throat angle

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pjifl
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Re: throat angle

#16 Postby pjifl » Sat Jan 12, 2019 10:19 pm

OK. I will come clean !

Originally I mainly shot a 6.5 Rogue. Usually with Lapua 139s but some SMKs. Always a Tangent Ogive.
I dabbled with different jumps and favored about 20 to 30 thou but found it was very tolerant and this was the time I accidently applied 160 thou which also gave excellent results.

Then went to a 284 using 180 grain VLDs. Again dabbled with jump and settled on 20 thou. Up to then I had never used a VLD. I did try other different jumps but came back to the 20 thou. Never did like a jam for a lot of reasons.

Eventually built up two 7 SAUMs as described previously. Both Bartleins. It was natural for me to try my usual 20 thou jump and never saw the need to change. I honestly found the 7 SAUM a very easy cartridge to get running well. Some of this was because I benefited from others' experience with loading but ended up doing something different from them with VLD jump which worked for me very well and even better than some other shooters were getting. And eventually I shot Hybrids with the same loading just as well.

I can appreciate that a Tangent Ogive may be more forgiving on entry in theory, but I never found this in practice in my SAUM 5R barrels. Neither SAUM barrels are really badly worn out yet. About 1/2 way - perhaps a little more - through their life. Perhaps this is another factor. I never shot Hybrids in my 284s because at the time they were as scarce as hens teeth and anyway I was satisfied with the VLDs and had a reasonable stockpile. Could a 5R give less resistance to and damage a projectile less on entering the lands ? They are full profile barrels. I also found that these two Bartlein 5R barrels seemed to give a slightly smaller Vsd than many other barrels. After about 100 - 150 shots there seemed a window of opportunity where V was incredibly constant for at least 300 shots. Maybe more.

Peter Smith.

Frank Green
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Re: throat angle

#17 Postby Frank Green » Sun Jan 13, 2019 2:14 am

Could a 5R give less resistance to and damage a projectile less on entering the lands ?

Peter Smith.[/quote]

Peter, I do believe in the 5R (or even a conventional 5 groove barrel) the lands don't directly oppose one another. So to me this helps distort/upset the bullet jacket less.

In conjunction with the angles on the sides of the lands I feel these two things do help fight bullet failure.

That being said I don't feel the style or number of grooves on one style vs. another has for the most part any accuracy and or barrel life advantage.

No I don't feel you get the benefit of a odd groove barrel like in a 3 groove. These have typially really wide grooves/lands and part of one land still opposes part of another land.

Later, Frank

wsftr
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Re: throat angle

#18 Postby wsftr » Sun Jan 13, 2019 6:37 am

Just out of curiosity can anyone point me to testing of a crooked chambering vs precision and improvement using jam. The theory of jam makes sense I just haven't seen any testing around it. Don't worry not asking for proof just curious if there is something in print I can read.

Also - what are the thoughts of lead angle being a factor in the new high BC .308 200grn bullets success or not? (Sorry Bruce I can start a new thread if this is too far off topic). Current testing and conversation show these bullets to be tough to get to shoot with many giving up on them.

Would a 2.5 help?

bruce moulds
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Re: throat angle

#19 Postby bruce moulds » Sun Jan 13, 2019 7:15 am

wsftr,
you are not off topic.
maybe 0.5 degrees would help.
200 gns is heavy for 308, and the lower angle might offer a gentler start, allowing for some realignment with a crooked chamber.?
or the opposite?
the trouble with issues like this is that even frank who has more access to quality barrels, and pete who is the ultimate investigator cannot do the ammount of work necessary to relly solve these issues.
the u.s. military might be able to, but they have no interest in pushing this as far as it needs to go.
until then we all talk and share info which helps some.
ultimately is boils down to a mindset that will be open enough to keep trying anything until the best is found for that combination.
perhaps this is one of the resons we suffer lack of numbers in this type of discipline.
people now want instant gratification, and do not want to go the full monte.
bruce.
"SUCH IS LIFE" Edward Kelly 11 nov 1880
http://youtu.be/YRaRCCZjdTM

wsftr
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Re: throat angle

#20 Postby wsftr » Sun Jan 13, 2019 7:23 am

Thanks Bruce. Yes agreed that a lot of testing is needed and I appreciate that potentially no one has done that.
I am after just thoughts at this stage.
I have other 200s working very well (in .308) and I am playing with one of the new gen High BC .308 200s. The 1.3 plus all of my other tricks can't get past the "promise" of the bullet. It shows very clear windows of where it wants to shoot but no cigar.
I am curious if there is a theory to support a different leade angle to get them to shoot.
My thoughts are two shallow angles are more likely to allow a crooked entry to remain and its not until the bearing surface is fully engaged that it might straighten. The new 200s have very little bearing surface.
Last edited by wsftr on Tue Jan 15, 2019 5:02 pm, edited 1 time in total.

