The effect of bearing length on accuracy

[SteveOak]

To bound the consideration a bit, lets assume a goal of .5 MOA as sufficient accuracy. Discussion of accuracy tighter than that are certainly interesting and appreciated, but please indicate if your thoughts are intended to exceed the goal of .5 MOA accuracy to help me sort things out. .5 MOA is sufficient for what I have in mind.

Thanks!

Is there a relationship between bearing length and accuracy?

If the bearing length is less than a certain percentage of the bullet length expressed, say, in calibers, is the potential for accuracy adversely affected?

If bearing length to bullet length adversely affects accuracy, is this related to the ogive of the bullet? Does the ogive shape and/or rate of curve change the answer?

I may think of more questions but this should get me started. Your input is much appreciated.

 

[ASbobcat]

To bound the consideration a bit, lets assume a goal of .5 MOA as sufficient accuracy. Discussion of accuracy tighter than that are certainly interesting and appreciated, but please indicate if your thoughts are intended to exceed the goal of .5 MOA accuracy to help me sort things out. .5 MOA is sufficient for what I have in mind.

Thanks!

Is there a relationship between bearing length and accuracy?

If the bearing length is less than a certain percentage of the bullet length expressed, say, in calibers, is the potential for accuracy adversely affected?

If bearing length to bullet length adversely affects accuracy, is this related to the ogive of the bullet? Does the ogive shape and/or rate of curve change the answer?

I may think of more questions but this should get me started. Your input is much appreciated.

Your question is a good one. Maybe a professional ballistic expert has a definitive answer, but from my own personal experience, accuracy suffers if the bearing length is shorter than the diameter/caliber. Some bullet manufacturers sacrifice bearing surface for a longer ogive in an effort to boost BC. From my experience that's a negative tradeoff.
If the bearing surface is less than diameter, the potential for "wobble" in the bore is naturally greater.

 

[GLShooter]

I've never noticed issues based on bearing length as long as the twist was right for the overall length. I haven't run into anything that had less than a caliber of bearing surface as I like seating deoths of at least that.

 

[SeabeeKen]

That makes sense but it would also seem that it would improve the chance that the bullet could start into the rifle crooked with less chance of it straightening out. Just my thoughts on it.
I shoot a 175 hammer hunter in my 338 which has a very short bearing surface but it shoots under .5"

 

[gunsandgunsmithing]

With the .5moa caveat, I don't think you see that much difference but these things have been tested in sr br and little things do matter...just not likely to .5moa with a quality bullet and gun. Center of gravity and center of pressure are two. My friend Randy Robinett makes a fantastic bullet but I had a Broughton bbl that just plain would NOT shoot his 30 cal 118 10 ogive at all. Around .5 moa IIRC. The bbl shot other bullets well and those bullets shot great in another bbl. I'd be guessing as to why. Very short bearing surface and 5c rifling, maybe...or maybe just coincidence. IDK.

 

[dcali]

Conventional wisdom says that if it's long enough to keep the bullet from tipping in the bore, it's long enough. When you look at it that way, the ogive shape matters - a tangent ogive will help keep things straight a little better than an aggressive VLD. I've been warned by people smarter and more experienced than me about making long bullets with a bearing surface less than a caliber. I haven't tried it yet, but bullets in the neighborhood of 1 caliber are capable of excellent accuracy. It'd be far easier to make bullets using a short jacket and just try than to theorize about acceptable minimums.

The reason you have to keep the bullet straight is exacly why you'd think. If it comes out crooked, it gets all wobbly and bad things happen. I could get more technical, but it wouldn't add anything to that description.

The downside to a long bearing length is that in order to get it, you have to shorten the nose. Doing that hurts BC significantly. (The minimum drag shape is essentially zero bearing surface). Everything in bullet design is a tradeoff.

 

[Infrequent Shooter]

The 7.5Swiss bullet is pretty darn impressive. I don't have the specs handy. good example of older design that got it done. called GP11

Swiss cartridge GP 11

The legendary Swiss cartridge “GP11” (7,5 x 55mm, Image by Carteach0)

swissgunblog.wordpress.com

 

[Infrequent Shooter]

more on GP11, Swiss

 

[K22]

I am also in the 1/2 moa club meaning that is good enough for my purposes. In my experience, shooting a 1/2 moa consistently is not easy even with the high-quality equipment and tuned reloads.

