Another Odd Day at the Range

[lugerstew]

Just got back awhile ago from the range, was testing some proven loads at 300 yards. After retrieving my targets, I was shocked at this one that was supposed to have 2 groups of 7 each of the same load, but it only had 2 groups of 4 each, 3 rounds from each group must have disintegrated. Wow, this is from my 243, with a 1 in 8 twist 26 inch barrel, the velocity of the rounds were 3522fps.
Does the rpm of a bullet slow down with distance? I would have thought so.
I have shot these dozens of times, but only at 100 and 200 yards, and they shot great, in fact one of my favorite bullets for this gun. But if they are going to disintegrate occasionally after 200 yards, then I might have to not use them any more.
These are 80 grain Berger FB varmint bullets, with RL15, 40 grains, coal of 2.700.
Any suggestions?

 

[Shadow9mm]

I'm guessing your pushing them too fast with too much twist. From what I have seen 1:9 to 1:10 is a common twist rate, you are running 1:8, and they are a thin jacketed varmint bullet. Berger recommends a 1:21 or faster. I would contact berger and see if they have and velocity recommendations for the bullet or if it might be quality control issue on the lot you have.

you listed 3522fps, did you check velocity on that day, or when you worked the load up? if the latter I would run a few over the chrono to double check.

 

[scatterbrain]

I have a 244 with a 26 inch barrel, 1 in 12 twist. Using 85 grain bullets I run at 3200 fps and would not go faster, the primers are starting to square up on the edges. 70 and 75 grain bullets at 3500 fps will back the firing pin dent out flush with the face of the primer, maybe a little past flush, that's hard on the brass and gun.

 

[hounddawg]

I run those same bullets in my 6BR. From 3050 - 3150 FPS (29.5 - 31.3 gns of 8208) I get bugholes at 100 with single digit SD's. My barrel likes them at about .040 off touch and at 300 it will get me .3 - .5 MOA. On calm days I have shot them out to 850, but after 500 they are so light the wind pushes them around a lot

 

[std7mag]

What is your twist rate, Hounddawg?

 

[hounddawg]

1- 8 Criterion.

Luger if you can get ahold of some 8208 it really works well and anything between 2900 and 3150 gives a good group. Here are the load workup targets and a composite of 29.5 - 30.3 that shows where a .8 gn powder spread of 25 shots had a POI of only .7 MOA. You could throw this stuff from any powder throw and go hunt pdogs with it. It worked just well with 100 gn Hornady interlocks also

 

[lugerstew]

Thanks hounddawg, I looked and I have about a half pound of 8208, I don't know why I haven't tried it yet, so I will definitely do a workup with it and these bullets.
This bullet is so nice, I have gotten 1/4 inch 3 shot groups at a hundred yard workups with about any powder I try, like 4831, H4350, Imr 4064, RE16.
Wow that is an awesome 25 shot .7 moa group, like you said, I think I will try to keep them under 3300fps and find my best load, and then try again at 200 and 300, that's all my range goes to.
I wonder if you started to have bad groups with these at 500 yds. not only because of the wind and being a fairly light bullet, but the bc is not awesome on these, and also, I have read that a FB bullet tends to loose accuracy after 3 or 4 hundred yards compared to a boat tail bullet?
I didn't have a chrono that day, I was only going by my initial load findings, but I can't imagine much difference in the fps, because the load was exactly the same and the temperatures were probably within 10 deg., and I also try to keep consistent with my aiming etc, and also never leave the round to cook in the chamber much more than 30 to 45 seconds.
Thanks again

 

[Unclenick]

The main reason for accuracy loss with increasing range is wind blowing lower BC bullets around more. Rotation does slow with flight time, but it slows much more gradually than forward velocity because the cause of the slowing is just air friction due to the surface speed of rotation, which, in your case, is just 336 ft/s at the muzzle. As a result of the more gradual rotational speed loss, the bullet stability factor normally increases with range, which increases the yaw of repose, which is what causes spindrift to hook a bit harder at long range.

