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Posted: 1/3/2014 10:51:20 AM EDT
| With Match grade barrels becoming popular what makes a barrel get a tighter grouping. I hope that isn't too broad of a question but I did hear a guy talking to a gunsmith. The Gunsmith said a tighter rifling makes a big difference but the liability goes up. Well obviously my nosey ears didn't catch the whole conversation. |
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with barrels,
type of line (chrome, chrome moly, or steel unlined) giving the hardness of each material, and the chance of uneven application of lining, unlined barrels tend to be the most accurate, but require more upkeep. barrel profile-the thicker the barrel, the less harmonics from heat, and shooting will affect it, a slim profile barrel will vibrate at different speeds depending on how warm it is, this vibration will cause second, and third shots (and subsequent) shots to have a point of impact shift, while groupping will be consistant, the groups will generally move. crown- the crown vents the final gas as the round exits the barrel, a poor crown will cause uneven venting and cause inconsiatant accuracy with rounds the speed of rifling, that is how many full rotations of teh rifling are there within 12 inch, this is usaully measure and listed on the barrel, usaully something like "1 turn in 10 inches" (1:10 inches). the heavier the bullet weight, the slower you want the rifling(it could be the opposite) most ar platforms are rifled at 1x7, 1x9, or 1x6, the longer and heavier the bullet the faster yuou want the twist very specific information hammer forge vs other rifling, now rifling depth is standardised for conventional amunition, but the method this rifling is put in the barrel effects accuracy, most rifling is done via pulling a "razer" and shaving the materail away in the rifling pattern, but hammer forgeing means the rifle is hammered from the outside onto a rod with the rifling already engraved on it, this cause the engraving to be mirrored on the barrel giving the barrel its rifling, however this method tend to produce stress points on the rifling and even uneven rifling and could prevent consistant accuracy. 
rifling type aka polygonol or conventional. polygonol create a better gas seal and thus is the more accurate, however produces more lead fouling ann require more upkeep to keep clean, conventional is normal rifling. the last that affects accuracy is the throat how well a round transitions into rifling, think of the crown but in reverse. |
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One small correction. It is not bullet weight per se but the bullet length. Let's use the 458 caliber for an example. A short 250 gr. lead bullet needs a slower twist than a long 500 gr bullet which supports what was said, but a super long 100 gr aluminum bullet, which is as long as a 500 gr lead bullet, needs the faster twist like the 500 gr bullet, not the slow twist of the lead 250 gr. bullet.
Long bullets need a faster twist, short bullets need a slower twist, which normally means heavies need a fast twist and light bullets need a slower twist, but it really boils down to length. |
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Quoted:
Define what you mean by "tighter" rifling. Well it basically was explained to me that there was no wobble and the casing fit real tight in the barrel. It is supposedly not very smart to have such a tight fit in regards to liability. I think I need to google it. |
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Ideally, the bore must be cylindrical and the rifling geometry the same down the length of the bore.[35] Some airgun rifles have a short cone towards the muzzle to improve the speed of the bullet.
A slight gain in the rifling twist, or a slight tapering down of the bore, will still ensure the tight fit of bullet to bore, so when selecting a barrel, the tighter or faster twist end should be chosen for the muzzle.[35][36][37] The twist of the rifling must match the intended ammunition for best accuracy. Rifling with too slow a twist will not stabilize long bullets, causing them to precess in flight; at its worst, this can result in the bullets tumbling in-flight and keyholing, where bullets strike the target sideways. Too fast a twist can also be a problem, as it can magnify problems in the bullet. A bullet whose center of mass is slightly off-center will diverge at a rate proportional to the rifling twist, so excess twist will result in greater dispersion.[38] In practical terms, this is only a problem for rifles chambered for common military calibers where a variety of different loads exist. For example, the M-16A1 rifle is unable to accurately fire bullets that are heavier than 55gr due to barrel twist that is too slow to stabilize heavier bullets.[39] Precision rifles typically come with barrels that are either tailor-made for a specific ammo load or made according to the buyer's specifications. Barrels can also benefit from lapping, not only because it makes the bore a more consistent diameter, but also because it polishes the bore. Barrel lapping should be done with the tool moving in the same direction as the bullet will move, so that any imperfections in the barrel will be smoothed so that they don't interfere with the passage of the bullet. A smooth, polished bore will not only hold the bullet better, but also reduce barrel fouling.[35][37] Chamber[edit] Of great importance to accuracy is the fit of the ammunition to the barrel. The chamber should be concentric, and the throat sized just slightly larger than the bullet diameter.[35] The cartridge must headspace correctly, be held firmly in place, concentric to the bore, and the bullet must fit the bore and be guided to engage the rifling cleanly.[3] Once the bullet is engaged in the rifling, with a good seal and coaxial fit, it must stay that way. It is often possible to shorten a barrel slightly by removing material from the breech end and re-cut the chamber, which can correct many problems in the original chamber.[36] |
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