Frank Green
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Re: throat angle

#21 Postby Frank Green » Sun Jan 13, 2019 12:37 pm

Bullets themselves I believe can be a problem. O.K. I’ll come out and say it....some are a problem. A couple of things to think about....the balance point of the bullet and the design of the bullet.

I’m not a bullet maker! I’ll base what I say from test data that I have seen and customer problems that I’ve worked on and in talking with bullet makers.

The balance point of the bullet. To radical of a design/balance point and I feel there are some bullets you will never get to shoot good. This is happening in conventional jacket bullets and as well as a lot of the new solids. To me the solids can be another topic but I will reference them here a time or two because I feel it’s part of design that over laps with conventional jacketed bullets.

What looks good on design paper doesn’t always work in the real world as we all know. I won’t name any bullet makers publicly either.

Also the thought of running a faster twist and or in conjunction with a higher velocity will help the bullet design and help stabilize them. I don’t agree with that thinking.

We made a .338 cal. barrel back around 12 years ago. Mac the gunsmith here in the U.S. did all the gunsmithing work and some of the testing for the customer. If I remember correctly it was suppose to be a F Class gun. The barrel was a 7 twist and chambered in .338 LM. With out any load work 300gr SMK would hold right around .5moa at 300 yards. The rifle was built in mind to run a new copper solid bullet.

Mac said 1 or 2 out of 5 rounds the solids would go thru the target sideways. I believe this was even at 100 yards. The ones that didn’t go thru the target sideways/keyhole and would leave a out of round black ring on the paper. Mac knew right away it was a stability problem and it wasn’t in the barrel and or the twist rate. So he took some of the bullets bored out the heel of the bullet and pressed in an aluminum slug to change the balance point of the bullet. They got them to shoot and actually shoot some decent groups but it confirmed what Mac thought. Balance point of the bullet was way off.

Jacketed bullets with a real radical point and long taper along the lines of a secant design I’ll say for just reference I feel can be more tempermental. The more the long point tip is pushed to be more lets say aero dynamic the more problems will have with them. I feel there is a point of diminishing returns.

I cannot say who did the testing but I have test data that was done a few years ago. Caliber where 6.5mm, .30 and .338. In the combined calibers I’m probably not exaggerating that about 30 bullets where tested in different twist rates in each caliber as well as in different chambers. So for example in .30cal in chambers from .308 win. Thru .300 Norma. In twist rates from 12 twist and as fast as 7 twist. Weights ranged from 155 to over 200+ grain bullets.

These where tested out to a 1k yards and further and or till the bullets went subsonic and went unstable. Recorded data was drag and velocity. What was interesting for one example was the 175smk. Didn’t matter if it was a 12 twist barrel, 11.25 twist barrel, 10 and I want to say 8 twist barrels where tested. The bullet had a real smooth drag curve and was stable etc....the whole time till it went subsonic. It didn’t dance around or do anythign weird.

Another bullet though in the over 200gr range and is a secant design. It didn’t matter if it was shot out of a .300wm at 2750fps our out of a .300 Norma at 3000fps (yes at 3000fps that’s way over pressure) the bullet never really went to sleep/settled down or what I will say was completely stable. The higher velocities helped it but didn’t cure it. Same with twist rates. It was tested in 10, 9 and 8 twist barrels. Didn’t matter. It really danced around when it hit the transonic range. The 175SMK didn’t do anything weird even when it crossed the transonic range of velocity. When I seen the test data as well as some other bullet/ammo makers the conclusion was the same. Bullet had/has design issues.

