Thread: bearing length

I have a rifle that doesn't group well or consistently. At this point I'd like to thank R93 , Jit and others that have helped with their advise. It's a 7rem mag with a 1 in 9.5 twist and this is where I believe the problem lies. I have wanted to use 162sst's and the like but they group terrible. I spent a lot of time trying all sorts of things with powder, seating depths etc but not until I had used 162 eldx's, 168 berger classics, amax's have I come to a conclusion based on a bit of measuring. To start with I believe I have a finicky barrel. I also believe 9.5 twist is boarder line for those heavier projectiles. Some rifles with this twist work and some don't. Reading stability tables it seems the longer the projectile the faster the twist needed or words to that affect. This got me thinking on which shot the best in my rifle so out came the verniers and hornady comparator. In order of accuracy it would go ;sierra tmk 3/4 inch, 168 berger 3/4-1 ,amax 1-1.5inch, eldx 1.5-2 inch, sst 1.5-3.00inch. Groups are usually consistant for each bullet just some days I shoot a bit better but the averages are the same. The tighter projectiles usually have two very close and one away to open up a group. I've had the barrel measured and checked as well. What I found tonight was interesting and sort of makes sense. I had a feeling bearing length might have something to do with it as much or more than length. I put the comparator on the vernier and measured the ogive length and the turned the bullet and placed the boat tail end in and measured that distance , then subtracted one from the other to get a bearing length and also measured the entire projectile. this is what I got;

Sierra tmk 160 oal 1.430 bearing .444
berger 168 classic 1.393 .347
amax 162 1.426 .524
eldx 162 1.475 .502
sst 162 1.394 .515

These are measurements from the boxes of projectiles I have and averaged over several from each box. You'll notice that the least bearing the better accuracy I get and that length seems to be a lessor factor. Those of you will notice the amax is out of place but I beleave that they have changed and my first shooting was probably done using the earlier ones which did shoot better than the boxes I have now. I got them off a friend who'd had them for years and the boxes were different and I understand there was a change to them at least once in their time.
So any ideas as to whether my method is any thing like valid or useful and like wise my findings?

your worried about 3/4 inch groups is it a hunting rifle or a bench rest rifle you've got

I think there is a lot to your experiment.
However fast twist rates are still required for the longer bullets to reduce the loss of stability about a projectiles axis when and if it yaws.

Bearing surface issue could be subject to rifling type and a number of other things.

I suspect one of my barrels is fickle like yours. It shoots the old Amax well. But will not shoot the new stuff at all.
Interesting stuff.

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It's a hunting rifle. Problem being that it likes soft target bullets which are fine at 300yards + but not so flash up close. I wanted to use a stiffer bullet like the sst with a good bc out to 500 yards or so and steel etc further out. I shot a small 70lb boar at 143 yards about a week ago with the tmk. It hit behind the shoulder and created a big entry and massive lung damage but didn't exit.I know velocity was still high but what if it had been a big boar twice that size or a large stag and I'd hit leg bone etc? Also point of impact changes a bit and I believe that atmospheric conditions have more effect on projectiles that aren't stabilized properly. This all leads to a lack of confidence it the rifle which isn't a nice thing

I shoot a brick of a bullet BC wise, out to 500. Tested out to 600 on steel and shot great.

It is in fair conditions it is just a few clicks on the dial.

If you are not planning to shoot further there are a number of bullets that will be excellent in your caliber if you find one that shoots.

I wouldn't get to hung up on BC unless I was shooting further.





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True. But I feel having a 7mm mag and only using low bc and light bullets defeats the purpose of the cartridge. My practice target is at 650 and have access to targets a lot further out. As for animals 5-600 would be plenty far enough but if the situation arose and conditions were right and a longer shot was needed then I'd like all the help from the cartridge I can get. It's looking like it's time to seriously look at another faster twist barrel. hopefully one that isn't so fussy. Also my try or see if anyone can some bearing measurements on some other projectiles. eg 154sst ,160 abs perhaps?

Flat base bullets should give you more bearing surface

Could I ask if you have shot any 140gr-class bullets and found that you do in fact get the accuracy you are after - confirming your line of reasoning?
The Nosler 150grBT gets good reports on here as a game bullet and alternative to the heavy-weights, and might be a compromise worth trying.

