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Youth / Low Recoil Hunting Rifles Part 2

In part 1 of this post last week I looked at recoil and report, as well as muzzle velocity when comparing a typical 20″ .243 Win hunting rifle with a 16″ 6.8 SPC AR-15. Both of these rifles are mild recoiling and excellent for youth hunters, or really any hunter who wants an easy to carry rifle for medium game like deer, black bears, cougars, and hogs. The results shown in the last post indicate that the .243 has the edge in recoil and velocity for 95 grain bullets, though the 6.8 SPC will be a little easier on the ears. In keeping the 6.8 SPC to 16″ barrel length, the addition of a silencer, typically adding 5 to 7 inches of length to the weapon, will be easier to carrier and shoulder than the 20″ .243 Win.

We turn our attention to the down range performance of the two rounds. Since the last post, I’ve learned that Barnes isn’t making the 95 grain .243 caliber TSX anymore so I’ve replaced it with the 95 grain Hornady SST, so all the analysis of the previous posts for internal ballistics is the same. The G1 BC of that bullet is .355, while for the 95 grain 6.8mm TTSX the G1 BC is .292 and the 110 Accubond has a G1 BC of .370.

 Muzzle velocity (fps)300 yard velocity (fps)400 yard velocity (fps)300 yard drop (inches)400 yard drop (inches)
.243 Win 95gr Hornady SST BC .35528342108189312.930.4
6.8 SPC 95gr TTSX BC .29227071865162715.837.7
6.8 SPC 110gr Accubond BC .37025311878168717.239.8

The table shows that the .243 Win shoots flat. Most youth will keep shots on deer well under 300 yards, but I’ve used 300 as a good benchmark as that’s a shot you want to be able to make. The 6.8 SPC shooting both the 95 and 110 grain bullets is no slouch and both the .243 and 6.8 will be able to hit game at 300 and 400 yards. How do these rounds stack up in terms of energy?

 100 yard energy (ft-lb)200 yard energy (ft-lb)300 yard energy (ft-lb)400 yard energy (ft-lb)
.243 Win 95gr Hornady SST14021152938756
6.8 SPC 95gr TTSX1221953734558
6.8 SPC 110gr Accubond12931060862695
6.8 SPC 110gr Accubond 20" barrel 2600 fps13681124916741

Again, the .243 Win has the edge over the 6.8 SPC, though the 6.8 hangs in there with the 110 grain Accubond, and either of these calibers will take medium size big game out to 300+ yards. Note that in a 20″ barrel where the 110 Accubond can push to 2600+ fps, the energy of the round has effectively caught up to the .243 Win. The tradeoff then is terminal performance vs overall length of rifle. A 16″ AR-15 is very ergonomic and easy for a hunter, especially a youth hunter, to carry and shoulder in the field. The addition of a silencer to the 16″ weapon isn’t as cumbersome as when added to a 20″ weapon.

However you compare them, the .243 Win and the 6.8 SPC both make excellent choices for mild recoiling hunting rifles for medium sized game. Personal preference for energy down range, flat trajectory, rifle size and weight, report sound level, and those intangible aspects like personal preference for a given caliber, are all valid reasons to choose .243 Win or 6.8 SPC. There are of course many other calibers, like the 6.5 Grendel and .300 Blackout in AR-15 platforms, and .260 Rem, 6.5 Creedmoor, and .25-06, etc. in bolt action platforms, that make for great mild recoiling hunting rifles. One thing is for certain, hunting is a great American tradition and pastime, and there is no lack of choices when it comes to rifle calibers that get the job done.

 

 

 

Youth / Low Recoil Hunting Rifles Part 1

Updated June 24, 2017

The 6.8 SPC caliber makes for great medium sized game hunting from hogs to deer to black bears and more. The 6.8 has mild recoil, and a lightweight rifle chambered in this caliber is ideal for hunts in which you put a lot of miles on your feet, up and down in hilly terrain. The mild recoil is preferred for anybody, but especially so for young hunters, women hunters, or anyone who wants a lightweight easy shooting rifle. My 18″ 6.8 is what I choose when I’m hunting deer in central and eastern Washington State. I’m not considering the ultra-mild recoiling .223 because it’s not legal for big game hunting in many states, my home state of Washington included.