What’s interesting is the bullet has had some pretty good success in F Class and FTR. One time a customer and FTR champion shooter and I where talking when he was in the shop about this as he had shot some of the bullets but had given up on them. He told me at Raton, NM which is almost at 7k feet elevation he said his rifle shot excellent and could hole the x ring/10 ring with no issues. He went back home and went to a local match just a couple of weeks later (that range is at 600 ft elevation) and he said he did all he could to just hold the 9 ring. Now we just thru another variable in there....the environment and Mother Nature. He went back to a previous bullet he use to use and had no issues. He also said he noticed the gun doing weird things out around 800 yards. When we looked at the data on the bullet from testing he started jumping around and said oh my god. That’s exactly what that bullet does.

In this example the throat angle isn’t going to help the one bullet. Also as another bullet maker and testing facility pointed out to me was again especially if you are required to shoot box ammo you cannot chase the wear of the throat of the chamber. The more that bullet I described above has to make more of a jump or if you want start crooked into the barrel the more of a wobble that will be put on the bullet and if the bullet/ammo already has a premature wobble the more it will be amplified. It will be harder to keep the gun in tune for a longer period of time and as that throat wears the worse it will shoot.

Sorry for the long winded post! Hope it makes sense. :shock:

wsftr
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Re: throat angle

#22 Postby wsftr » Sun Jan 13, 2019 4:13 pm

Thanks Frank. Assuming there isn't a CoG issue a lot of it seems to come down to not really understanding why a tangent is friendlier than a secant as a general rule..exceptions agreed and understood. I do work on the basis that a bullet manufacturer has put in due diligence and can actually get the bullet to shoot before releasing for production...boutique makers aside as I can see them not having the resources but a bunch of helpful friends.

I'm curious as to why 1.3 is so common for the leade I assume its from testing over time.

The tune window and its duration is another subject and to me still circles back to some degree as to why is tangent friendlier than secant. If we new that it could lead down a bunch of paths.
I'm not sure how but someone figured out the 168 SMK's 13 degree boat tail was an issue for stability but no one seems to state why the new 200 gen .308 bullets don't shoot because of a bad design but nothing that states what the bad design is yet.
It does feel like we accept they are too pointy so they won't shoot but we don't really know why (I'm not talking external ballistics here). I'm expecting a design application would have highlighted a CoG issue early on...but maybe I'm a little naive here.
Last edited by wsftr on Tue Jan 15, 2019 3:44 am, edited 1 time in total.

Barry Davies
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Re: throat angle

#23 Postby Barry Davies » Sun Jan 13, 2019 4:26 pm

Do projectile designers actually perform tests with various shapes and various C of G's or do they simply design a projectile that " looks good " shape wise etc and say " well that should shoot "
I am still convinced that the Sierra 2155 is more accurate than some of the later so called Higher ballistic projectiles --and has less variations batch to batch.

bruce moulds
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Re: throat angle

#24 Postby bruce moulds » Sun Jan 13, 2019 6:06 pm

and dynamic instability can bring gyroscopic stability (calculatable) unstuck bigtime.
bruce.
"SUCH IS LIFE" Edward Kelly 11 nov 1880

http://youtu.be/YRaRCCZjdTM

williada
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Re: throat angle

#25 Postby williada » Sun Jan 13, 2019 6:59 pm

G''Day Bruce, I've sat back on this one, but I have reported on Ozfclass before some of my findings using different reamer leade angles in testing I did for NRAA back in 2003 from the machine rest. The angles tested were 1/2, 3/4, 1.15 and 1.30 degrees for Palma. I have subsequently tested out to 3 degrees plus depending on the the tangent of the ogive of the bullet. Also sat Corby's pressure tester on them in 2003 and separately with pressure trace. Yes pressures are reduced with a reduction in throat angle, and so is felt recoil and as such the pressure peak whether it is forward or aft also affects barrel lift and is influential on a desired lift for positive compensation at long ranges and reduced lift for short ranges. As a rule of thumb I start testing on the 0.010" jam. This is really a convenient reference point only. Subsequent load development must explore the seating depth. I agree with Peter, more jam maybe covering up sloppy gunsmithing or big freebore diameters we had to run with using issued ammo or as barrels wear, then a 0.025" jam can be beneficial with current setups. The seating depth is also a timing factor for both muzzle lift and minimum bore size at bullet exit in terms of shock wave impact on bore size. Critical is the diameter of the freebore, this must be minimum and a chamber nip assists here because cleaning and shooting wear the freebore and throat. Concentrically loaded and fitted ammunition is essential to accuracy.

