need 308 advice

[Unclenick]

Slamfire,

Look here.

Originally Posted by mhancock

…I hope my 168s will do as well. Mk has always done well in my stw. I'm still learning what this 308 likes. We have some 168s too but I'm going to try the 165s at range today…

For F-class, it depends on the range. If you are using 500 yard targets, then you'll be fine with the 168 grian SMK's. If you try to get past about 700 yards though, be prepared for keyholes and tumbling and having trouble even staying on paper. I've watched this happen to whole groups of shooters and rifles twice now. If there's enough side wind, a dynamic instability will turn this bullet head over heels. Bryan Litz also comments on this problem with this bullet.

If you want to get to 1000 yards without a long barrel, I'd recommend the 175 grain SMK's, as they don't have this problem. The current 155 grain Palma SMK's are another possibility, but they have a short bearing surface, so you need to determine whether your rifle likes them or not.

[Jimro]

Just a little background on the 168gr SMK.

It was developed to be a 300 meter free rifle bullet, and it set a couple world records and brought home the gold at a couple Olympics.

However the sharper boat tail, 13 degrees, has known issues in the transonic range. That is something that isn't an issue at all at 300 meters. Sierra found it simply happy coincidence that the 168 SMK shot well at 600 yards and quickly grew to be a favorite for High Power and Service rifle competitors.

Honestly most shooters I know have limited resources to shoot beyond 600 yards, so the 168 SMK (and all the various clones out there) still enjoy good commercial success as they are generally very accurate at all practical ranges.

There are newer 168gr bullets with different ogives and boat tail angles (and lengths) that may fly a lot better through the transonic range than the 168 SMK, but I'd have to defer to someone like Brian Litz for that.

Jimro

[Bart B.]

Jimro,

I agree that magnitude of the barrels whip and wiggle frequency changes with the force applied to it. But the metalurgy properties don't change enough with the heat they go through to make any difference in the frequency they do it at. When heat makes a difference, the barrels's soft enough that even a reduced load would blow it apart or at least crack it at its back end.

The most common reason shot impact moves as the barrel heats up is external forces on it change as it expands from heat. Most often, its the point where the barrel tenon shoulder is hardest against the receiver face. When the receiver face ain't squared up with the chamber axis, one point will bear harder against the barrel when it heats up bending it from the additional pressure at that point. If this were not the case, then squaring up a receiver's face would have never caused a good barrel to no longer walk shot impact as it heated up.

In second place, an improperly stress relieved barrel will bend in the direction of stress lines as it heats up.

Too many folks have fired dozens of shots 20 to 30 seconds apart that have put all the bullets inside 1/2 MOA at long range. Arsenal match ammo's tested at 600 yards with 200 to 300 shots per group fired that fast; good lots go under 1 MOA with mass produced ammo using 172-gr. FMJBT bullets not as good as Sierra's HPMK's.

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Unclenick,

Sierra's 155-gr. Palma bullets shoot just as accurate in barrels with the same bore and groove dimensions used for heavier 30 caliber HPMK bullets up to 250 grains for best accuracy; there's a very big spread in bearing surface across these bullets. The difference in the barrels is the twist rate and muzzle velocity needed. 155's need to leave at least 3000 fps to stay supersonic at 1000 yards and go through 1:13 twist rifling. Any faster twist can cause some bullets to jump off the muzzle axis enough upon exit that they'll not shoot accurate. This is the reason benchresters use rifling twists just fast enough to stabilize the bullet all the way to the target. 115-gr. 30 caliber bullets with very short bearing surface in the .30 BR case also shoot most accurate with the same bore and groove dimensions but with 1:17 or 1:18 twists. Therefore, I don't think bearing surface has any significant effect on attainable accuracy as much as twist rate does.

There does seem to be an improvement in the 155's accuracy when a 4-groove barrel's used compared to a 5- or 6-groove one. Heavier bullets seem to do well with 3 through 6 grooved rifling.

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All,

Note that Sierra's 190 HPMK's shot very accurate having an excellent track record in 1000-yard matches from 24 inch 7.62 NATO barrels in Garands. And in the 22 inch M14 barrel, Sierra's original 180 HPMK (same boattail as their 190's and heavier) shot just as accurate with equal success. Both had to use maximum loads to do it, though. But in the 1980's, Sierra made a change to the 180 HPMK.

To cut tool and die making costs, Sierra changed its 180 HPMK boattail to the same shape as the 168 HPMK. That way, the same dies could be used in the base forming operation of cored jackets for both bullet weights. The US Army Marksmanship Unit immediately noticed a loss of accuracy in their M118 7.62 match ammo rebulleted with Sierra's new, short-tailed 180's. No longer would they shoot 1 MOA at 1000 yards in the best M14NM service rifles; it was more like 2 MOA or bigger and that's not gonna cut the mustard in competition.

So, the US Army convinced Sierra to keep using the old boattail shape on special lots of 180 HPMK sold to them. All the commercial 180 HPMK's had the short boattail of the 168's but they did very well through 600 yards. Given a choice of Sierra's 168 or 180 HPMK's for use up to 600 yards, I'd pick the 180 as it's less effected by subtle crosswinds.

And this scenerio was the reason Lake City ammo plant changed the bullet in their M852 match ammo from Sierra's 168 to a new 175 grain HPMK bullet from Sierra with the same, old long boattail shape the 190 and heavier bullets use.

