7mm Twist Rates

[Matt P]

I'm looking at building a 7mm, probably in 7x57AI and if I can't get the velocity I'm after then a 7mm x 270AI. What twist rate for 175 SMK ??
Has anyone used either of these calibers ?? If so what velocity did you get ?? Will be used mostly for F Class and Fly and don't want to run on the edge of stability or have to push the bullets really hard.

Thanks
Matt Paroz

[a.JR]

Hi Matt, We have run a lot of 7mms in the north for a long time now ..A coupla observations ,1/ i don't think that the 175 Smk or the 180 Ber are the bullets to set your sights on..Tony z is still running 2 types of 7mm(hopefully he will chime in) and niether respond on a consistant basis with the heavy bullets,i gave up on a big case because of inconsistances ,although i won athe QLD FLy champs with the 7mm Redneck it is not a patch on the 30 cal version..One L/G cartridge i came up with is based on the excellent lapua 9.3X62 brass because we were haveing trouble getting any RWS stuff..This 7x 62 imp runs a 175/180 to 2950 with ease using the 4831 style powders ,very easy to get along with and you can get the brass anytime.. That being said i would use the bullets in the lighter area at 162/168,these will go 3000fps all day..If it were my pick i would still use the Maddco 7mm 10 twist with the 168smks,i have used over 10000 of these at around the 3000fps and don't believe i ever suffered when the wind was up .. If your heart is set on the 175Smk then i have been involved with 4 different guns that have won with that bullet (1 of at 3.4in group at 1000yds) in a Maddco 10 in twist..Hope this helps..JR..Jeff Rogersquote="Matt P"]I'm looking at building a 7mm, probably in 7x57AI and if I can't get the velocity I'm after then a 7mm x 270AI. What twist rate for 175 SMK ??
Has anyone used either of these calibers ?? If so what velocity did you get ?? Will be used mostly for F Class and Fly and don't want to run on the edge of stability or have to push the bullets really hard.

Thanks
Matt Paroz[/quote]

[Matt P]

Jeff
Very interesting I'm being told I need a 8 twist barrel. The reason I wanted to go away from the 6.5 was to get away from the fast twist although I've had lots of success with the 6.5 I still get shots out of the "group". I thought a 9 would be as slow as I could go for the 175, not at all interested in the 180 cant afford to loose 10 points in 1 shot.

Matt

[a.JR]

Matt ,Probably since tony and i started shooting the 7mms in 1997 we definatly used more of the old design 168 than any other projectile(true world BC is about .550) .. Tony had a good run for a long time with the 162 Amax (about .590) ,still swears by em ,Dave Goodridge has shot a big bunch of small one's as well at 1K bench,i never really got on top of them ..My old 7mm short mag when we 1st started a 1000yd in 2000 was a demon for small vertical dispersion using the Sierra,it would regularly shoot 10 shot groups in the 2 to 3 inch area (VD)which was at the time very noticable compared with the other guns, i set a 6 match group agg record for AUS the 1st year we tried the new distance.. This was done with a 10 twist maddco ...I would not like to try it down your way but we have shot all the small bullets in an 11 twist just to prove it could be done .. I hope TZ chimes in because i think he used a 10 twist 162 combo to shoot the 1st 250 FLY score recorded in AUS.. I my experiance if your wanting to get rid of the 9 1 syndrome then forget the 175Smk,it runs real good when it's calm and when the crappy winds are on it will kick the one out just like your 6,5 does.. 1 thing that the books don't tell you is a .550 BC at 3000fps in a 10 twist holds it's own against the winds a lot better than theory should allow it to ....JR..Jeff Rogers....................

Jeff
Very interesting I'm being told I need a 8 twist barrel. The reason I wanted to go away from the 6.5 was to get away from the fast twist although I've had lots of success with the 6.5 I still get shots out of the "group". I thought a 9 would be as slow as I could go for the 175, not at all interested in the 180 cant afford to loose 10 points in 1 shot.

