Can rifling affect BC

[oaken]

Does anyone know anything about the BC of a bullet being affected by the rifling?

Fashion sees those looking for an edge trimming their meplats (I would too, if I

could afford a trimmer) but surely the damage inflicted to the bearing surface of

a bullet can make a significant difference too.

Can a 3 groove barrel create a more efficient bullet than a 5 or 6 groove and do

the “c” and “r” style rifling profiles contribute anything to higher BC’s. Does

a worn barrel create less damage than a crisp freshly cut one?

Or am I just being daft?

Rup

[6.5shooter]

Good question Rup.

Have often wondered the answer to this myself.

Lost river bullets made a 338 lap bullet with an extremely high bc and it had ribs the whole way down the bullet. It looked like a ribbed condom

They claimed this sealed the bullet so the rifling didnt change the bullets shape therefoe not effecting its high bc.

So there must be some truth in it.

 

Garry

[Andrew]

I guess that if you measured and calculated the BC of a single bullet, fired from identically dimensioned/twist barrels of differing rifling forms, at exactly the same velocities, at exactly the same moment, you might get an inkling of the answer.

 

The problem is that most shooters never calculate the actual BC of their chosen projectile for the locality they shoot at, and in the environmental conditions they shoot at, anyhow. If you're not going to do that, then pondering rifling form effect is a moot, eh?~Andrew

[The_Dogge]

I guess that if you measured and calculated the BC of a single bullet, fired from identically dimensioned/twist barrels of differing rifling forms, at exactly the same velocities, at exactly the same moment, you might get an inkling of the answer.

 

The problem is that most shooters never calculate the actual BC of their chosen projectile for the locality they shoot at, and in the environmental conditions they shoot at, anyhow. If you're not going to do that, then pondering rifling form effect is a moot, eh?~Andrew

Not necessarily, if there was a systematic effect, like sharp rifling reducing BC by roughing up the bullet (I'm not saying it does, but if it were a known phenomenon), then we'd choose microgroove or something wouldn't we? You don't need to know your exact BC value to gain an advantage from it being higher do you?

 

I very much doubt it would have a measurable effect for a pointed supersonic bullet, drag is dominated by the shockwave drag, the skin drag is less important and the effect could be counterintuitive, think dimpled golf balls....

 

From the title I thought you meant rifling twist rate, which can theoretically affect the BC if the nose of the bullet doesn't track the trajectory because it is overstabilised, so presents a slightly larger frontal area, though this is a theoretical effect which I don't think has any real consequence. Bryan Litz covers it in one of his articles here http://www.appliedballisticsllc.com/index_files/Articles.htm

 

Has anyone seen Ben Taylor's "Smooth Twist" rifled barrels? A prototype was supposed to shoot very well on a ruger 10-22, though I think they were primarily developed for air rifles. They were supposed to improve the BC of pellets by giving them a smooth edge, though a subsonic pellet has very different aerodynamics to a supersonic bullet.

[Andrew]

What I was saying is that if you don't know the BC you're working with to begin with then, any reduction would go unnoticed. I wasn't commenting on whether or not it actually happens. Everything the bullet experiences affects BC.~Andrew

[c18rch]

As you say Gareth, the nose up effect from over stabilisation would have more effect than damage to the bullet if present. I have thought about this in the past and wondered if, as you say, it actually improves BC by delamination. I also seem to remember reading somewhere about different bullet designs in history and one person trying a sprial screw around the outside of a bullet to give some sort of propulsion to the bullet. A kind of corkscrew effect, which go me wondering if the rifling imprint might have a similar affect.

 

But also as above these things are all very difficult to prove. The main difficulty would be firing a bullet accurately without leaving any rifling marks on it to use as a comparison.

 

Interesting you should also mention smooth twist rifling. Is this similar to polygonal? My .30-06 has a polygonal rifled barrel and I have to say I am inpressed with it. It does seem to live upto some of the claims for polygonal rifling such as higher velocities. One of the claims is also less bullet jacket damage, although this is do with the bullet jacket holding together better and improved accuracy by not umbalancing the bullet as much more than improved BC.

[The Gun Pimp]

 

 

Interesting you should also mention smooth twist rifling. Is this similar to polygonal? My .30-06 has a polygonal rifled barrel and I have to say I am inpressed with it. It does seem to live upto some of the claims for polygonal rifling such as higher velocities. One of the claims is also less bullet jacket damage, although this is do with the bullet jacket holding together better and improved accuracy by not umbalancing the bullet as much more than improved BC.

 

If a barrel is producing a higher velocity, then something is creating more pressure. All things being equal - cartridge, barrel length etc., then the internal finish may be creating more friction due to surface finish or rifling 'squeezing' the bullet more or the actual bore may be tighter than your previous barrel.

 

Cheers

Vince

[c18rch]

The claim is that polygonal barrels create a better gas seal which results in more velocity:

 

http://en.wikipedia.org/wiki/Polygonal_rifling

 

I know this isn't the best source but I have seen it elswhere too.

 

I have nothing to compare it against so I can't verify my claim. Especially as mine is a .30-06 AI. But having looked at the available reloading data and comparing it to what I get and also quick load results I seem to be getting about 50-100fps more than the data suggests I should be getting.

 

This is taking into account barrel length and environmental factors.