I expect the most likely cause of your problem was something making the gun shoot a little faster that day. If the ammo had been loaded for a while, the start pressure could have been increased by cold bonding between the bullet and case, for example, but other causes are possible. At your velocity and twist rate, you might have those bullets on the ragged edge of core stripping. That is where the jacket is spun up so rapidly, the bullet core slips inside it, so the main bullet mass never gets up to spin rate. This leaves you with an unstable bullet whose mass symmetry is poor, so it hooks off and into the dust somewhere on the way to the target. But if the bullet core is maintaining its bond to the jacket, then you may simply be on the edge of disintegration at your test velocity and then be randomly surpassing it with some bullets because of whatever was causing a little bit of extra velocity that day.

A less likely cause would be that as the yaw of repose increases with range, a dynamic instability becomes apparent in the bullet. But this usually happens in the transonic range, and not at the 2500-ish fps your bullet would have at 400 yards.

The way to test for the latter situation is to set up a 400-yard target and see if more bullets disappear on their way to it. If not, the first cause is more likely.

 

[lugerstew]

Thanks Unclenick, I should try the 400 yard suggestion, just have to get to that place someday, because my shooting range maxes out at 300.
I definitely need to do a bit more testing with that load, I had them loaded for less than a week, so I don't think it was the cold bonding, and thanks for that new to me term,,"Yaw of Repose" , I had to look that up, because I had never heard of that before.
Have a great day, always helpful to hear your advice.

 

[lugerstew]

Just talked to a Berger Tech., He said my rpms were around 308000 and they like to stay under 300,000 rpms, so I was on the hairy edge with that.
Also, he asked how many rounds did I use before at 100 and 200 yards and I said 3 at 100 and 4 or 5 at 200.
Because I shot more rounds of the same fast load, he suspects because the barrel got hotter, it worked the jacket more and caused the failures probably on my last 3 rounds of each 7 shot group, the barrel would be hotter.
To kind of test this, he said shoot at 100 yards with around 10 rounds of the same load at 10 different bullseyes, at my normal pace, similar conditions, and see if I have a problem of hotter barrel and fast rpms acting on the jacket and making some rounds not hit the target.

 

[Unclenick]

Interesting idea. Might go to weakening the bond between the jacket and core.

Another thought is, if the barrel has a tendency to build copper fouling quickly, that could raise pressure and velocity and spin rate along with it, too.

 

[lugerstew]

Yes Unclenick, the Tech did mention a dirty barrel, how many rounds have been through it and maybe a cause of higher pressures, I did have about 50 rounds through the barrel before this, but I cleaned it today, going to load the exact rounds and try his 100 yard, 10 round, 10 bullsyeye theory.

 

[Unclenick]

Let's see what that does. It would be interesting if some simple copper build-up were responsible.

 

[Bart B.]

Formula to calculate bullet spin rate in rpm.....

(Velocity in fps X 720) / (twist in inches)

Many times, I've shot 24 rounds in 50 seconds with 30-06 or 7.62 NATO barrels and never had bullets come apart. Neither did several dozen others doing the same thing at the same time.

 

[lugerstew]

I agree Bart B., and I bet many other people have as well, and gee, what about full auto barrels that people claim they had seen them get red hot?
I guess I'm just trying it because the Berger Tech. told me, that those are varmint bullets and have a thin jacket, which could be a reason for what happened.

 

[lugerstew]

Well I did the 10 rounds at 10 bullseyes at 100 yard test today. I'm not sure I learned anything positive from it, because, it all went perfect and no bullets disappeared.
Either the bullets disintegrate at farther than 100 yards, or maybe it is the dirty copper fouled barrel that the Berger Tech. asked about. It was clean this time and had about 70 rounds through it when the bullets disappeared.
It was roughly the same temperature outside, and I had the chrono on, they averaged about 3500fps.
The only things I can think of doing is shoot them at 400 yards like Unclenick suggested, or just stay away from them and test similar slower loads that I have worked up, and see what they do at 300 yards.
Thanks for any suggestions