RPM Threshold

[Nueces]

Yup, that's it, or, the same thing; 720 (which is your 60 times 12) times velocity divided by twist.

Mark

[KYCaster]

Velocity in ft/sec X 60 = velocity in ft/min X 12 = velocity in in/min, divided by twist = RPM.

Jerry

[andrew375]

Andrew375

That is a nice group, especially for a belly shoot. The velocity is chronographed? As you say in that thread that one group does not accuracy make so does it do that well consistantly? You obviously have a nice load there at any rate.

Larry Gibson

All my loads are chronographed. I don't make any claims as to consistency as there are too many variables muddying the water introduced by the shooter. The rifle may be consistent but I'm not! I am getting better though.

[joeb33050]

Again, if the RPM threshold is based on what folks are shooting, then I think we're making a mistake.
The Production class revolves around a Savage .308 with a 10" twist and a bullet going 1800-2100 fps. This isn't because folks choose the gun or the twist or the cartridge, it's because that's what's out there and available.
If we had our choice, the Savage might be in 30BR or 7.62 X?? or 30/30-smaller cartridge case; and the twist might be 12 or bigger #.
One 2007 competitor used a 18" twist barrel in 30BR at 2750 fps with a 108 grain monotype bullet.
The CBA experimenters/rifle builders seem to be going toward the 30BR Score jacketed direction without-maybe-thinking this through.
The SS breech seaters are shooting almost as well at much lower velocities with heavier bullets. Why aren't the experimenters going toward long heavy bullets in gain twist barrels at 1400 fps?
joe b.

[felix]

Reason: because the longer the boolit is, the more distortion it receives in a gain twist barrel. If the barrel is going to be made with cut grooves, then a very, very small amount of gain twist might help the cutter mechanism do a more consistent cut job. ... felix

[JudgeBAC]

This may be off topic and, if so, I apologize. Is the following observation a factor of velocity, RPM, a combination of the two, or some other factor? My model 29-10 6.5" bbl with a 255 gr. Keith bullet over 9 gr. of Unique places these 1" above the sights at 25 yards and to a similar place at 50 yards. A 277 gr. MM bullet over 9 gr of Unique places the bullets 3.5 to 4" LEFT of the sights at the same distances. What gives?

[felix]

Position of impact at 25 yards with a revolter has more to do with how the gun recoils more than anything. The left-right business at that distance might have something to do with how the boolit style makes it through the forcing cone based upon mechanical conditions. I have found that the Unique speed of powder needs to be very close to being right on. If you vary the powder by half grain on either side of a proverbial sweet spot, and the accuracy goes out by a visual amount, then pick another powder. So, try to adjust the loads between each boolit. This is more of a question for the 44man and others who play hard ball with pistols. ... felix

[JudgeBAC]

Felix, thanks for your thoughts. I moved this query to the wheelgun forum.

[Larry Gibson]

All my loads are chronographed. I don't make any claims as to consistency as there are too many variables muddying the water introduced by the shooter. The rifle may be consistent but I'm not! I am getting better though.

Understand, I've shot quite a few "belly" matches myself.....some days are diamonds, some are stones!

Larry Gibson

[joeb33050]

Reason: because the longer the boolit is, the more distortion it receives in a gain twist barrel. If the barrel is going to be made with cut grooves, then a very, very small amount of gain twist might help the cutter mechanism do a more consistent cut job. ... felix

I don't think so. The bullet is going to turn and get engraved no matter what.
Smith gain twist barrels are used by the SS guys with good results.
I think the CBA fixed experimenters are like all people, doing what's new and popular without stopping to think about it deeply.
Gain twist has worked for 150 years, although a good fixed twist barrel works fine too.
joe b.

[felix]

There is a difference in knowing so and thinking so, Joe. However, on your behalf, there is safety in numbers. ... felix

[Larry Gibson]

Woke up this morning and since the wife was sleeping in I turned on the Mag 20, put some 60/40 WW/Lino alloy in it, degreased two Lyman 2 cavity moulds (311291 and 311466), put a pot of coffee on, turned on some smooth jazz on the radio and commenced to cast 16 lbs worth of bullets.

My plan is to test identical loads for for three .308 rifles; a M788 with 10" twist, a M70 target with 12" twist and the M98 Palma with the 14" twist. The goal is to observe accuracy improvement or deterioration as velocity is increased from around 1700 fps up through 2500+ fps with each rifle. The reason for the test is to observe what the average guy may get accuracy wise with the .308 based on different twists. I chose the 311291 because it is readily available and a proven design. The 311466 was chosen because it is Lymans' fastest .308 Winchester bullet (2920 fps with 748) in their new Cast bullet Manual.

Bullet selection will be by visual inspection for defects with only the bullets with complete fillout and no wrinkles or voids being used. I do not intend to weight the bullets or sort them by cavity. Right now the 311291s are .309 - .310 so it looks like a .310 sizer will be used. The 311466 drops at .315!!!! Not sure whether I'll test them at .314 or size them to .311 (after lubing) for the test. Gas checks will be Hornady's and they'll be seated using the GC seater on the 450 then the 311291s will be run through a Lee .310 sizer then lubed in a Lyman .310 H die. The 311466's, after seating the GCs on the 450, will be run through a Lee .314 then lubed in a Lyman .314 H die in the 450. It may be shot as is or then run through a .311 Lee sizer.