With that said, the aspects of bullet bearing length that I have experienced are that it makes a difference depending on the rifle's twist. For example, for many years I have shot 12" twist 223 bolt rifles using 50 to 55 grain bullets. I was able to achieve fairly consistent 1/2 moa groups at 100 yards with tuned reloads and my higher quality rifles / scopes. When I entered the world of 8" twist 223 Rem via the Tikka rifles, the 55's just didn't shoot as tight. I tested the 60 Vmax flat base bullets which have a longer shank and I was back in the 1/2 to sub 1/2 moa area.

The other aspect I have noticed is that flat base bullets, while not popular with a lot of shooters, almost always shot better than the boattail bullets out to about 200 to 250 yards. One exception is the 85 Sierra BTHP in the 243 Win which has a very pronounced boat tail. This is the most accurate bullet I have ever tested in the 243 Win, and I've tested a bunch in the last 50+ years. Also, the Nosler BT's have a very slight boat tail, almost a bevel rather than a boat tail. They shoot as good if not better than any flat base bullet I ever tested.

So, the bottom line in my experience is that you need to do your own testing to determine which bullet performs well in your rifle. I don't think there are any absolutes.

 

[Tesoro]

On a side note: not being a competitive shooter, I get a little confused on this .5 moa or whatever definition that is thrown around. I understand this is an aggregate number, but how many shots is the norm? Is wind, or the occasional bad load or pull, included in the statistic? And is there is standard range? I would shite can any barrel that wont shoot consistently under .5 at 100, excluding my induced errors, but after stretching out the distance then the other factors can over ride the rifle's precision capabilities for moa 'averaging'. I assume accuracy is a misnomer and not part of the 'moa calculation' as this is due to skill and not physics. So what is the true scoop?!

 

[TaperPin]

The more even and consistent the barrel is, with appropriate twist to stabilize the bullet, the easier a 1/2 moa rifle is. Until the barrel was shot out, my first 1/2 moa gun, a 7 lb 7 mag hunting rifle shot every hunting bullet and powder tried into 1/2 moa. My advice to other hunters is the fastest route to 1/2 moa isn’t at the reloading bench as much as it is the barrel and chamber. I cringe every time someone buys a Bartlien, Brux, Krieger and then has a loose max dimension saami chamber cut by someone who simply runs the reamer in with minimal effort expended. Those saami specs on modern cartridges that show nice tight .0005” throat clearance and good clearance around the case, lose their appeal once the +.002” acceptable tolerance is added on.

 

[ASbobcat]

I've never noticed issues based on bearing length as long as the twist was right for the overall length. I haven't run into anything that had less than a caliber of bearing surface as I like seating deoths of at least that.

They are rare, mostly in the 6mm 95gr. That's where I found the problem. Berger 95gr hunting VLD is significantly shorter than caliber, and it was a shotgun in barrel that shot 105's just fine.
But others here have not had a problem.

The upside of short bearing surface is less friction and more velocity. So it's a tradeoff, if you can keep them under .5moa.

 

[J_idaho]

I found using 6mm 95gr vlds in my sons 243 win that they shot really good when seated deeper than the mag length would allow after trying the hybrids which is the 95gr classic hunter they shot very consistent with more jump at mag length I had to give up 1/4 moa from 1/2”-3/4” groups at a 100 yrds but in a gun that’s not going past 500 yrds that’s fine.

 

[Zero333]

I had a great load with the 240 gr SMK (no longer made) and RL-33 in one of my 300winmags that had a fairly short freebore, and I wanted it to fit reliably in a 3.5" magazine. So I loaded it to 3.450" COAL.

The 240 SMK has the longest bearing surface I've ever seen on a .30 cal bullet. It was seated really deep in the case. I still got great accuracy and velocity. Everything from 75 gr and 77 gr of RL-33 had the same Point of Impact. Thus I went with 76 gr @ 2,770'ish fps from a 26" barrel.



220 yards 4 shot group...



Didn't need to burn up the barrel any more. Load development took 20 bullets ( 4 shot groups @ 75 gr. 75.5 gr. 76 gr. 76.5 gr & 77 gr.) and saved a bunch of barrel life.

 

[Bill Norris]

Differences in bearing surface length will certainly get into the differences in the amount of drag vs. bullet weight vs. speed vs. barrel condition etc. and the list goes on. It would require lots of testing and possibly expensive equipment to determine the answer.

