do a bit of homework. What is the most velocity that Barnes gives load data for? We can check Barne’s Website to look up some load data. For example they list a max load for 30-06 which gets 2900fps. This yields a RPM of 208,800 about 20% lower than our 1:7 twist 16in barrel would yield. It does not appear that they list loads for 300 RUM, which is unfortunate as that would be the ideal cartridge to test bullet integrity in.
Given I know for a fact Barnes Tests bullet integrity, for the off chance someone does what we are doing, I am comfortable using QuickLOAD to check velocity should we load this bullet in a 300 RUM. QuickLOAD Estimates as velocity of 3188fps using N570, this gives us an estimated RPM of 229,536. We are still 12% lower then what our 16in barrel will spin that bullet at.
Not wanting to give up on this, I checked another online resource, Hodgdon Reloading to see what data they might have. Their data was generated using a 150gr Sierra Soft Point Boat Tail (SPBT) bullet.
Since Hodgdon data is actual test data and not simulated the velocities listed here are likely closer to what QuickLOAD predicts. With the Sierra bullet they were getting max load velocities of ~3500fps, this gives an estimate RPM of 252000 or within 3% of what we’d get shooting the same bullet 2535fps from 1:7 twist barrel. The question is an obvious one, and still hasn’t been definitively answered. Will the bullet hold up?
We can’t compare the Sierra SPBT bullet to the Barnes Match Burner as the Soft Point likely indicates that Sierra intended this bullet for use as a hunting bullet, thus it is likely to have a thicker jacket and may be of a stronger construction. In order to definitely answer the question I need to see a bullet of a very similar built I.E a Sierra Matchking, or a Hornady Match.
We can get around these concerns a few different ways.
- We can use a barrel that is a 1:8 Twist instead of a 1:7 Twist. This reduces the bullet spin to 228,150rpm. This is only 10% higher then what the bullet will see if fired from 30-06 @ 2900fps with a 1:10 twist barrel.
- We can stick to bullets with a thicker jacket, such as the Nosler Partition, or the Sierra Game King.
- We can shoot monolithics such as the Barnes TSX/TTSX, or the Hornady GMX which have 0% possibility of coming apart the twists and velocities we’re dealing with.
- We can shoot either reduce the load, or shoot heavier bullets, both options will lower to the muzzle velocity and will thus lower the RPM’s the bullet spins at. Keeps us from using factory loads.
Barrels are fairly inexpensive. With this project we will likely stick with the 1:7 twist barrel for now, but when we get to the test range we will recognize the risk, and test for it. If we find that some bullets do come apart, we’ll have to make the decision to either limit what loads we use in the rifle or switch to a 1:8 twist and reevaluate.
Action Length
We had originally intended on using an Old Mexican Mauser Action for this build, but as fate would have it, I managed to twist the receiver while removing the barrel. (I did something stupid, more on a seperate write up) I have been able to source a Rem 700 Short Action that I will use instead. For the vast majority of the loads I plan on shooting the Short Action will work fine.
SAAMI allows 0.795 ogive length for the .308 Win which the Remington SA is designed around, the A-Tips have an Ogive Length of 0.975. Thus the SA is too short for me to load the Hornady 250gr & 230gr A-Tip bullets and still have them feed in the magazine. I will have to single feed those. Being that those are expensive bullets, I am unlikely to shoot a lot of them unless I can Swage a similar bullet.
Alternatively if I really want to shoot a steady diet of subsonic heavy bullets, I may have a custom cast mold made with a 6 or 8s ogive, for that purpose. Nothing prevents us from using a long action in the future, but we have to start somewhere.
External Ballistics
This rifle is intended to have an effective range of 0 to 500 yards. Which is short, given that most people consider a 308 Win having an effective range of 800 to 1,000 yards. Again the purpose and scope of the build isn’t to have a long range shooter. I have rifles that can make those shots. We are stealing something from Jeff Cooper on his ideology of the Scout Rifle.
When we do a direct comparison between 300 Blk, and 308 Win with the heavier 250gr bullets, we can see the increase in muzzle energy is ~12% higher. If we go with a heavier bullet, such as a 300gr bullet we could boosty muzzle energy by ~28%. While the 1:7 Twist will stabilize a 300gr bullet, no one is currently making a 300gr .308 Cal bullet, nor have a found a mold for casting such slugs. This may be something we try in the future.
Conclusion
Part of doing a proper design evaluation is taking a hard look at the details for a given project. That is what we attempted to do here. We wanted to set some goals, mainly build a 308 Win Rifle that could match or exceed the performance of 300 Blackout, and still perform as a scout rifle.
In doing so we identified a potential issue with the fast barrel twist, and then went about trying to understand the issue, and discussed ways we might try to mitigate the problem. We also discussed the issues with using a short action instead of a long action, and touched on how the external ballistics may be affected.
As I alluded to in the article we made mistake with the mauser action that scrapped the action. This unfortunate turn of events lead us to source a Rem 700 Short Action for our build. Before I spent more money on the project I wanted to take a moment to really make sure I understood the potential issues and roadblocks that could make or break the project down the road. Hopefully you learn a thing or two and can apply it to your own future builds.