The standard youth deer rifle appears to be a bolt action .243 Winchester with a 20″ barrel. I’ve never shot one but I’m told it has mild recoil, which I find surprising given that the .243 Win is based on a .308 parent case. My goal with this post is to compare the 6.8 SPC to the .243 Win. I want to compare external ballistics, recoil, and report / sound level – as every good parent wants to protect their child’s hearing as much as possible. Using QuickLoad as the tool for internal ballistics, and Hornady’s external ballistics calculator for down range performance, we can compare performance against several metrics.

Recoil

For most of us dad’s out there, we want our kids to have fun hunting and shooting. We start them young, and the last thing we want is a flincher because we started them with too much gun. Turns out the 6.8 SPC and .243 Win are great choices for youth hunters and shooters in terms of recoil. To see why we can compare the recoil force due to the impulse imparted by shooting on the rifle and shooter. The entire process of powder ignition to bullet exit at the muzzle takes about 1 millisecond. The force imparted to the rifle can be estimated from the impulse based on the following formula:

    \[ J = F  (t_2 - t_1), \]

which is the impulse due to a constant or average force, F, applied over a timespan starting at t_1 and ending at t_2. An impulse is a change in momentum so we can also compute the impulse J as

    \[ J = mv_2 - mv_1 \]

where v_1 is the starting velocity and v_2 is the end velocity, and m is the mass that we assume does not change for this analysis. From the previous two equations we can make the following equation:

    \[ mv_2 - mv_1 = F (t_2 - t_1) \]

And then we solve for the average force that would result from the given impulse

    \[ F = m (v_2 - v_1) / (t_2 - t_1) \]

which, given that v_1=0 and t1=0, and letting t2 = t, the total time from ignition to uncorking, simplifies to

    \[ F = m v_2 / t \]

Report

The magnitude of the report is primarily due to the sound pressure at the muzzle the moment the bullet exits. The ratio of pressures is captured by this expression (using the reference pressure P_{ref} = 20\mu Pa which is regarded as the smallest sound pressure change the human ear can detect):

    \[ dB = 20log_{10}(P_{exit}/P_{ref}) \]

This is the sound pressure right at the muzzle, which would instantly destroy anyone’s hearing if their ear was right at the muzzle. At a distance of 1m the sound drops considerably

    \[ dB_{1m} = dB_{muzzle} + 20log_{10}(r_{muzzle}/r_{shooter}) \]

We’ll take 3\mu m as the distance at the muzzle and 1m at the shooter to avoid taking the log of zero to get numbers that are typical of rifle report measurements.

Now we are armed to compare recoil between the 6.8 SPC and .243 Win, and for added fun we’ll throw in the .223 Rem and .308 Win to see how they both stack up to a high power round. To get a true apples to apples comparison for recoil, we’ll assume 16″ barrels for internal ballistics. When we look at external ballistics, I’ll leave the 6.8 SPC at 16″ and use the more common 20″ barrel for the .243 Win and .308 Win, and an 18″ barrel for the .223. These barrel lengths will also be used for comparing the report from the rifles. Especially great for comparison, the 6.8 SPC and .243 Win have shoot similar weight projectiles. In this case we’ll compare the 6.8mm 95 grain Barnes TTSX against the .243 95 grain Barnes TSX. Using Quickload with similar near max safe pressures we find the following:

Rifle/Bullet.243 Win 95 gr TSX 20"6.8 SPC 95 gr TTSX 16"6.8 SPC 110 gr AB 16".308 Win 165 gr AB 20".223 Rem 75 gr SMK 18"
Velocity (ft/s)28312707253026302700
Time to Exit (ms)1.080.8020.8771.1650.88
Exit Pressure (psi)16076103909880980911152
F-average (lbf)1160142014061846937.7
Report (dB)140.6140.2144.4139.4141.2

Note that the values for the report in the table are estimates, but useful for relative comparison. Barrel length is representative of typical youth hunting rifle barrels. As the barrel length increases, so does the muzzle velocity, and the report at the shooter decreases as the exit pressure is lower and the point of exit of the bullet is further from the shooter.