Cases will be once fired (through the M70) LC 92. Necks will be inside reamed in a Lee Target Loader for unifromity and concentricity. Primers will be WLR and Remington 9 1/2s. Lube will be Javelina as I've not had any lube failure with it up through 2600+ fps.

Powders will be; 4895 with a Dacron filler used until loading density is 80%+, RL19 and H4831SC.

Testing will be done at Tacoma Rifle and revolver Club which has very sturdy cement benches. All loads will be over the Oehler 35P chronograph. Targets will be at 100 yards. The end result will be NOT to compare the accuracy of the rifles against each other (I already know which rifle will win that one). The test will be to observe accuracy of each rifle as it increases or decreases through increasing powder charges and how accuracy responds as velocity/RPM is increased. Testing identical loads in the 3 rifles with 3 different twists should then give us some indication on the effects of RPM.

Any other suggests? Remember this test is to replicate those cast bullet loads that the average cast bullet shooter would produce. As the average cast bullet shooter doesn't get too anal in reloading technique/detail neither shall I.

Larry Gibson

[felix]

Good show, Larry! Should be a fun experiment. No hurry. ... felix

[sundog]

Hooah, Larry. Over a chrono, but what are you gonna do? Groups for each load for each rifle?

My take is that your test will actually produce enough data to tell something. I'll reserve my SWAG until after your data is compiled (although I have my suspicions). Looks like you'll be getting some quality trigger time!

[Blammer]

got any 311284 boolits? Them would be nice to test too.... lol as if you don't have enough covered.

[Pat I.]

I think the CBA fixed experimenters are like all people, doing what's new and popular without stopping to think about it deeply.
Gain twist has worked for 150 years, although a good fixed twist barrel works fine too.
joe b.

Of course this is right. I spent 250 bucks on a 17 twist Shilen barrel, 150 on a reamer, 125 for a LBT mould plus the RCBS 22 cal mould blocks I used to make another bullet, brass, dies, powder, time putting it all together, and a bunch of other things and did it all on a whim with no thought involved. Since there was only my 17 twist and the other fellas 18 twist, which you failed to mention he used to win the unrestrcted class with, 30 caliber barrels on the line I don't think either of us chose it because it's new or popular. His is a 30 BR mine is a 30x47, lots of those being shot isn't there.

[waksupi]

Harry Pope, and his contemporaries, never did really settle on wheter gain twist was better or not. I'm with them.

[ssn vet]

this is really interesting......especially for newbs like me....

reminds me of the velocity limits published by the plated bullet mfg.s

Berrys normally states a limit of 1,200 fps on their pistol bullets and on the 150 gr .308 FN I load for my Marlin 336, they say 1,700 fps (numbers pulled from memory, it's been a while).

The best explanation I've heard for this is because the copper plating starts to "spin off".....but I wonder

These plated bullets have a pretty soft swaged cores, I wonder if that comes into play....

sorry for the thread hijak, I find the topic very intereting

[felix]

No hijack... Yes, a heavy (and not too tightly woven) core will want to push off a jacket. Any hole found will let loose a bomb, re 220 swift, 22-250 types using the thinner jacket bullets. Maybe a depleted uranium core will take 2 times that rotational velocity before even thinking about getting nervous. ... felix

[joeb33050]

Of course this is right. I spent 250 bucks on a 17 twist Shilen barrel, 150 on a reamer, 125 for a LBT mould plus the RCBS 22 cal mould blocks I used to make another bullet, brass, dies, powder, time putting it all together, and a bunch of other things and did it all on a whim with no thought involved. Since there was only my 17 twist and the other fellas 18 twist, which you failed to mention he used to win the unrestrcted class with, 30 caliber barrels on the line I don't think either of us chose it because it's new or popular. His is a 30 BR mine is a 30x47, lots of those being shot isn't there.

Pat;
You seem defensive. There's no need. Maybe it's a coincidence, but the trend to slow twist short bullet rifles in CBA seems-seems-to follow that trend in jacketed score shooting.
Comparison of results in CBA NM shooting has shown the PB breech seaters doing virtually as well as anyone else, when there are enough of them attending.
Perhaps the slow twist short bullet fast plan is the route to ultimate accuracy with CBs, but I wonder what could be done with long bullet appropriate twist slow bullets with ?gain? twist barrels.
You've got the "where we are now", then two diametrically opposed alternatives that present themselves. Movement seems to be toward one of those alternatives, with no movement toward the other.
Maybe, as I said, maybe, this is happening without the experimenters thinking this through all the way.
And maybe not.
Regardless of that, my impression is that the slow twist fast short bullet guys do not go to the slow twist because they percieve an RPM threshold ahead.
joe b.