Tony Z wrote:Dave a while before i gave it away i built and trialled this devise because the issue of off center pin holes was really bugging me. I built it and fitted it to the old 30 cal HG. Shot it once and had very good results then sold the gun and use this as a paper weight now.
It has a parabolic cone to split the blast from the muzzle as the bullet slips though the bore which has around ten thou clearance on a 30 cal bullet. The cone sat clear of the muzzle by around a 100 thou and this in itself was to be a subject of trial and error to find working parameters. The cone diverts the blast through the vent holes and outward at 5 degrees and away from the bullet while it looks to depart its second muzzle. The vent holes are also angularly set at five degrees to minimize torque. The only gas, in theory, that is near the bullet is the gas trailing it. Vaughn talked about this theory of a bullet in free flight being accelerated without bore friction and also being free of disruption. He did it by boring the barrel and venting the gas sideways. That constitutes a brake where at the time of this experiment brakes were not legal for heavy guns in IBS rules.
Mine is not a brake but allows the bullet to be free of the gas envelope. It worked with 200 SMKs very well. Unfortunately i never got around to trialling it with a true slippery VLD. I know the concept works but unfortunately i no longer have the targets to show the result. Never got used in competition.
The other benefit was around a 100 fps velocity increase similar to Vaughn's experiment. That in itself was well worth the effort, but unlike his attempt i did see an accuracy gain and even looking bullet holes.
Does anyone else think this sounds like the job an air stripper claims to do for air rifles?
The jury is still out, as I have a fly target shot with 200SMK, which have a boat tail, which has centred pin holes. The question remains, is it the boat tail, or is it VLDs, which the old 200SMK certainly isn't.
When I get the HG back up I will test 187FB, 185 Rebated BT, and BT, non VLD, as well as BT VLD. This might put the issue to bed in terms of it being muzzle blast causing dispersion of bullets, or simply the VLD bullets never really going to sleep, as theorised by Ellison and other notable shooters.
Apparently some of the vld doubting Americans have also found that targets shot with flat base or hpbt's with tangent ogives are providing central pin holes.
Secant ogive vld's while producing some extremely consistent and small groups are exhibiting slight instability and random pin hole orientations. Even for groups in the 1 to 2 inch range at 1000yds.
Tony Z wrote:Dave a while before i gave it away i built and trialled this devise because the issue of off center pin holes was really bugging me. I built it and fitted it to the old 30 cal HG. Shot it once and had very good results then sold the gun and use this as a paper weight now.
It has a parabolic cone to split the blast from the muzzle as the bullet slips though the bore which has around ten thou clearance on a 30 cal bullet. The cone sat clear of the muzzle by around a 100 thou and this in itself was to be a subject of trial and error to find working parameters. The cone diverts the blast through the vent holes and outward at 5 degrees and away from the bullet while it looks to depart its second muzzle. The vent holes are also angularly set at five degrees to minimize torque. The only gas, in theory, that is near the bullet is the gas trailing it. Vaughn talked about this theory of a bullet in free flight being accelerated without bore friction and also being free of disruption. He did it by boring the barrel and venting the gas sideways. That constitutes a brake where at the time of this experiment brakes were not legal for heavy guns in IBS rules.
Mine is not a brake but allows the bullet to be free of the gas envelope. It worked with 200 SMKs very well. Unfortunately i never got around to trialling it with a true slippery VLD. I know the concept works but unfortunately i no longer have the targets to show the result. Never got used in competition.
The other benefit was around a 100 fps velocity increase similar to Vaughn's experiment. That in itself was well worth the effort, but unlike his attempt i did see an accuracy gain and even looking bullet holes.
Does anyone else think this sounds like the job an air stripper claims to do for air rifles?
Very much.
when I first looked at the photo I thought Air rifle air stripper,
Gary
Hi guys. Not familiar with "air stripper" from an air rifle. Could someone post a pic of one.
While not exactly about the discussion directly, this is interesting and may question spin rates and stabilization having a direct influence on BCs.
Nathan Foster posted this on a thread i found doing a net trawl about the 180 grain ELD M.
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"Under the conditions here, the G1 BC's for the 180 ELD-M at sea level at 59f from a 9 twist Practical are as follows:
From memory, I believe Hornady rate the G1 BC for the ELD-M as .796 from their recommended 8 twist. But in the above field data, you can see the BC was achieved with the 9 twist via its muzzle velocity. "
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Hornady claims this bullet can attain a BC of 0.819 in a 7.5 twist at Mach 2.25, around 2600 fps. Foster has stated at 7mm Practical speeds using an 8 twist accuracy drops off and one in three bullets disintegrated.
By Hornady's charts and recommendations a 9 twist should not be used with this bullet.
So it comes back to two things i have said for years, recommended twist rates are only a guideline, often over conservative, and real world data always trumps theory.
So where does it leave this discussion? Faster twist rates by various sources are said to garner higher BCs which leads to the notion less drag is attained by reducing frontal area. But mine and Fosters findings are too quick a twist rate leads to accuracy issues mainly from jacket failure. Hornady's 180 ELDMs BC of 0.819 sounds very attractive, but is probably limited in application to 7/08 velocities if you are looking to get the best accuracy potential.
So the discussion may be leading us into an area of being counterproductive where optimizing for BC may lead to issues with accuracy not stemming from stabilization spin rate, but from mechanical issues because of the spin rate.
We may well spin them fast enough to get the pin holes centered and optimize BC only to find that one in three bullets become bird shot.
Hi All,
As stated previously I did load up the 4 different type of 7mm. projectiles and shot them at Townsville. I shot the 180 M/K's and the 180 Berger V.L.D's at 1000. Both produced reasonable groups and the V.L.D's showed the off centre pin holes where as the M/K's did not. I shot the rebated B/T's and the F/B's at 600. Both had centred pin holes. Have obtained 10, 180gr. hybrids to try next time in Townsville. Will shoot them with M/K's to get a comparison.
Obtained the hybrids from Marty Lobert and showed him the results of the two 1000yd.tests. He is as mystified as we are about the cause. He took photos of the 2 groups and was going to send them to Peter Smith for his views on the subject. Will advise of the reply from Peter.
Cheers,
Trevor.