Matt

[dg]

matt

presently am attempting to finish assembly, case forming and run in on the following projects.

purchased 2 tobler 9.5" barrels. used the 280 ackley reamer, run in short,
heavy gun - match savage action -chamber to suit a 7x63( 7mm 30-06 improved)
and a
light gun - barnard action-7mm gtuy ( suit rws fire formed 7x57 imp case length).

due to unsuitable weather lately, progress has been slow, but 175's from both barrels do get to 500metres ok and show good promise.

hopefully in next few weeks, will get the chance to finalise tuning and try them at 1000 yards.

one good thing i learn't from previous disappointing 7-270wsm project was that the 175's @2800fps would get to 1000yds and shot under 6 inches in 1-10" maddco, which surprised me somewhat.

will catch up at the fly

cheers
dave g

[Tony Z]

Dave, have you had a chance to run those Amaxes you got sent yet?
I will email you shortly the results of the 160 FB and Amaxes in the new barrels.
Tony Z.

[dg]

tony

i have been delaying testing the 162 a- maxes until the weather is more shooter friendly.

as i probably mentioned, they look good ( have not found any loose tips yet) and weigh and measure pretty good too, but as you are well aware, it's on paper results that matter most.

be in contact soon

dave g

[Tony Z]

JR, i just re-read this post and that bit about the 250 score is not right. The highest score i have ever shot with a 7 mm is 233 in Springsure in 1998. That was also using the 168 SMK in a 10 twist. The small group i shot in Cantberra was also with the same combo.
The 175 in 5 barrels of 4 twist rates will not agg. The 9 through to 11 twist make no difference with the 175. The Amax will group and agg and drift less than the 175, as will the 168. The 160 FB will group best of all but the drift is quite different.
Marty Lobert showed me all his test groups with an 8 twist and 180/175and they were horrible once the speed went up above 2700 or so. I suggested the 9.5 twist he uses now and he is having good results . My two 9.5s shoot the occasional good group with the 175s, but the agg is way over 3 inches in Fly, the drift is horrible and the score is way off my 230 average for last year(185). Might be OK for F Class, but no where near good enough for Fly or 1K.

Tony Z.

[AlanF]

Tony,

When you say wind drift on 7mm 168s is better than 7mm 175s, I presume you can't be talking about Sierras? According to my calculations, the 168 would need to be travelling about 450fps faster to get inside a 175 in the wind.

Alan

[Tony Z]

Alan, i know this is going to throw dust up in some peoples faces, but there are a number of us that have shot a lot of the 175s and we are not finding the "holy grail" that we hoped it would be. Recently one of our 1K shooters was at wits end and shot the 168 SMK vs the 175 SMK at the same time. The 168s were nearly 2 minutes further up the paper, the wind drift was less and the group was tighter.
The BC claimed for the 168 was originally .619 and more recently it was adjusted to the high .48 area. Some who have contacted Sierra direct have been given a figure around .550. All i know is that the 175 will not work for my expectations or needs and others up my way have also given them the flick. Of the three batches i know of brought into the country, i have shot all of them.
Calculations are calculations, and all i can tell you is that i will not shoot a 175 in competition, the 162 Amax and 168 SMK group, agg and don't do strange things when the wind flicks around.
What the real world BC of the 175 is, i don't know, but the real world BC of the 168 is not in the .4s, but either way if it don't group consistantly, it ain't worth a cracker.

Tony Z.

[a.JR]

The day that i get beat in competition by a Ballistics calculator or a BC guru is when i just might take some notice of all the clap trap that is bandied about by a bunch of people that get on forums ..Until then i will believe only what i see happen on the target and what i see is that a 9.5 or 10 twist 7mm with 160 to 168 bullets will consistatly win more matchs than a 8.5 to 9 twist with the big bullets .. As a mater of fact if i were to go back to a 7mm it would be with Bob Cauterucio's 156s in a 10 to 10.5 twist..JR..Jeff Rogers

[AlanF]

Jeff,

As you know, I don't shoot BR, but from what I can gather, wind-bucking ability is probably less important to a BR shooter where you can pick a steady condition and machine-gun without having to wait for marking, particularly if you have only 5 record shots. In F-Class Open,I don't know of anyone having much success in windy conditions at the longs with anything under about .52 BC.

On another matter, I tried a new load in an old barrel last weekend, and got the best elevation I've ever had at 800yd. I couldn't measure it but I reckon that all 20 shots were in less than 2" elevation. And this might cheer you up, I used your meplat trimmer for the first time in a while. Scores were only 92 at each range, due to a little mis-reading of the wind, but I was grinning from ear to ear.

