.308 Win 220gr vs. 175gr SMK for Long Range Target

[robrob]

Or post 20.

[zeke]

"This means the gyroscopic stability factor increases as the bullet goes downrange. That's what was vexing Stoner. The M16 rounds tumbled just fine when they hit a target up close, but far enough away they were they settled out and would just make pencil holes. He was trying to get them to tumble further out."

Find this very interesting, although for a slightly different subject.

[Jim Watson]

Eugene Stoner would have disagreed with Berger's number. He wanted the military to go from a 12" twist in the M16 to a 14" twist to get the stability factor down nearer to 1.

Does this mean that the history I read which said that the M16 started out with the 14 twist varmint barrel and was increased to 12 to stabilize the FMJ boattail in cold dense air was wrong? (As Jeff Cooper said of another gun, rumor has it that it gets cold in Russia.)

I have variously read that the wounding mechanism was "tumbling" or a one-time yaw turnover or the bullet breaking in two at the cannelure. And that the standardization of a 14 inch barrel has done more to reduce velocity dependent effects than changes in rifling pitch.

[edward hogan]

The 185gr Berger will serve with distinction; and is mag-length capable (from what I understand).

I have 168gr SMK and Nosler BTHP; not like I'm trying to get the last 50yd capablity. And they work...

[Bart B.]

Bullet spin rate in rpm doesn't slow down as much, percent wise, as velocity as it goes down range.

Read Unclenick's comments and use Kolbe's formula in

https://thefiringline.com/forums/arc...?t-556530.html

[Jimro]

You really need to think about what you are trying to do with a rifle and load.

For target work at known distance, the 220 SMK from a 308 is a fine option. The additional drop at 1k yards is irrelevant since the known distance ensures that you won't see vertical dispersion problems caused by distance estimation errors, and the 1.4 MOA advantage in wind drift for standard 10mph 90 degree cross wind will help you stay in the ten ring.

For tactical work, it's the opposite. The 175gr SMK has 3.6 minutes less drop at 1k (that's about 36 inches), which on a torso sized target is pretty significant. That makes it easier for a sniper or dedicated marksman to use holdover/under stadia lines on a scope reticle (smaller holds are generally better, keeping the target closer to the optical center of the scope).

The 220 SMK load is dropping 79 inches between 800 and 900, and 98 inches from 900 to 1k. The 175 is dropping 74 and 90 across the same distance. I know that 5 inches and 8 inches doesn't seem too significant, but the average human torso length is between 17 and 21 inches, that additional 5 to 8 inches of less vertical drop becomes helpful in making distance errors less of a factor for success of a shot where you might only get one chance. It's about a 25% advantage when you run the numbers.

So this is why the 175 SMK has survived as the DOD's projectile for sniper ammunition, despite other options doing better on the competition line. It performs equally well through bolt and gas guns, has a tangent ogive making it very jump tolerant.

If you run the numbers for across the course high power (200, 300, 600 yards) it's clear that the 175 SMK is probably the better choice for most shooters (faster action time and less recoil, minimal difference in ballistics).

So I hope this provides some context about how a dedicated target rifle shooting known distance for score across a string of shots is just different from a tactical rifle shooting unknown distance only going to take one or two shots.

[robrob]

Even for tactical situations wind drift is much more of an unknown than range. Reticle ranging will get you close enough that wind drift becomes the dominant factor. In most sniping situations you can laser range find, especially if the sniper gets into position before the target arrives.

[Unclenick]

...and assuming the target decides to stand exactly at the spot you pre-ranged. But I suspect there are more than just trajectory ballistics involved in the choice. The new 6.5 Creedmoor SOCOM sniping rifles were selected in part because the snipers they tested it with got a significantly higher 1000 meter hit percentage with them over the .308 sniper systems. More recoil increases the recoil moments that set up barrel vibration so it can produce an undesirable accuracy influence. The military doesn't get to tune loads to the individual rifle.

[Bart B.]

Originally Posted by Unclenick

...and assuming the target decides to stand exactly at the spot you pre-ranged. But I suspect there are more than just trajectory ballistics involved in the choice. The new 6.5 Creedmoor SOCOM sniping rifles were selected in part because the snipers they tested it with got a significantly higher 1000 meter hit percentage with them over the .308 sniper systems. More recoil increases the recoil moments that set up barrel vibration so it can produce an undesirable accuracy influence. The military doesn't get to tune loads to the individual rifle.

If those military sniper rifle barrels are all the same profile and free floated, they'll all have the same vibration frequencies and amplitude. If their stocks are all the same, they'll all shoot the same load equally accurate. Each cartridge version could have its own barrel and stock shapes.

However, the 7.62 versions have more recoil during barrel time so they're harder to shoot as accurate as 6.5 versions shooting lighter weight bullets producing less recoil.

It would help to know what each versions accuracy tests were. If both tested equally accurate, then tests with field positions would be valid.

[Unclenick]

I don't know how closely the all-else-being equal applies. Boots Obermeyer developed the 5R rifling and made the barrels for the M24 system for awhile, but then someone else got the contract, so I don't know if that introduced any subtle differences. The M40 is based on a Remington action like the M24, but doesn't use the 5R barrels. Indeed, its barrels and their sources have changed with the different version upgrades. The stock is different.

The 6.5 CM seems to be being adopted as a new medium cartridge. If you search for military articles it looks like a sniper system, an assault rifle, and a machine gun have all been selected to use it.

[Metal god]

A 220 is too long for the .308 Win.

I would say this is true if your COAL is 2.800 . I however am shooting/testing the Berger 200-x gr bullet now and my COAL is something like 2.925 . The fact the bullet tip is so long .

Here it is compared to a 168gr smk


At a 2.925 COAL the cartridge is to long to clear my ejection port if I try to eject the round with out firing it and obviously must be hand fed into the action . So does that make it to long for a 308 ? I say no but think it's reasonable for others to say it is . It depends on if your pro's out weigh your cons .

The interesting thing about this 200gr Hybrid bullet is that it's baring surface is somewhat short for it's length . Resulting in the ability to get it down the bore quite quickly for it's weight .

Left to right - 175gr tmk , 195gr ELD , 200gr Berger , 190gr smk , 175gr smk



As UN points out the heavy for cartridge bullet does seem to take a bit more tuning to get right and that's where I'm at right now with these bullets .