Youth

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 .355	2834	2108	1893	12.9	30.4
6.8 SPC 95gr TTSX BC .292	2707	1865	1627	15.8	37.7
6.8 SPC 110gr Accubond BC .370	2531	1878	1687	17.2	39.8

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 SST	1402	1152	938	756
6.8 SPC 95gr TTSX	1221	953	734	558
6.8 SPC 110gr Accubond	1293	1060	862	695
6.8 SPC 110gr Accubond 20" barrel 2600 fps	1368	1124	916	741

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.

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)	2831	2707	2530	2630	2700
Time to Exit (ms)	1.08	0.802	0.877	1.165	0.88
Exit Pressure (psi)	16076	10390	9880	9809	11152
F-average (lbf)	1160	1420	1406	1846	937.7
Report (dB)	144.4	140.6	140.2	139.4	141.2

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.

Youth / Low Recoil Hunting Rifles Part 2

Youth / Low Recoil Hunting Rifles Part 1