 

[jelenko]

On a side note: not being a competitive shooter, I get a little confused on this .5 moa or whatever definition that is thrown around. I understand this is an aggregate number, but how many shots is the norm? Is wind, or the occasional bad load or pull, included in the statistic? And is there is standard range? I would shite can any barrel that wont shoot consistently under .5 at 100, excluding my induced errors, but after stretching out the distance then the other factors can over ride the rifle's precision capabilities for moa 'averaging'. I assume accuracy is a misnomer and not part of the 'moa calculation' as this is due to skill and not physics. So what is the true scoop?!

Great question.

From my own experience and reading what others have said/reported:

Reported/claimed groups range from 3 to 10 shots. Unless someone says the number of shots in a group, you really don't know. My perception is, if the number of shots is not mentioned, it's a three shot group.​

Group size is generally reported as the actual size on the target - including any effect from environmental conditions. If the reported group size does not include a shot that was off call for some reason, most people will mention that.​

As one's skills improve, we build confidence in the results on paper. Accuracy as discussed on this forum is meant to be about the rifle and ammo - not about the shooters skills or about environmental conditions per se. That said, actual results depend significantly on the shooters skills in positioning the rifle, executing the shot and reading the environmentals.​

 

[gunsandgunsmithing]

A teen rifle doesn't always have a teen shooter but in a match, where it matters, they measure them all. If you look at a match report, nobody cares if someone shooting factory bbl, action, trigger etc finishes last while a bat, kreiger and bix wins. Nobody asks why one wins and one loses. But reverse that...A factory gun wins and all of a sudden, conditions must explain it..or it was a fluke, or whatever.

Bottom line, if I say I have a teen rifle, I'm not counting my mistakes but rather, what I can do with that rifle MOST of the time, in average conditions. I don't cherry pick conditions as that's a waste of time anyway. The flip side is, if I pull the trigger in a switch, that has nothing to do with the rifle...UNLESS i'M IN A MATCH.


Every shot counts but if we're measuring what the rifle truly is, I say it's what the gun does consistently and in average conditions. In a match, the best gun can finish last due to one shot. Doesn't mean it's bad or that it's great..but that's what it was on that particular day. IOW, I did or didn't do my part. Famous words! Perfect conditions and bad conditions are both bad times to honestly assess a rifle. It's nice to shoot teeny tiny zeroes but they mean nothing unless they are in a match. The question is a good one and IMO, it's about being honest with yourself about your equipment. Wallet groups are a waste of paper they are printed on most of the time. Any decent rifle can do it once! A teen rifle does it a lot. I mean, a whole lot! Unexplained "fliers" are the gun/load until you prove otherwise. This is much of why so many experienced shooters emphasize using flags...EVERY TIME YOU SHOOT.

 

[jelenko]

It's nice to shoot teeny tiny zeroes but they mean nothing unless they are in a match.

!!

 

[searcher]

I think the short answer to everything you asked is yes.

 

[Straightshooter1]

To bound the consideration a bit, lets assume a goal of .5 MOA as sufficient accuracy. Discussion of accuracy tighter than that are certainly interesting and appreciated, but please indicate if your thoughts are intended to exceed the goal of .5 MOA accuracy to help me sort things out. .5 MOA is sufficient for what I have in mind.

Thanks!

Is there a relationship between bearing length and accuracy?

If the bearing length is less than a certain percentage of the bullet length expressed, say, in calibers, is the potential for accuracy adversely affected?

If bearing length to bullet length adversely affects accuracy, is this related to the ogive of the bullet? Does the ogive shape and/or rate of curve change the answer?

I may think of more questions but this should get me started. Your input is much appreciated.

A while back I had the same kind of questions when I started precision reloading. I had sorted a batch of 168 SMK's and found some with a .033 difference in bearing surface length (BSL). So, I took 20 of each and loaded them up to see what the difference might be out of my RPR's 1:10 - 20" barrel. As it turned out, there was, what I consider, significant difference in velocity as well as a POI shift at 100 yds.

Note too that I had weighed them and also measured the BTO's to see if there was a significant difference there, but there wasn't much and certainly not as much difference as the BSL difference (also measured the OAL's, labeled BL for bullet length, that showed some corresponding difference to the BSL's).

IMHO, it take a large difference in BSL, like we have here, to see a difference on target for short range . . . but at extreme distance shooting, that's something that should be considered.

Here's how all that looked: