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domer_pyle · Senior Member

http://www.chuckhawks.com/rifle_barrel.htm

Interesting info. I wonder if a shorter rate of twist would help with accuracy from pointed pellets too.

domer_pyle · Senior Member

I don't know what I was thinking, meant to put this in the rifles section, sorry

DavidS · Banned

Hi Domer,

This is an interesting subject which I almost wrote about before and one that may have some bearing on the accuracy we are able to achieve with our air rifles but there are a number of other factors that also enter into the equation.

Twist rates in firearms are generally determined by the weight of bullet an individual is intending on shooting for any given caliber, usually meant for individuals that reload so they can pick an appropriate bullet for a given barrel and the twist rate of the rifling. Lighter weight bullets require a faster twist rate to maintain stability in flight while the heaver bullets of the same caliber don't require as fast a twist rate to maintain the same stability as they have more kenetic energy to keep them stable and on their flight path.

I'm not sure just how much effect the slight variation in pellet weights actually has to do with accuracy in flight related to the twist rate of the barrels in a pellet rifle? We don't have much choice anyway. Now between a Pellet rifle and pistol with generally a much shorter barrel there may well be some bearing due to the shorter distance and less than one actual revolution that takes place so a heaver pellet may do better there?

Considering that in a break barrel rifle we basically should get a fairly uniform power output with each cock of the rifle so it would then seem that variations in weight between pellets may cause a bit of a different flight pattern but also their design and uniformity would seem to enter into the equation too.

I weighed 16 Crosman Hunting Pellets on my RCBS Electronic Powder Scale and found that even amongst them there was as much of a variation as .4grn, most were 7.5 grn. but only in a couple out of 16 pellets were as much as 7.9 grn. but at the least the heavier would tend to drop a bit faster and give a different flight path in trying to get a group it would seem. The way around this would be to weigh each pellet of a scale
electronic preferably and group them so you'd know for sure all being shot were indeed of the same weight. A bit of a pain, I'll admit.

Being shot from a barrel with a 1 in 12 twist rate means the pellet twists or rotates Once in every 12 inches of travel times the velocity at which it's shot at. So for the ease of figuring lets just say it's shot out of the barrel at 1,000 ft/sec which equals 1,000 revolutions per second.

In my target shooting it seems that the time elapsed from when the Benjamin fires to when the pellet strikes the heavy cardboard target just over 100' feet away is only about 1/10th of a sec., estimated, so even in that short space of time the pellet would rotate 100 times so there is a considerable amount of gyroscopic action taking place to hold the pellet on course.

So it seems to be a process of trial and error not to mention possible shooter error as well or the breeze if your not aware of it that might effect pellet flight? I did notice that some of the newer Gamo pellets did have a smooth finish to the pellet, as opposed to the ribbed skirt of the older pellets, which may be more beneficial? Also no only were some ribbed
but even contain casting irregularities that could affect flight and hence accuracy it would seem?

Very probably, accuracy is determined more by pellet design and uniformity thereof as well as consistent weight which is probably more of a determining factor in a pellets flight as well as consistency of power output.

I'm unsure but rather doubt that the small weight difference would have much bearing if fired from a barrel of a different twist rate?

rsterne · Moderator

Slavia · Moderator

domer_pyle · Senior Member

I've heard of that, read a bit about it after finding that article. Maybe twist rates can affect BC as well. I can't remember what thread it was even in but a couple rifles shot at almost the same velocity but one had a bit better BC, or something like that.

It would be cool to get a few barrels custom rifled to some really slow up to really fast twist rates, some at 1:50 up to 1:9 and do some BC and accuracy testing. To find out what, I don't know but I'm sure it would yield some interesting results

rsterne · Moderator

Mandrels for custom twist rates are available for $300.... and then the barrels (hammer forged) are about $5 per inch after that....

Step right up, the line forms here!.... I'll volunteer to do the testing and documentation if someone funds the project.... Mr. Green

Bob