Alan

[Tony Z]

Alan, i neither agree or disagree with the point you made about the BC requirement in LR BR. The real emphasis on BC becomes an issue when you make a mistake in judgement of the conditions as you release the shot. A higher BC might give you one ring less disaster score wise or a couple of inches less in group size, but none the less you have made an error. Jeff went to a 7/300 Win Mag a couple of years back because he felt that the new 175 SMK driven at 3100+ fps would give him a clear edge over most. Grouping capability of the cartridge aside, the real world shot placement was less predictable than the Bibs at both 500M and 1K. The wind drift i will let him tell you about. The barrel was removed the moment he arrived back from winning the Brisbane Fly, (yes you read that right) and sold off and he returned to the 30 Redneck and the .525 BC Bib which to most in the LR game would be seen as a backward step. He then went on to win last years SOTY in Fly with a record aggregate score. There have been only three 250+ scores shot in Fly, and all three of them were done using a bullet that is rarely used because of its low BC, namely the 200 SMK in 30 cal. Then there is the case of Rod Davies at last years Atherton OPM F Open class where he shot a 22 BR with an 80 grain FB made by John Little that went on to win the overall at 300, 500, and 700 metres. This was all the more remarkable as he beat off a red hot Marty Lobert with his 284 and 175 SMKs by using a projectile having a BC of less than .400.
Now i am certainly not advocating for all F Open shooters to go out and build a 22 BR, but the point is that in all but the 7mm example (not picking on the 7, but i do have all the details of this gun in my head as i was the one that built it) the key was repeatability. You can have all the BC in the world like the guys who built 338s using the 300 SMK and its near .700 BC, that have never threatened to win a Fly or dominate 1K, as it is no good to you if the bullets integrity is suspect. On paper this 300 SMK is the bullet to have, but rarely does it show up in the winners circle.
Now i don't want to sound condescending, but if one was to look more closely at what is where in the 1k game, or even the 600 IBS for that matter, it would be of more benefit than perusing the whos who of Yank F Class and their assocciated target with its 20 inch bull and 10 inch X ring at a 1000 yds and a line full of 6.5/284s. Any 6.5/284 that does not shoot a possible and lots of Xs should be used for hand warming purposes once doused in fuel and set alight. With Bisley in 2009 and the F champs, if all the teams turn up with 6.5 somethings, its gunna be a duel of the coaches where the rifles and their shooters become irrelevant. But if we are smart like the South Efrikens and their version of the 280 AI and the wonderful 168 Berger (10 twist barrel at 3000 fps shot like a dream but hard to get bullets here) that cleaned up the Yanks on the final day, haha, we can make the coaches job a lot easier.
If you can get consistancy and a high BC then good luck to you. My view on BC is changing as i find the higher the BC, the more one has to trade off to use it and whether its 1K or F Open i don't see this as a viable option. Accuracy first and foremost, and BC, within reason, second. The five barrel test and 800+ 175 SMKs has told me that this is not consistantly accurate and the BC is not the .630 quoted. If you asked me today what to take to Bisley tomorrow, i would say 10 twist Maddco, 280 AI with RWS brass and 168 SMKs. No fuss tuning, no odd vertical shots, groups like a demon and gives nothing away in wind drift to any 6.5 pill i have yet to see. Barrel life will be well beyond 2000 rounds. This is what i intend to shoot for my first full years venture into 1k light gun. And Alan, every shot fired will be with a meplat cut bullet as per the last 5 years
Tony Z.

[Tony Z]

Matt P, that 6.5 that flicks the odd shot out, does it have a heavy profile barrel like say 1.000" or maybe more at the muzzle or does it have a fullbore style profile? I have a theory that may change the way i view fast twist barrels and the reason they flick the odd shot. It may be nothing to do with twist rate but more to do with barrel muzzle moment and the length of the bullet shank. I placed a stretcher tube on John Lorimers 6.5 x 55 AI barrel recently and he is yet to see a wild shot that plagued this barrel before and the groups have shrunk dramatically.

