Spin rate test

[Fuj]

I had'nt planned on doing this but I was doing a powder work up
between H-4350 and Varget with on one my 7mm wild cats. I have
a 9 twist barrel that is getting pulled but decided to leave it on to
get more fire forming done, and using up old stocks of accumulated
outliers of bullets and primers. I have a fist full of 130gr SMK's, both
blems and pulls I'm using.......To start, I was able to run up to 3191 fps
with the H-4350 with a little more room to push it over 3200 fps.......
With Varget, I'm currently at 2989 fps and no pressure. Next nice day
I'll push the Varget further with this 130 SMK.......Doing a calculation,
It looks like I'm at 256000 RPM and no blow ups. This will be as far as
it gets with H-4350, but may go beyond with Varget if pressure lets
me.......Now, years back, I had worked on hunting loads using Hornady
and Speers, and could not even come close to this spin rate without
blowing bullets up with the wrong twist rate......I have this 12 twist Hart
I plan on chambering up for my wild cat project and still be able to run
the 150's. For what it's worth, JBM is giving me a stability factor of 2.545


So, just for "arguments sake"......How much spin rate has anyone pushed
bullets before rapid disassembly ?? I have shredded in mid flight, both
Hornady and Speer hunting bullets with claimed heavier jackets with way
less spin........

 

[dusterdave173]

This question and hearing others experiences attracts me too--My first and only blow ups were with Hornady 22cal SP SX They are advertised as thin jacket blow up varmint bullets--at 3200 in a 9 twist factory Savage barrel they turned to grey smoke BUT...I have a Hawk Hill 7 twist that will shoot 40g Vmax at 3600 that has to be near 360K or so and they shoot nice tight groups zero issues--I think that is way beyond what folks say will live--That barrel is smooth as glass inside and the Savage looked like a hollow log inside--does that matter? I don't know anything about all this except what I have observed
My 7 twist barrels in 223 the Hawk Hill and a Wonderful Urban 223 ISSF barrel have taught me a lesson--they both shoot 40g through 85.5 Bergers like a Champ! Yes I know the lighter stuff is way over spun according to bullet makers advice and conventional wisdom etc but that does not keep them from doing it
Jacket thickness and or barrel surface- has to conspire with FPS to make the blow ups in my VERY limited experience maybe when I learn better I will not shoot them spinning so hard but so far seems fine to me Those barrels have also shown me that a mile long throat for heavy bullets will still shoot shorter lighter bullets jumped a Long way just fine.
I am a rookie --I don't know 1/10th what folks on here do--Just telling what I have seen in my rifles

 

[Rtheurer]

something to ponder, the amount of time the bullet spends in the barrel, for example a 30" barrel vs 24" barrel and the amount of friction creating that heat on the bullet jacket that transfers to the lead core as it turns to a less than solid state that cause's the bullet to come apart.
I hear folks blowing up 88gr ELD 22 cal Hornady bullets. Yet I am pushing them at 3000fps and have not had one let loose yet out of 24" barrel. While the next guy ( Ned) pushing them out of a 30" barrel had many let loose at lower velocity dispite both being 1:7 twist.
RPM I don't believe is the whole story or even a quarter of it maybe.
In the Prone Sport Eric Stecker at Berger Bullets was the first to my knowledge to try and figure out what actually was creating the blow ups. This was back in the late 90's early 2000's if I remember correctly????

 

[Zero333]

Jacket thickness and barrel temp matters quite a bit.

 

[FeMan]

Jacket thickness and barrel temp matters quite a bit.

Good point. Condition of the bore and bullet OD/rpm matter as well.

I have had blow ups as low as 240,000 rpms with 7mm 180 eldms in a hot, tired bore.

 

[dellet]

Tough question, complex amnwers.

My experience is that it’s mostly twist rate. When you start crunching the numbers, the spin you gain by changing twist, can almost never be duplicated by velocity. The big exception to that is subsonic shooting in the same barrel where, your velocity spread can be a couple thousand fps. Working the 300 Blackout it became somewhat of a challenge to blow bullets up giving feed back to manufacturers.

Then bullet design comes into play, jacket thickness, tip design, how it was designed to expand. The easier or more violently it expands, the more prone to failure.

Powder choice, heat generated, rate of acceleration comes into play.

Rate of fire, how quickly the barrel heats up and retains that heat.

Quality of the barrel, rough spots or a poorly drilled gas port, can compromise the jacket.

Starting with the obvious, heat and over spin. Using the 110 Varmageddon bullets in a 8” 1/5 barrel. 2350 fps= 340,000 rpm. Thousands of shots at the rate of 1 per minute, multiple double taps.
Around 40 rounds into a carbine class walk through, maybe 4-5 minutes, and bullets were coming apart within 5 yards.

Expanding solids that are pre-stressed by skiving. Designed for subsonic they can come apart below 150,000 rpm. Most are designed for a specific rpm and velocity, so directly asking the manufacturers what that window is, is vital. A 300 Blackout bullet in a 308 may not be as big of problem as a 308 bullet designed for 1/10 in a Blackout sporting a 1/5.
Spin them too fast and they open up in flight.

Brass turned bullet with aluminum tip, skived for expansion and rated to spin over 300,000 rpm, 3,000 fps in 1/7. Coming apart in a 1/9 at 2600 fps. Never could prove exact cause of failure, but rapid acceleration was best guess. Changing to a slower powder, lower start pressure and longer time to peak pressure allowed 3000 fps in the same barrel.
Same bullet in a 1/6 held together at 2750 fps, almost 100,000 rpm faster.
Too Rapid acceleration likely caused the relatively heavy tip to set back into the bullet initiating expansion in the barrel. Bullets came apart before crossing the chronograph.

Barrel imperfection can score the jacket. A good example is a gas port drilled half in the groove, half in the land.
Poor transitions in the chamber leaving a copper ring behind where you would expect a carbon ring is another example.

Over crimping damaging the jacket.

Any combinations of the above.

Yes, I’ve blown up a few bullets and sent them back to manufacturers.

 

[Rtheurer]

All great answers to why just Spin rate it self is not the complete answer at all.

 

[CNC-Dude]

Ned told me also, especially in the .223 category that the depth the rifling engraves the jacket is critical as well. Many times shooters blame the bullet manufacturers for blow ups without realizing the unseen dynamics that are also contributing to the issues....

 

[Ranger188]

The ONLY time I've had bullet blow-ups was with a 22 Creedmoor.
I do clean my barrel and didn't run long strings of fire.
I was pushing them to 320,000+ Mostly 75g - 90g.
Started at about the 300 round count and just got worse from then on.
Couldn't go more than 5 rounds without one going poof.
I even backed it down a bit, tried different companies bullets.
Never tried a 22 creed again.
Haven't had a bullet go since then.

 

[Zero333]

80 gr Berger Varming in a 8" twist 243win @ 3,220 fps work great in the sub freezing temps. Once it gets warmer outside and the barrel really heats up, 25 % of them blow up just passed 100 meters.

Same 243win 8" twist... with 58 gr V-max @ 3,935 fps... are more accurate in the cold winter than in the summer, but don't have any blow up's.

So this is why I said jacket thickness and barrel heat matters just as much if not more than RPM.

The 80 gr Berger Varmint are explosive on varmints while the 58 gr V-max are not very explosive and sometimes just go in the varmint front and exit out the top or bottom. Sure it kills the varmint, but not very dramatically.