As we all know, the recoil from a .223 Rem is very mild and this data agrees. The .308 has significantly more recoil, and the .243 and 6.8 are relatively mild, with the .243 being almost as tame as the .223. From a recoil point of view, either would do but the .243 is best for typical hunting calibers. Muzzle velocity is also the best for a 20″ .243 Win, though the report is the worst of the bunch. Lesson – wear hearing protection when you hunt.

Speaking of hearing protection, a silencer is best, or electronic ear muffs. The 16″ 6.8 combined with a compact silencer makes a great gun, decreases recoil further, and isn’t so long as to be uncomfortable for an American youth to carry in the field. My oldest son has been hunting deer with a 16″ 6.8 AR-15 with an Ops-Inc silencer since he was 11 years old.

In the next post I’ll compare the external ballistics of the 6.8 SPC and .243 Win.

June 2 2017 Range Time

Took 4 rifles to the range on Friday:

18″ 308 Fulcrum

18″ 223 Fulcrum

18″ 6.8 Recon

22″ 6.8 Heavy

Except for the 223, all shooting was with hand loads. Today’s post relays the results of 20 rounds of 308. The rifle was wearing a Vortex PST, decent but nothing fancy.

18″ 308 Fulcrum Results

My version of the 168 SMK FGMM

Load 1:

168 SMK over 42 grains of H4895
Federal cases, CCI large rifle primers
OAL 2.80″
MV 2577fps

Result: 10 shot group at 100 yards

Sigma = 0.249MOA (0.183 to 0.358 – 95% confidence)
Extreme Spread = 0.82 MOA
P1-0.5 = 91.8% (72 to 99% – 95% confidence)
P1-1.0 = 100% (99 to 100% – 95% confidence)

Pretty good. Indicates high probability that 7/10 shots are expected to be within 0.5MOA of true point of aim, and 10/10 shots should be within 1.0MOA of target. For an auto loader this is great.

Load 2:

168 SMK over 43 grains of H4895
Federal cases, CCI large rifle primers
OAL 2.80″
MV 2611fps

Note: First shot was 2″ below the center of the rest of the group. I’m calling this an outlier, but I don’t like it.

Result: 9 shot group at 100 yards

Sigma = 0.389MOA (0.281 to 0.547 – 95% confidence)
Extreme Spread = 1.69 MOA
P1-0.5 = 66.9% (35.2 to 86% – 95% confidence)
P1-1.0 = 97.7% (79.5 to 100% – 95% confidence)

Not as good as the last group, and my shooting ability is an uncertain factor. Still, this data indicates high probability that 3.5/10 shots are expected to be within 0.5MOA of true point of aim, and 8/10 shots should be within 1.0MOA of target. So the question is: did my shooting fall apart and produce this less precise group, did the additional 1 grain of gunpowder cause the degradation, some combination of the two, or something else?

For fun we can combine the two groups by overlaying at the “center of mass” of each group

There is clearly a cluster in the middle and then two outliers. I’ll never know the cause but this is interesting. To get more insight, separating the groups by coloring them differently shows the contribution to the blob above from each (ignore the numbers, they’re for a single group):

The 17 rounds clustered in the middle imply something… the rifle is clearly capable of excellent accuracy for an auto-loading weapon. Did those two shots come from bad shooting? Bad loading? Fatigue? Are they truly representative of the weapon itself?

Possible I wasn’t as careful with the second batch of 10 rounds as I was with the first while charging the cases or seating the bullets. Or I was tired as this was later in the day after shooting the other rifles. And what was with that shot that was 2″ low? Clearly, more range time is warranted.

I’ll get to those other rifles in the next post.

New SHIRTS are here!

Our new line of shirts is here just in time for summer – available in sizes Large, Extra Large, and Extra-Extra Large! These are olive green “Hanes Beefy Tee” cotton shirts, with the Bison Armory logo in white on the front, and a 4 color graphic on the back – with the bullets done in proper bullet colors! Here’s a link to the product page: http://bisonarmory.com/bison-armored-shirt/

medium new shirt

Perfect for Cougar watch in the driveway… seriously, we had a cougar in the driveway a few months ago!