Here goes. With a fast twist barrel the common denominator is a long bullet shank in the high BC bullets. The vibration of a large capacity case, unlike that of the 6 BR is quite violent and due to reasons too long to explain in this post a barrel generally osscillates in a vertical or narrow figure 8 plane. Therefore most groups that are in need of tune are vertical rather than horizontal when shot in mild conditions of course. The vibration has both amplitude and frequency, but most importantly the vibration terminating at the muzzle causes movement (moment) in that vertical plane. The amplitude will give the muzzle a velocity in the direction of that vibration plane. Now with the frequency, the bullet may be exiting the muzzle and before the muzzle changes direction, the bullet is clear and on its way to the target. But, with say your 6.5, like Johns the bullet shank is longer than most, and if both the freqency and amplitude are at the wrong timing or possibly the frequency is low with a high amplitude, or vice versa, the bullet may be almost out before the muzzle moment has changed direction thus tipping the tail so to speak. In other words your cam timing is out and you have just clipped a valve by a couple of thou. The bullet could then wobble causing a larger frontal area and reduce BC and drop out of the sky or be deflected to another point and not be subject to a loss in BC. Or the inbalance in the early part of the flight makes the bullet suseptible to wind variations like that claimed by some experienced shooters. Of course what i have just described is able to possibly happen in any old gun, but the nature of the VLD with its javelin shaped profile at or near the edge of instability could mean that such an event could be disastrous to the final termination point of flight.

The tube gun has everyone puzzled as to why exactly it works in the way it does with its ability to make a shooter out of a piece of crap. I have done many over the last ten years and have yet to see one failure. From an engineering point of view, the tube guns are just a way of strengthening a somewhat flexible structure, but the thing that has everyone puzzled is the bit as to why it works no matter what tension is placed on the lock nut. Many have tried all manner of things like spring loading and liquid filling etc, but the tried and proven simplest way is just with a tube and enough tension not to come undone when the barrel lenghtens from heat during firing.
Put simply, i reckon the muzzle due to its surrounding structure is less mobile and less affected to a vibration moment and therefore a short or long shanked bullet will not be kicked out of orbit, and statistically the groups from a good barrel shrink, and the bad groups before stretching are no longer visible to the same extent.
In JRs tube gun in 300 Redneck, there are only good groups and better ones and his tuning is limited to what the brass can sustain for a number of shots. This attribute is mirrored by a couple of Yank tube gunners using not just the Bibs but other calibers aswell.
The view that the tube gun alters the frequency and amplitude of vibration might be right, but the fact that the tube and barrel of dissimilar metals do not hold a constant tension throughout the course of fire as things get warm, puts a huge hole in the belief that these vibrations are constant throughout. My view is that the muzzle being controlled in its ability to sling about is the real reason why this setup works and may be the cure for a long shanked 6.5 pill.
I have no way of proving this yet, but i have yet to see a more plausible explination. My very first tube gun was a 7 mm that slung a shot about one in ten before i did a number on it, and the results from it are the direct reason why JR has a tube gun today. His tube gun coupled with the near perfect 187 BIBs is a truely wicked combination. A 50 point win in last years Brisvegas Fly is proof of that.

Tony Z.

[a.JR]

Al ,Glad you got some use out of the trimmer..The problem i find with a set up like yours (long bullet/fast twist)is in the consistancy, like when you go from 20 shots with a tight water line and next target you get 1 high and 1 low and a big bunch in the middle .. It is in all circles passed off on the dreaded conditions ,well i tell ya i think it's in the gun 95% of the time ... On the subject of shooting styles /disciplines ,I think you got to bite the bullet one day and actually go to a LR Bench match to compete..There you will find a large majority of competitors that pick there way through the conditions just like F/Class people do, cept most of us don't have someone in the butts telling us where the last shot went..Since 2003 i have had a gun that can put 10 shots away in less than 30 seconds ,in that time i have shot about 40 registered matchs, out of those comps i have only ever once had the chance to RUN 8 of the 10 shots on the same target ..The weather up my way has for years not allowed the machine gun technique to be of any use even with 5 cartridges..There is someone from down you way coming to F/class match shortly with a slow twist /flat base bullet combination ,let me know what you think after he has had a coupla matchs under his belt..JR..Jeff Rogers.....quote="AlanF"]Jeff,

As you know, I don't shoot BR, but from what I can gather, wind-bucking ability is probably less important to a BR shooter where you can pick a steady condition and machine-gun without having to wait for marking, particularly if you have only 5 record shots. In F-Class Open,I don't know of anyone having much success in windy conditions at the longs with anything under about .52 BC.

On another matter, I tried a new load in an old barrel last weekend, and got the best elevation I've ever had at 800yd. I couldn't measure it but I reckon that all 20 shots were in less than 2" elevation. And this might cheer you up, I used your meplat trimmer for the first time in a while. Scores were only 92 at each range, due to a little mis-reading of the wind, but I was grinning from ear to ear.

Alan[/quote]