|
|
|
Discussion Forums
Forum Index
> Rifles general discussion
> Gain Twist Bore
Gain Twist Bore
@ 06:14 am (GMT) |
Matthew MorrisonLooking for some general discussion on this. I have built a custom 300WM with locally manufactured action and barrel The Barrel is a 26 inch Stainless 14 - 10 Gain twist 5R. I expect my velocities are going to settle in at 2700 FPS with a 208 gr ALCO Bullet in front of 74-75 GR of Hogdons H1000 So I am going out next weekend with some virgin brass to fireform and get a feel for how the gun shoots. Has anyone had better worse results with a gain twist barrel? Anything to advise in terms of max velocities and expected Max Velocity spread using gain twist. Max velocities to work up to with Gain twist. |
Replies
@ 07:45 am (GMT) |
Chris MurphyRe: Gain Twist BoreHi Matthew I haven’t used gain twist but it’s nice to hear of a sensible twist rate usually they are fast to stupid fast. But with the 300 be prepared to go as high as 78-79 grain H1000 to get the most out of it work up from 74-75 in .5grain increments watching for pressure. The few 300s I’ve done have had sweet spots round 78-79 but as l said WATCH FOR PRESSURE |
@ 08:54 am (GMT) |
Nathan FosterRe: Gain Twist BoreHi Mathew, a gain twist bore can certainly help reduce bullet jacket upset. However the throat geometry of the chamber reamer is equally important. If the leade angle is steep, it may to some extent negate some of the benefits of the slow starting twist.There are two ways we can approach twist using the gradual gain. The first is as you have done, starting very slow and then arriving at a standard twist rate. This is the most gentle approach. A second and now common method, is to start a bit slower than than the standard twist and finish at a twist rate that might otherwise be considered faster than normal. If we take the 7mm magnums as an example, the standard twist for a long time has been 9 to 9.5. More recently, the 8 and 8.5 twist barrels have become common. However if these are used with the magnums, the rifles can prove finicky (thats the stupid bit Chris mentioned above). To be sure, there are many dud custom magnums at present, utilizing these twists. The more potent the magnum (muzzle velocity), the more we may see problems. An 8 twist can work extremely well on a 7mm-08, but can prove to be a disaster on a 7mm Practical. But by starting with a 10 or slower twist, it is possible to arrive at 8.5 with fewer issues and with a measure of bullet weight versatility. We have number of readers spread throughout the UK, Canada and the U.S currently using gain twists in this manner. In this instance, the goal is not so much extra muzzle velocity but rather the maintaining of a high BC during flight. But as Chris alludes to, it can be wise to refrain from pushing things too far. These things aside, do keep in mind that there are many other factors which can effect outcomes. The chamber leade angle is one factor. Rifle bedding is a major factor. Unfortunately, two tubes of ali or stainless do not equate to pillar bedding. This is a common misused term to fool gun buyers, utilizing the word bedding after the word pillar. The term aluminum bedding block can also be misleading. I tend to use the word chassis to differentiate. The contour of the barrel can also effect outcomes which can also tie back into bedding methods. Then finally we have the issues of load development, concentricity and shooter ability. All too often, these variables muddy the waters, effecting reports. It is therefore wise to treat most reviews as heresay, especially these days where rifles are bought or assembled and trotted off to the range in the space of a few days prior to a blog post being vomited out under the guize of a 'sniper rifle review'. There is little point in me quoting potential loads or velocities etc. The individual bore dimensions, your altitude, scales calibration and brass thickness will each effect outcomes. Just work your loads up step by step in half grain increments watching for pressure signs. Hope that helps a bit. |
|
@ 06:58 am (GMT) |
Matthew MorrisonRe: Gain Twist BoreHelps a lot, Nathan. I have read your entire library and some more than once and bought the complete set twice more for Christmas Gifts to my pals. (I'm the guy on the beach in Maui with Bikini Wife but choosing to read your book instead of her eyes)I have done the full bedding job on this gun and have once already done a round of load development with an different bullet. Shot just under an MOA I did make an error in the initial post. The bullet is the Alco 200 gr. (not 208) Interestingly, I am getting the impression I have bought and imported an orphan discontinued bullet as I cannot now find the 200 gr ALCO Rebated Boat Tail aluminum tip for sale anywhere nor can I find the Ballistic Coefficient for this bullet. I do have a line on two more boxes and have done all the federal importing paperwork on it. So, once I have them, I will be left with a total of 300 bullets I imagine that will be plenty to work with Anyway. no need to reply Thank you both for your feedback Matt |
|
@ 08:44 am (GMT) |
Nathan FosterRe: Gain Twist BoreNo worries Matt and yes, I remember your holiday photo. Steph and I love the photos sent to us, showing folk reading the books in various settings. |
|
@ 10:19 am (GMT) |
Nathan FosterRe: Gain Twist BoreJust some additional info in line with Chris's intent and for the sake of other readers. There are a wide range of variations in sweet spots with the .300's. As I mentioned before, there is no point in me quoting loads due to the many variables, but do keep in mind that sweet spots can run as high as 2925fps (200-208gr from a 26" barrel) or thereabouts. Provided we are acting in a safe manner and reading pressure signs correctly, it pays to test the full gamut.Obviously, information can easily be taken in the wrong way. A guy may try to chase 26" barrel velocities, ignoring the fact that he had his magnum barrel cut to 22" to make it handier when fitted with a muffler or brake. He may compound issues further by denying himself the sense to read pressure signs and blame the chamber as being rough if cases won't extract, when instead his loads are simply too hot when approaching common 26" barrel velocities. Such situations are certainly not unique. In contrast to this, a barrel that is long but very lean may have its own unique and limiting harmonic factors. More often than not, the light weights tune up at the lower nodes while also being easier on the shoulder. But such loads can still create a lot of heat and groups can open up quickly. There are certainly many variables which can amount to variations from rifle to rifle of up to and around 300fps. Regarding throat geometry, I find that factory rifle reamer designs do vary, resulting in differences in max OAL's of up to 3mm or .118". For every 1mm or 40 thou, there is the potential for up to +/- 50fps, athough again, other variables greatly effect results - hence the word potential. As a side note, I would one day like to create my own take on the .300 Win Mag which incorporates both SAAMI OAL's but with shallow leade angles and slightly better neck tolerances (as per the 7mm Practical). The goal, like the Practical, would be to create something that is easier for end users to work work with. I would also like to do the same with the 7mm Rem Mag. But this will take some experimenting with and will require both David Manson and myself to donate time for this. Its not as easy as one might expect. Over time, I have come to believe that these small (but difficult to exact) changes would make life easier for both gunsmiths and shooters. |
@ 11:25 am (GMT) |
Martin TaylorRe: Gain Twist BoreMaybe my Rem 300wm is a freak having found this cal a joy to tune and use with pills from 168 to 210 grains. The only thing l would like to change would be a little longer neck length to let the brass grow into but saying that l have no issues with runout etc.All brass is neck turned and F/L sized every time using Redding S dies & minimum shoulder bump. This sub .3moa combination still cannot be faulted at any range after years of field service. I like the Idea of a gain twist barrel and it would make a great project getting 2 Bartlien's, one gain the other fixed of the same contour etc. then chambered with with the same reamer. If only l had money and a gunsmith with to much time on his/her hands............If only! |
Please wait
ABOUT USWe are a small, family run business, based out of Taranaki, New Zealand, who specialize in cartridge research and testing, and rifle accurizing.
|
KNOWLEDGEBASE |
KNOWLEDGEBASE |