IMG_3489 (547x640)

Back graphic detail

IMG_3496 (640x157)

Front graphic detail

6.8 SPC and Twist Rate

I cross posted this at 68 Forums and I’m adding it here for posterity.

Here’s the rub: Twist rate makes ZERO difference to barrel performance.  My earlier theoretical analysis appears to be supported and I expect that twist rates equal to or slower than 1:4 will see no appreciable difference in pressure or muzzle velocity. So far I haven’t seen any difference with 16″ 4-groove barrels in 1:7 vs 1:11 twist.

The numbers (mv in average of several shots):

16″ 6.8 BSP”

SSA 115 OTM: 2492 fps

SSA 110 TSX: 2547 fps

SSA 85 TSX TL: 3010 fps

16″ 6.8 SPC II 1:11″

SSA 115 OTM: 2493 fps

SSA 110 TSX: 2575 fps

I had exactly 5 rounds of the 85 TSX TL and didn’t have any left for the SPC II 1:11″ twist. I’ll get those next time. I didn’t measure accuracy closely but I was shooting about 1.25″ to 1.5″ at 105 yards with everything. Neither barrel showed better accuracy than the other. The brass looked exactly the same. With a suppressor or without, didn’t make any difference.

More testing will follow with the SSA Tactical loads

 

18″ SPR 7.62mm / .308 Winchester Range Report

We put our new 18″ SPR 7.26mm/.308 Winchester rifle through its paces at the range this week. This rifle features our .308 Winchester compatible 7.62mm NATO chambered barrel. The overall performance was flawless. The Chrony was set up 15 feet from the muzzle, and we loaded up with Hornady 155 gr A-Max and Winchester 168 gr Ballistic Silvertip ammo. The A-Max achieved a muzzle velocity of 2685 fps, and the Silvertips hit 2365 fps. Reliability was perfect. At the end of the session we had a nice pile of .308 brass sitting at 3:30 about 10 feet away. Our MA-TEN based BR-X rifles were designed to go bang every time the trigger is pulled, because we know your hunting trip or your life may depend on it.

The first session showed good accuracy with 1.5 MOA 5-shot groups. Took the rifle out for a second range session because we knew it could do better. The first trip out we found that the trigger was terrible. It was a basic GI single stage trigger, but something was obviously wrong with the mating surfaces between the hammer and the sear. This time out we put a Timney single stage trigger in the rifle with a 4.5 lb pull. What a difference. Here’s our setup:

18" BR-X on the bench

 

And the view down range:

BR-X on the bench, downrange view

Targets are 105 yards from the muzzle. No Chrony on the second day as it was raining and miserable. This time we brought several different types of ammo including 150 gr Hornady SST Superformance, and hand loaded 150 gr SMK and 150 gr Barnes TSX

The Bank

And the results

Range results

Starting at top left: the first group was fired with 150 gr Sierra Match King over 46 grains of H4895. The first shot is for sight in, then moved a few clicks left and called it good for testing. As you can see, there are many three shot groups that would be great, but three shot groups are for sissies, so I manned up and shot 5 in each group. This is more a testament to my shooting still than to the rifle. Our professional shooter Dave wasn’t available for this range day so I had to do the dirty work myself. First group came in at 1.36″, measuring the groups as the largest distance between the centers of any two shots in the group. This produced a respectable 1.34 MOA.

The second group at the top right is 150 gr Barnes TSX over 46 grains H4895. Woof! I didn’t even bother measuring that one. From the left at the bottom: 150 SMK over 43 gr Varget, 150 Hornady SST Superformance, 150 Barnes TSX over 43 gr Varget. The rifle obviously likes the SMK’s, and with some time put into load development, should be able to push MOA or better. This will be our focus in the future. I’m also looking forward to the weather warming up and a nice sunny day at the range. Once I get a good load dialed in, it will be time to call Dave to put up some real groups.

Bison Carbine with Ops Inc 12th Model Silencer

Silencers really make shooting fun. We brought a lightweight 16″ carbine to the range for some fun with an Ops-Inc 12th model silencer.

BR-15 carbine, Ops-Inc 12th Model silencer

Silencers are expensive, the wait for the paperwork to go through is a drag, but they’re worth the trouble. Once you shoot suppressed, you’ll never go back. And you’ll never move to California.

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