The throat angle on the 1/2 degree shot well but eroded quickly. But realize all barrels are individuals, bore friction differs not only with bore size but land configuration as well as finish and in this day and age of small targets load development has to be optimised around what distance you are trying to shoot as well. That implies a tight group can be formed with any configuration, but ask yourself will it hold up over different distances and how long will the angle be retained given wear and tear? The advantage of the tangent ogive setting was to reduce setup tension and to centralize the projectile in the bore more effectively. The trade off is with SD, with the jam setting. A longer jump is effectively a bigger expansion chamber so the variances in pressure appear as a smaller percentage. That was the theory when larger chambers were used with greater headspace dealing with the poorer quality issued ammo at Bisley. I endorse Frank's remarks re 5R and also Barry's with the 155 Sierra. Everything is a compromise and a balance of competing ends.

pjifl
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Re: throat angle

#26 Postby pjifl » Sun Jan 13, 2019 11:49 pm

wsftr wrote Just out of curiosity can anyone point me to testing of a crooked chambering vs precision and improvement using jam. The theory of jam makes sense I just haven't seen any testing around it. Don't worry not asking for proof just curious if there is something in print I can read.

Although it will not answer all of your question, I believe the book 'Rifle Accuracy Facts' by Harold Vaughn is really worth a read. It may be hard to get now. I think some (not much) of the material is flawed and dated but it is a book that gets one thinking and he certainly addresses crooked chambers using an experimental approach and measuring effects on targets.

Peter Smith

Frank Green
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Re: throat angle

#27 Postby Frank Green » Mon Jan 14, 2019 2:43 am

Thinking about it some more I can give an example or two of crooked chambers or what I will call chambers that where cut off center line of the bore.

Bill at work back around 2006 we rebarreled his factory Rem. Varmint in .223 Rem. No matter what he did he could never get the gun to shoot. Pillar bedded the laminated wood stock, crown was good, bore finish was o.k. for a factory gun and didn’t foul abnormally bad etc...best it would shoot was about 1 1/4” groups at a 100 yards. When we pulled the barrel to fit up a new one I took a good look down the bore from the breech end with my naked eye. I picked up the throat of the chamber was a full .003” off center. I confirmed it with looking in the bore scope. Prior to helping him put the new barrel on I never thought of checking his brass for runout etc...looking back on it I wish I would’ve thought of it. I still have his original barrel in my office.

.300wm SAAMI spec chamber. I’m not a fan of the factory chamber and others like it. There is no straight/parallel section in the freebore area. Starts out .315 diameter and just tapers in a cone shape with a 1 degree 26 min angle on it. Don’t have a reamer print here at home that I can copy and paste. To me a chamber with a throat like this in conjunction with the type of ammo being shot can have huge effects on accuracy. The barrel itself is usually the first thing that gets blamed and it might have not have anything to do with the barrel itself.

Take a look at the .300 A191 chamber spec. I’ll post prints tomorrow. Has a decent freebore section with a parallel throat. We can debate freebore diameter but compared to the SAAMI spec night and day different.

We’ve made a lot of ammunition test barrels in both chambers. The word I always got back was the A191 chamber consistently shot better than the SAAMI spec.
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Last edited by Frank Green on Tue Jan 15, 2019 1:56 am, edited 2 times in total.

DannyS
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Re: throat angle

#28 Postby DannyS » Mon Jan 14, 2019 6:47 am

Two rifles, 4 x 308 barrels, all shooting berger 155.5s, 1 seating die on 1 setting used for all barrels, jumping anywhere from 20 to 60 thou. Can’t see any difference in accuracy.

scott/r
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Re: throat angle

#29 Postby scott/r » Mon Jan 14, 2019 7:26 am

DannyS wrote:Two rifles, 4 x 308 barrels, all shooting berger 155.5s, 1 seating die on 1 setting used for all barrels, jumping anywhere from 20 to 60 thou. Can’t see any difference in accuracy.


Does that mean that they are all crap? :D
Just kidding.
Scott.

Frank Green
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Re: throat angle

#30 Postby Frank Green » Tue Jan 15, 2019 1:57 am

I just attached the drawings to my previous post.


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