Rifling Surface Area

[Steve S.]

I am working up .308 150 grain loads using (at present) the Hornady SST and BTSP designs. The SST has a longer rifling engagement surface than the BTSP - I am wondering if the increased surface area inherently makes for a more accurate design - should I lean toward the longer shaft designs or are there too many other variables to favor just that one? Thanks.

 

[Nature Boy]

I am working up .308 150 grain loads using (at present) the Hornady SST and BTSP designs. The SST has a longer rifling engagement surface than the BTSP - I am wondering if the increased surface area inherently makes for a more accurate design - should I lean toward the longer shaft designs or are there too many other variables to favor just that one? Thanks.

I don’t think the longer bearing surface of the SST makes it inherently more accurate, but it usually generates more pressure compared to bullets of the same weight with shorter bearing surface. That’s why you can’t just assume the same powder charge works across same weight bullets of different makes and designs

The SST is a hunting bullet. My experience with it shows it exbits good hunting level accuracy but it’s not a match bullet

 

[Archie]

The old timers shooting cast bullets tended to think the longer bearing surface improved accuracy. It could be the lead bullets needed the greater land to bullet contact than jacketed bullets.

From what I've heard claimed, it seems some gun writers think it does and others think it don't. I can't recall anyone claiming accuracy was worse.

It seems - and don't take my word for gospel - the increased bearing distance will increase interior pressure. But not a lot. If one begins at the starting level and works up, one is not likely to get into trouble. This assumes the loading manual doesn't print 'special' loads and dire warnings about 'that certain bullet'.

In any event, I advise against working up a load with the same weight bullet to max or close, then changing bullets without being the process over. Do not replace much anything at the top end.

Serendipity. More chamber pressure should deliver greater velocity. At the cost of greater interior pressure, which may accelerate wear.

 

[Walkalong]

I believe that, in general, a longer bearing surface is good for accuracy, but there are other more important variables, with jacket uniformity and overall bullet uniformity being paramount.

 

[Slamfire]

I believe that, in general, a longer bearing surface is good for accuracy, but there are other more important variables, with jacket uniformity and overall bullet uniformity being paramount.

There is one parameter that we, the reloading public, take for granted, and that is bullet concentricity. If the center of gravity of a bullet is not within the axis of rotation, then the bullet will be unbalanced, wobble, in flight. This is not condusive to best accuracy. Devices have been built for shooters who wanted to determine just how good their bullets where, but these devices are expensive:

http://www.bulletinspector.com/vern_juenke_history.htm





You can buy a this version for $1850.
http://www.bulletinspector.com/products.htm

This is an interesting quote from the seller:

No one here at BulletInspector has any idea of how this works.

I have a number of "vintage" rifles and when shot with modern ammunition, the groups are better on the average than what I have seen reported in pre WW2 and even, post WW2 literature. I am of the opinion that this is due to the microprocessor revolution wherein manufacturing technology is making better, more consistent bullets. Before Total Quality Management, manufacturer's allowed a certain percentage of junk to go out the door. Production lines were not under control, it was just accepted that a certain amount of junk would be caught by QA, and some would be shipped. You can see this in the tale of the low number M1903's in Hatcher's Notebook. The Army was aware that it was shipping defective rifles, but instead of fixing their production lines,figuring out why they were making bad product, the Army decided to raise proof pressures at the end of the production line. Their idea was to induce a greater stress on the rifle and weed more weak actions. But they were smart enough to understand that they were still shipping defective rifles, that would pass the proof test, and fail when enough regular pressure rounds went down the barrel. It was a lazy man's way of fixing a termite problem: just paint over the termite infested wood and leave the termites be!

This was the thinking of the times, and I will say, into the 1980's, when the Japanese, following Demming's theories on Quality Management, were beating the stuffings out of American Manufacturers. Competition proved that producing junk was unprofitable, even if the quantity was small and the average consumer was unable to discriminate between good and bad. Manufacturer's cannot stay profitable, for long, in today's world, unless the production line produces good product, each and every time. Microprocessor technology and modern manufacturing engineering have created for us, much better bullets, than what used to be available at the same price level. Old guns can shoot very well if fed good bullets. Our modern bullets are just great, even the hunting bullets.

 

[Demi-human]

...are there too many other variables to favor just that one?

Yes.

I find the ELD, VLD type to be very accurate, despite the shorter bearing surface on a longer bullet.

 

[hdwhit]

SteveS asked:
I am wondering if the increased surface area inherently makes for a more accurate design - should I lean toward the longer shaft designs or are there too many other variables to favor just that one?

All other things being equal, a longer "shaft" would be inherently more stable as it provides less opportunity for the bullet to "wiggle" as it is driven down the barrel.

Unfortunately, not all things are equal in this case. As already noted, the longer "shaft" has more material in contact with the barrel so it generates greater friction thus slowing it down.

As Slamfire pointed out, different bullet designs and products of different manufacturers may do a better job of getting the center of mass and center of buoyancy to coincide and this effect can swamp any greater accuracy resulting from by the longer "shaft".

Of course, for bullets of the same weight, a bullet with a longer "shaft" has a shorter nose and probably a shorter length overall and this can impact whether the rifling's twist rate provides optimum stability (i.e. the longer the bullet, the faster it needs to be spun, but there are limits).

And don't forget that each barrel and each rifle are individuals with their own tiny manufacturing anomalies, so the best way to find out which one works best in your gun is to load up some of each configuration at different powder charges and go out to the range and test them.

 

[Walkalong]

A match bullet is no better than the uniformity of the jacket thickness. If it is thinner on one side, even by the tiniest amount, the lead core is off center and unbalanced.

 

[Jim Watson]

A long bearing surface is favorable for accuracy.
A long spitzer nose and long boattail are favorable for trajectory.
Life is a compromise.