A different twist on RPM Factor. Australian overun Roo bullets.

[madsenshooter]

Not a shot at your pet theory Larry, just a cute title. Doesn't even have anything to do with cast boolits. I got a good buy on some Australian contract overrun Nosler 55gr softpoint 6mm bullets some time ago. I had some loads worked up using the Swiss 4198, SPP 210, that worked fairly well, but it wouldn't cycle the action until the load was hot enough that I was getting ejector marks. I decided that wasn't the optimal powder for the bullet and loaded a bunch with 2015. In between, I'd put some risers on the scope mounting system of my Obermeyer barreled AR 15. I went out today to get it sighted in. Got it bore sighted and let fly. First shot hit the target, but was way off. I cranked and cranked on the adjustments, couldn't even hit my backboard. Bore sighted again. Then I put a small piece of wood on the very large backstop. I wasn't even hitting the backstop! What? I know they're coming out the end of the barrel cause it hasn't blown up yet. Finally I shot a round with both eyes open. About 30yds in front of the muzzle there was a grey poof of smoke. I know this is nothing new to many of you high velocity varminters. But it's the first time I ever experienced it. I guess they weren't designed for the high RPM of a 1/8 twist. I can only estimate the velocity at around 3000fps, based on the previous load of 4198 being around 2900fps IIRC. I think I loaded a couple hundred of them. I have some really good up close self defense rounds! Splat! I boresighted again and had another load with 87gr V-max bullets. They impacted close to the boresight and I got a good zero for them. Once I understood what was happening with the 55gr Noslers, I decided to shoot a few and watch. Sometimes the poof wasn't so big and the bullets hit about 6ft below my 87gr zero, but if any of them made it to the back stop, it was on the first hop. IIRC they were going around 2900fps with the 4198. I think I may have gotten over 3000fps with the 2015. It sure had me scratching my head for awhile! God I hate to pull all those down! Wonder if any of our down under friends have ran into problems with these bullets? Since most would be shooting a larger cartridge than the 6x45, I'll bet they have.

[DIRT Farmer]

The only time I have seen it was trying to shoot a varmit with a 22-250 in a rain storm. not a single one made it to the varmit
IIRC, they were 52 grn Berger HPs

[Larry Gibson]

grasshopper (madsenshooter)......

learn to walk the rice paper leaving no foot prints and you can shoot those bullets as fast as your thinking will allow..........ignore the laws of physics and ballistics......let mind go and the force (or is it technique?..it has all been posted, you must find it.......) will be with you.......or so we're told[smilie=s:

Yup, RPM it was and too much of it.....way beyond the RPM threshold for those bullets, eh?

:drinks:

Larry Gibson

[6.5 mike]

Only one I've seen that happen with was a 220 swift :popcorn:.

[madsenshooter]

One would think they take more RPM than what 3000fps would give them. All in the twist I guess. Yea, RPM threshold, wrong word in my title. These will be loads for when you can see, not the whites of their eyes, but the color of their iris! CQC loads!

[Calamity Jake]

I have a Rem. 700 in 243 that copper fouls to beat the band, a 1000 stroke lap with JB helped a little. I guess I need to fire lap it!!!
With Speer TNT's after about 40 rounds without cleaning I start getting blowups, had a buddy spotting at a Pdog shoot years ago discover it, I kept wandering why I was missing and didn't see any dirt fly.

[PAT303]

Those bullets were made soft to suit pro roo shooters down here as roo's are quite light framed and a soft explosive bullet was the type the shooters wanted. Pat

[Larry Gibson]

RPM at 3000 fps;

Twist;
7" = 308,570
8" = 270,000
9" = 240,000
12" = 180,000
14" = 154,285

Thus we see a logical progression toward much higher RPM as the twist rate increases. The 7" twist gives twice the RPM.....lots of difference in centrifugal force on minor defects between 155,000 RPM and 300,900 RPM. At the 270,000 RPM with your 8" twist it's not any wonder the thin skinned bullets came apart!

