RPM for projectile disintegration ?

[Josh Cox]

Interested to know what RPM 308 projectiles blow apart ?

[KHGS]

I have never seen a 30 cal bullet come apart (that doesn't mean that they don't). I have only seen it in fast twist barrels 6.5 cal and below. it depends on twist rate, usually 8" twist and faster. Barrel condition and jacket thickness are factors.
Keith H.

[tachyon]

The ballpark figure for copper jacket projectiles is 250,000 RPM but different manufacturers use different alloys and different jacket thicknesses so there is no single answer.

RPM = MV * 720 / twist

From that can calculate the twist required to get to 250,000 RPM for various Muzzle Velocities

2400 6.9
2500 7.2
2600 7.5
2700 7.8
2800 8.1
2900 8.4
3000 8.6
3100 8.9

As Keith says jacket failure is rare for 30 cal because we don't get to the necessary RPM at the velocities we typically see with the twist rates we typically use.

I have seen (heard) it with 7mm F-open projectiles being pushed hard in fast twist barrels. As you may note 2800fps in an 8 twist barrel is just over 250,000 RPM.

Given bullets were designed by engineers it should come as little surprise that the jackets (which reduce the sectional density compared to lead) have been made just thick enough to tolerate the stresses they are exposed to with a modest safety margin.

[ErrolW]

Nosler 175 rdf projies explode going down range.

[macguru]

The 2 instances of projectiles blowing up that I had were both with nosler projectiles, and both with savage factory barrels. That combination and no other. The first was a 7 twist 223 savage and 80gr nosler bullets. The second was an 8 twist 6.5mm 260 remington savage barrel and i think 140 gr nosler custom competition projectiles. The noslers had thin jackets, and the savage barrels have terrible scoring across the lands that must disrupt the bullet jacket and set it off. You could see them blow up half way to the target !! Interestingly, the 223 barrel was still deadly accurate despite this scoring across the lands. (with sierra projectiles that did not blow up)

[PeteFox]

I have an 8twist, 4 groove 6.5mm Bartlein barrel. with a 260 Rem driving 147gn ELDM Hornadys @ 2800 fps. It will occasionally blow these bullets up.

My mate with a 6.5 PRC (over 3000fps) using the same 147's does not blow up bullets but it is an 8 twist 5R Bartelin barrel. In his previous 4 groove 8 twist barrel it was a very regular occurrence.
Something about the engraving of the bullet?

I have since changed to Lapua 139's @ 2900 fps with no blow ups.
Pete

[Weairy]

I've never heard of that, that's full on.

[Malcolm Hill]

Had a friend that played around with a 22.250 1 in 8 barrel a few years back and it was a demon for blowing up 80gn Amax's at around 3400 fps. About half the projectiles disappeared in a haze around fifty metres out from the muzzle. Sierra's fared much better with only a few failures by comparison. Sectioning the projectiles showed the Amax to have a much thinner jacket causing the problem. I recall working out the spin rate to be well over 300,000 at the time. That barrel was rechambered back to 223 and has had no further failures with any projectile. Regards Malcolm.

[Barossa_222]

I have seen 2 shooters blow up 7mm pills with regularity. One was a saum the other a custom 7mm built on a 375 Ruger case. Fast twist coupled with speed, heat and fouling. They weren't Noslers from memory. Bergers and Sierras if I remember rightly.

[RDavies]

I have an 8twist, 4 groove 6.5mm Bartlein barrel. with a 260 Rem driving 147gn ELDM Hornadys @ 2800 fps. It will occasionally blow these bullets up.

My mate with a 6.5 PRC (over 3000fps) using the same 147's does not blow up bullets but it is an 8 twist 5R Bartelin barrel. In his previous 4 groove 8 twist barrel it was a very regular occurrence.
Something about the engraving of the bullet?

I have since changed to Lapua 139's @ 2900 fps with no blow ups.
Pete

Ive done a bit of reading up on fast twists in 223s with the 90gn pills and it also seems that the 5R Barteins kept the bullets together but the regular type rifling more often often caused the bullets to come apart.

[RDavies]

I have seen 2 shooters blow up 7mm pills with regularity. One was a saum the other a custom 7mm built on a 375 Ruger case. Fast twist coupled with speed, heat and fouling. They weren't Noslers from memory. Bergers and Sierras if I remember rightly.

I have also seen 7mms blow up Bergers at 2950fps in 8" and 3080 fps in 9" but this was shooting pretty quickly, so likely barrel and bullet core temps getting high, coupled with fouling. I havent yet seen it with Sierras so not sure if they have tougher jackets?

[Longranger]

A 1 in 10" twist in a 30-378wby tests the integrity of 155gn bullets at 3650fps...

[Chopper]

just for fun
i have a 22BR with a 6 groove 8 twist tapping 3950 all good , but have gone to 4050 and split necks
believe or not, 40g Nosler BT And Very Very accurate ,Chop

[Josh Cox]

Thanks Gents,

I am playing around with a 300WSM (F Open).

I've shot Berger 155.5 at 3050 from an eight twist in FTR / TR, with great results (274K RPM).

I have two barrels for the 300WSM, one on ten and the other in eight twist.

It will be interesting to see what can be done with the B155.5 at >3600 (as Longranger suggested) in the ten twist (around 260K RPM). I am quite confident the eight twist would likely be too fast (like Marty Lobert).

[pjifl]

Many older barrels were deliberately made slightly undersize. Newer ones of good heritage these days are spot on.

An overly tight barrel results in more bullet ruptures.