[Jimro]

Bart B., I love how you insist that the metallurgy doesn't change, then list all the ways that it does.

However, temperature does change resonant frequencies of steel, some alloys more than others.

http://asadl.org/jasa/resource/1/jas...s4?bypassSSO=1

Jimro

[Bart B.]

Jimro, I know where you're coming from. It's somewhere beyond the context of my comments.

I agree that a barrel's vibration characteristics will change if it gets really hot. Machine gun barrels, for example. But for most others, it does not.

When Tom Irvine wrote his software to calculate a barrel's resonant frequency on my request, his input requirements do not include barrel temperature.

http://www.vibrationdata.com/StructuralFE.htm

The range of temperatures most sporting barrels go through ain't enough to change it. If it did, then folks would never be able to shoot sub 1/3rd MOA 40-shot groups at 600 yards; they have. Nor would Lake City ammo plant shoot a couple hundred shots inside 6 inches at 600 yards with good lots of M118 or M72 match ammo with bullets not as good as Sierra makes; they have. Both instances starting with a cold barrel then shooting a round every 20 to 30 seconds; sometimes faster. Sierra Bullets shoots 10-shot groups testing bullet quality; one shot every 10 seconds or faster. Starting with a cool barrel, good lots of match bullets all go inside 1/4th MOA at 200 yards; I've seen some of there test groups of 30 caliber HPMK"s so shot all close to 1/10th MOA at 100 yards

Varmint Al's shooting page doesn't include barrel temperature in all of his barrel tuning and vibration analysis, either.

http://www.varmintal.com/ashot.htm

Note also that some folks cannot discriminate point of impact change causes as barrel temperature increases as to whether the increased powder tempertarure of some rounds sitting in the chamber too long or the increased amount of bore fouling in later rounds fired in the test are the cause of higher muzzle velocities causing bullet impact to go higher on target.

So please excuse my inability to be absolutely technically correct so nit pickers will have a way to discredit my comments. But it's probably OK for you to not absolutely technically correct in the context of the issue.

[Jimro]

Bart B.,

Let us think about this. If temperature isn't a variable then why do skinny barrels have group sizes open up quicker than heavy barrels when all else is equal? A group shift with heat is an indicator of stress, either from bedding or internal stresses in the barrel (or an out of center bore).

I agree with you that the frequency doesn't really change as that is mainly a function of length and material. What changes most dramatically is magnitude of frequency, which is why the groups on hot skinny barrels get larger, as the magnitude of deflection has increased. The when a 0.001 deflection of the muzzle translates into 1/6 MOA deflection for a 20" barrel you can see that it doesn't take much to get groups to open up as the barrel steel is affected by temperature. 1/6 MOA is insignificant for a High Power shooter on a target where the X ring can be 2 MOA, but it is pretty important for a benchrester where the difference between winning and losing a match can be measured in hundreds of an inch.

Those accuracy test devices you talk about have barrels thicker than my wrist for a reason, and that reason is to keep things rigid longer.

Jimro

[Bart B.]

Jimro, skinny/whippy barrel groups do not open groups up faster than thick/stiff ones do providing the barrel's fit properly to the receiver. If the receiver face has a high point (not square with the chamber axis), then it will definitely bend a bit as it heats up and that's the common cause of point of impact changing.

And those accuracy test barrels used by arsenals are no thicker than what high power competitors use; about 22 inches long and 1.1" diameter; smaller than your wrist. There is no difference in accuracy between long, skinny whippy barrels than short, thick stiff ones. Otherwise, a 32" long skinny Palma rifle barrel would not shoot just as accurate as a 22" short, thick arsenal test barrel that's a whole lot stiffer.

I've shot 20 rounds from such a long, skinny Palma barrel and all went sub 1/2 MOA at 800 yards; started with a cold barrel and after all shots were fired about 15 to 20 seconds apart, that barrel was very hot. And with a standard, whippy M1 barrel (more whippy than most rifle barrels), they've got a track record of holding 1 MOA for 20 to 30 shots at 1000 yards with good ammo starting out cold; I wore out four of 'em that shot that well.

Well made properly fit and installed barrels have never "walked" shots as they heat up regardless of their shape. But it's a popular myth that all barrels do so.

More important, a .001" deflection of the muzzle axis will cause more than a 1/6th MOA change in the direction it points. The muzzle's a few to several inches forward of the front zero vibration point, or node; there's another one close to the receiver. If it's 6 inches forward of the node, then a .001" movement translates to an angle of 1.67 MOA. Check out the following:

http://www.varmintal.com/amode.htm

Look at mode 2; that's the one that causes the greatest muzzle axis angle change when the bullet exits.

[superspirit]

I was just wondering if the OP had any more results with his loads, I am looking forward to his progress as I am trying to get started loading for a dpms LR 308 carbine.

[Jimro]

Bart B.

A Mann accuracy device was 1.25 inches behind the barrel collar which was bigger than that, although they weren't a "uniform" standard.

And one of Varmit Al's buddies records of barrel heating and deflection, specifically dealing with a pressure bedding system. Still, with the numbers we've thrown around for muzzle deflection (you insisted that the deflection was greater than I'd calculated, I won't argue that point as the amount of change isn't as important as acknowledging the change will happen).

http://www.rifle-accuracy.com/barrel-movement.htm

Note that the temp range covered is around 50 degrees F.

Jimro