Larry Gibson

[madsenshooter]

Thanks Pat, I was looking for some input from you fellows. You too Larry, I was thinking about the RPM they'd get in different twists, at different velocities, with more powerful cartridges, didn't want to fish out my calculator. I believe the majority of factory barrels in 6mm/243 nowadays are 1/9.25 or slower. Yep, CQC rounds for "at pistol" range. I started to file one of the 55gr bullets in half, the core is quite, scratch it with a fingernail, soft. Poof! Guess I'll pull them down and take out a grain of powder. Didn't have so many after all, around 70 to pull down.

[BD]

Madsenshooter, Your story made me a little better. I went through this some time ago trying to develop a "Varmint" load for my Bushy DCM. I started with 40 grainers, and when I couldn't hit the backstop I set up at 25 yards and got a group, and tried again. I was bewildered, but kept at it until on my third trip to the gravel pit there was another shooter there who clued me in. I didn't have any luck with that 1/8 twist barrel until I started into the 55 grainers. I saw this again a few years later in a friends .22-06 which had a 7 twist. He was dusting the 55 grainers. We both have since gone the other way. I'm using the 77's and he's shooting the 90 grainers. Over time I've come to realize that the ability to buck the wind a little better out weighs the higher velocity every time.
BD

[plainsman456]

I have a 22-250 that does the same thing with 55 grain bullets.
It tears up the "cheap"varmint bullets but truly loves those 60-90 grain bullets.

[madsenshooter]

I found my calculator. One of the best things I learned in college was how to use a graphing calculator. As Larry stated above, my 1/8 twist barrel would have been spinning that bullet 270,000rpm at 3000 fps. I was wondering how fast the bullet would have to go in a rifle with a slower twist before it got to the failure point. In a 1/9.25 twist, velocity would have to be 3469fps before rpm got to 270,000. The 6mm Remington can get above that velocity with a 55gr bullet, as can the 243 Win. So the roo shooters must be keeping their velocity down. If, on the other hand their rifles had a 1/10 twist, they could get up to 3750fps before reaching the failure point. Both cartridges can get there. 1/12, 4500fps, beyond the capabilities of both cartridges. If I could get an interface for this calculator to plug into my head, maybe I could gain some math abilities I always lacked!

[missionary5155]

Good morning
Now I woud be inclined to take this "Puff" another step. I would fire one of those "J" thangs at a low velocity & capture it in a very soft material. Next carefully slice it open to see how deeply your rifling is cutting into the jacket. Maybe your particular barrel is cutting so deep as to make the jacket unable to withstand the RPM´s...
Just another of those variables we face.
Mike in Peru

[Bob S]

"Spontaneous bullet disintegration" was not uncommon when the Sierra 240 grain .30 cal Match King hit the market, and 1:8 or 1:8.5" twist barrels in 300 Magnum were made to accomodate them. I can recall seeing this a few times on Range 4 at Quantico. It was deja vu all over again with .223 90 grain bullets with 1:6.5 twist barrels a few years later.

Resp'y,
Bob S.

[madsenshooter]

The jackets aren't all that thin, I discovered by filing one down. And, the rifling is 5R, only .002" tall, which shouldn't cut the jackets. They look odd where they transition from the full diameter to the ogive, like the material is stressed. I can see it with my eye, but it wouldn't show in a pic for me. I think it's just the fact that the core is dead soft lead. Heck, I've gotten 220,500rpm out of my babbitt bullets without blow ups.

[Larry Gibson]

Hornady rates their SXs at 3500+ fps before coming apart in 14" twist barrels. I've shot thousands of 55 SXs at 3200 fps out of 12" twists with excellent accuracy. They will come apart in a 9" twist at 3200 fps as will Sierra 50 Blitz bullets. Not always but enough. The Speer 52 gr HP won't take a 7 - 9" twist at 3200 fps either.