There's an article on AccurateShooter Daily forum right now on this exact topic. You cannot just use comparators back-to-back to get valid (relative) comparisons as there are other variables involved.

interesting topic

i use the double comparator technique to sort projectiles. i've marked the one comparator "base" and the other "tip" so they are always in the same orientation. as mentioned above, it is not perfect, but as a relative measurement it is the best option i have found so far.

i could get Berger 185 Juggernauts to stabilize fine out of 12 and 13 twist barrels, but no-go for Berger 185 Hyrids. would be interesting to do BSL measurements between those to see if it matches your results.

Originally Posted by r87mm

I have a rifle that doesn't group well or consistently. At this point I'd like to thank R93 , Jit and others that have helped with their advise. It's a 7rem mag with a 1 in 9.5 twist and this is where I believe the problem lies. I have wanted to use 162sst's and the like but they group terrible. I spent a lot of time trying all sorts of things with powder, seating depths etc but not until I had used 162 eldx's, 168 berger classics, amax's have I come to a conclusion based on a bit of measuring. To start with I believe I have a finicky barrel. I also believe 9.5 twist is boarder line for those heavier projectiles. Some rifles with this twist work and some don't. Reading stability tables it seems the longer the projectile the faster the twist needed or words to that affect. This got me thinking on which shot the best in my rifle so out came the verniers and hornady comparator. In order of accuracy it would go ;sierra tmk 3/4 inch, 168 berger 3/4-1 ,amax 1-1.5inch, eldx 1.5-2 inch, sst 1.5-3.00inch. Groups are usually consistant for each bullet just some days I shoot a bit better but the averages are the same. The tighter projectiles usually have two very close and one away to open up a group. I've had the barrel measured and checked as well. What I found tonight was interesting and sort of makes sense. I had a feeling bearing length might have something to do with it as much or more than length. I put the comparator on the vernier and measured the ogive length and the turned the bullet and placed the boat tail end in and measured that distance , then subtracted one from the other to get a bearing length and also measured the entire projectile. this is what I got;

Sierra tmk 160 oal 1.430 bearing .444
berger 168 classic 1.393 .347
amax 162 1.426 .524
eldx 162 1.475 .502
sst 162 1.394 .515

These are measurements from the boxes of projectiles I have and averaged over several from each box. You'll notice that the least bearing the better accuracy I get and that length seems to be a lessor factor. Those of you will notice the amax is out of place but I beleave that they have changed and my first shooting was probably done using the earlier ones which did shoot better than the boxes I have now. I got them off a friend who'd had them for years and the boxes were different and I understand there was a change to them at least once in their time.
So any ideas as to whether my method is any thing like valid or useful and like wise my findings?

And the Berger? Much larger difference in bearing surface compared to the TMK than the A-max to SST?

No never tried 140gn class other than some barnes 140's I had been given. They shot a bit over an inch off memory. But they being what they are are long for their weight anyway. Also I never got the rifle to shoot that weight as I would have stayed with what I had. I wanted to go from 130-150 at around 3k to 160- 180 at about the same. I would be interested to know if others measured the bearing surface on projectiles they found accurate in their rifles whether there is a link. I know there's variables such as secant or tangent, whether weight forward etc, softness or hardness of the jacket and core, throat lead and more. Not to mention on the eldx box it says 9.5 or faster. If I can lay my hands on a couple of say nosler ab's in 140-160 and 150bt I'll try them I only tried a few of the bergers so may have another go with them. But at the end of the day I've spent a lot and a lot of time chasing my tail and is it better to just rebarrel with a 8 twist?

ive always found by a lot of trial and error it what the gun likes not what you want it to like

True but you would expect a .270 to shoot 130gns, a.308 150's a .300 180's etc as they are bread and butter for the calibers. not radical very heavy or light for caliber bullets. I would have thought 160-162 would shoot in a 7 mag and maybe when you get 175-195 things might need to be accounted for like faster twists and maybe bigger cases to get the most out of them. I did see on another sight someone mentioned the more bearing surface the faster the twist needed. He believed thats why some of the bergers will stabilize using standard twists even though they are heavy and long for caliber. There bearing surface is shorter. Food for thought but needs more people to measure and record what works well in their rifles to see if there is a pattern I guess

What about trying the new 150eldx?