 

As you say it could be due to one of the factors you suggest. But I am not getting any pressure signs. I cannot prove otherwise which is why I said seemed to be living upto the claims.

 

I do take the advantage claims with a pinch of salt. I have not had the barrel long enough to form my own opinions yet. I bought it with the intention of seeing if they are as good as claimed. So far I am impressed.

[The_Dogge]

What I was saying is that if you don't know the BC you're working with to begin with then, any reduction would go unnoticed. I wasn't commenting on whether or not it actually happens. Everything the bullet experiences affects BC.~Andrew

I understood, but I disagree, I don't know the exact BC I've got* but if I started trimming meplats I would notice lower impacts even if I put the extra wind drift down to my miscalling the wind. I wouldnt know either value, but I would notice the difference.

 

 

* but it must be close, my software produces the right drop values

[Laurie]

I very much doubt it would have a measurable effect for a pointed supersonic bullet, drag is dominated by the shockwave drag, the skin drag is less important and the effect could be counterintuitive, think dimpled golf balls....[The_Dogge]

 

Right on! Rifling marks which are pretty small anyway could only affect surface drag which makes up a tiny percentage of the total drag in a supersonic bullet. It's so small that ballisticians just say ignore it for any ordinary design considerations. The vast majority of the drag comes from the shockwave that builds up ahead of the tip with secondary turbulence bands running off the bullet at the shoulder/body junction. A smaller component is tail-drag that is directly determined by the bullet's base area (hence boat-tail rear ends whose function is simply to reduce the size of that area). Nose drag is velocity dependant while tail and surface drag aren't. So, for any ordinary purpose where the bullet remains supersonic throughout its flight, think nose shape and meplat diameter first and foremost, tail shape second and don't worry about the rest.

[brown dog]

 

From the title I thought you meant rifling twist rate, which can theoretically affect the BC if the nose of the bullet doesn't track the trajectory because it is overstabilised, so presents a slightly larger frontal area, though this is a theoretical effect which I don't think has any real consequence.

 

One consequence is that the overstabilisation would increase (spin)drift.

 

 

I also think in terms of the skin friction ideas mentioned, heavy rifling marks would (and yes we're at the nerd level here ) increase the boundary layer thickness with a knock on effect of increased consequences arising from the magnus effect (someone appositely mentioned golf ball dimples earlier). With a clockwise spun proj with an equilibium yaw to the right, there's a gnat's nadger of left to right 'crosswind' that would, through magnus lift increase range by an incy tiny wee bit.

 

One weird thing that may, or may not read across from BIG bullets (I don't know) is that the disruption of smooth airflow along the surface may serve to actually increase BC (ie reduce the Cd).

Turbulence towards the rear of the proj can actually decrease base drag - this is a (perhaps counter-intuitive at first sight!) effect observed as a result of the turbulence created by the notches engraved by the rifling in shell driving bands.

[Gandy]

One consequence is that the overstabilisation would increase (spin)drift.

 

 

I also think in terms of the skin friction ideas mentioned, heavy rifling marks would (and yes we're at the nerd level here ) increase the boundary layer thickness with a knock on effect of increased consequences arising from the magnus effect (someone appositely mentioned golf ball dimples earlier). With a clockwise spun proj with an equilibium yaw to the right, there's a gnat's nadger of left to right 'crosswind' that would, through magnus lift increase range by an incy tiny wee bit.

 

One weird thing that may, or may not read across from BIG bullets (I don't know) is that the disruption of smooth airflow along the surface may serve to actually increase BC (ie reduce the Cd).

Turbulence towards the rear of the proj can actually decrease base drag - this is a (perhaps counter-intuitive at first sight!) effect observed as a result of the turbulence created by the notches engraved by the rifling in shell driving bands.

 

Ahh beat me too it, took the words out of my mouth.

 

Actually can't even read / spell most of the above sounds good though!

[CraigBenson]

I would expect that the BC would be effected by different rifling prifiles, the question is would it have a noticeable effect where POI would differ by more than a mm or so at the longer ranges that the rifle is used at? I suspect that there are greater forces acting on the bullet in flight so as to make any change in BC made by the rifling irrelevent.

[brown dog]

I would expect that the BC would be effected by different rifling prifiles, the question is would it have a noticeable effect where POI would differ by more than a mm or so at the longer ranges that the rifle is used at? I suspect that there are greater forces acting on the bullet in flight so as to make any change in BC made by the rifling irrelevent.

 

Craig,

 

Absolutely!

Just chatting through the ballistic nerditry of it

[oaken]

 

Just chatting through the ballistic nerditry of it

 

I did think the subject might bring on some severe nerdity, maybe even

propellerheadism.

It was C18rch's polygonal barrel got me thinking about the possible

advantages over conventional rifling. His rifle certainly shoots very

well and apparently takes minimal cleaning so it would seem to be all

good.

I am already contemplating next weeks

"tiro question requiring eXtreme nerdology"

Rup

[CraigBenson]

The thread is a good one and makes interesting reading. It makes you think about all aspects of balistics and that is never a bad thing. It was bound to get the boffins minds racing (as if they need any help with that.

[stillair1]

If you look from the base end of a .22lr shot through a smooth twist, it looks like a 5 side polygon with rounded corners. From the side you can detect the curved edges via. some magniifcation.

 

neil