Larry Gibson

[madsenshooter]

Well, now I know not to buy the varmint grenades. I'd thought I'd prefer heavier bullets anyway, in case deer became a menu item, and I have lots of 85-100gr bullets. The Russian 4895 makes a good powder for them in the 6x45. The roo bullets were just too good a deal to pass up at the time.

[Wolfer]

My old 9" savage 223 would shoot Sierra blitz 50 gr to about 2800/2900 before smoking anything over that and probably half wouldn't make it to the target. The 14" twist I have on it now does just fine. All the proof I needed for Larry's unmentionable theory!

[brotherdarrell]

I had a savage m12 .223 with a 9" twist and 26" barrel. I was sending 40 gr. sierra hp's over the chrono above 3900 fps. and 36 gr. varmint grenades on both sides of 4,000 fps. If I pushed 55 gr horn. sx much over 3400 fps they would disappear in mid-flight. For obvious reasons I won't go into much detail about the first two loads, but for whatever reason 'that' rifle with 'that' powder and 'that' primer combination never gave any high pressure signs. I stopped when I could'nt put any more powder in the case and still seat a bullet. Go figure?

brotherdarrell

[Rusty W]

I purchased some of those same 6mm/.243 Roo bullets, Shots I believe they're called. I shot a few from a Savage .243 in the 3500fps w/a 1-9.25tw barrel. Results were the same. I started at 100yds & couldn't understand what the problem was. Finally moved to 25yds just so I could hit the target. Wont work for what I have. Now I have 5-600 bullets just sitting in the corner waiting on a new 6mm gun to shoot them. Just can't decide what gun yet.
http://i269.photobucket.com/albums/jj51/RWoolever/02-19-11_1518.jpg
http://i269.photobucket.com/albums/jj51/RWoolever/02-19-11_1514.jpg
http://i269.photobucket.com/albums/jj51/RWoolever/02-19-11_1517.jpg

[madsenshooter]

Good to know I'm not the only one. Just slow em down a bit Rusty W. and use em up, should be ok down at 3300. Probably pretty devastating to anything you hit. I also had some 75gr Hornady HP varmint bullets that we leaving oblong holes at 200yds, but weren't coming apart, just tipping a bit.

[noonanda]

new reply to a old post, I am actually having the same problem. I am/was pushing them at 3840 FPS out of my .243 Win, some rounds would hit the target fine, others would impact like 4 feet low and left. I was stumped and thought maybe the B.C. was just so short that it wasnt stabilizing. Glad I wasnt the only one. but my twist rate on the barrel is the normal 1/10 or 1/11 (cant remember exactly right now)

[Eutectic]

RPM at 3000 fps;

Twist;
7" = 308,570
8" = 270,000
9" = 240,000
12" = 180,000
14" = 154,285

Thus we see a logical progression toward much higher RPM as the twist rate increases. The 7" twist gives twice the RPM.....lots of difference in centrifugal force on minor defects between 155,000 RPM and 300,900 RPM. At the 270,000 RPM with your 8" twist it's not any wonder the thin skinned bullets came apart!

Larry Gibson

The infamous .220 Swift factory load at 4140fps with a 48gr bullet..... To illustrate RPM vs. twist a little better this Swift load only turns up 212914 RPM with Winchester's standard twist of 1 turn in 14".

Eutectic

[Blammer]

I suspect that is why they are blems.

[Larry Gibson]

Some might even think they were overstabilized to the point of disintegration............:veryconfu

Larry Gibson

[leadman]

I have some of those also. been shooting them out of my Encore 15" barrel in 6PPC at over 3,300 fps with no problems. Barrel is a 1 in 10 twist. They do shoot more accurately at about 3,100 fps.
I'll have to try some in my 16.25" 243 Win barrel and see what happens.

I shot some factory 5.56 out of my 223 Contender that were Federal frangables. They did not make it to the 100 yard target.

[noonanda]

Im gonna load some up using the min load for Varget, also might work up some "plinking" loads using trailboss, thankfully I only bought a box of 500